Preface

At the beginning of the 21st century, problem of providing sustainable electricity for all in‐ habitants of the World is still very sharp. More than a billion people resides in off-grid terri‐ tories. Meanwhile, mankind has faced another scourge - global warming, which is the result of the indomitable consumption of fossil fuels and has resulted in gigajoules of low-grade waste heat and huge amount of greenhouse gases. Waste heat energy is estimated to be from 60 to 70 % of primary energy produced from the burning of fossil fuels. Waste heat is conditionally free-of-charge energy, but it is difficult to use, and is thrown away - a pity.

So, mankind should solve both problems of the deficit of electricity and the recovery of waste heat simultaneously. It is possible in principle. And, thermoelectric power generation can help here.

In accordance with thermodynamic and physical laws, the generation of electromotive force (EMF) by direct conversion of an alternative form of energy into electrical energy is possible in nonequilibrium systems only. The more alternative energy and the greater the deviation from the equilibrium is in the system, the higher is the possible conversion efficiency. There are two main ways of direct conversion: photovoltaic (PV) effect and thermocouple effect (widely used type is Seebeck thermoelectric effect). Due to physical reasons, it is impossible to create PV converters, effectively generating EMF through absorption of thermal radiation photons. But, PV converters, operating with solar radiation and thermoelectric generator modules (TEGs), operating with thermal energy can work in couple.

The efficient recovery of low-grade waste heat is an important and nontrivial task. Sources of low-grade heat are plentiful everywhere. As a rule, low-grade heat is simply dissipated without any benefit to people. This is caused by the fact, that low-grade waste heat is strong‐ ly localized near heat sources. Therefore, waste heat is difficult to use in a cost-effective manner for an intended purpose. TEGs are small-sized items that can be placed as close as possible to heat (thermal) energy sources, which can have temperatures of hundreds of de‐ grees Celsius. TEGs can operate anywhere (including indoors) and at any time of the day These factors are decisive for applying TEGs to recover low-grade heat. Therefore, in practi‐ cal applications, the TEG's structure will be exposed to systematic long-term heavy tempera‐ ture gradients, mechanical stresses (thermo-mechanical stresses) and high temperatures on one (hot) side. TEGs and, hence, the thermoelectric materials forming legs of thermopiles must withstand the abovementioned shock. To become attractive and affordable to custom‐ ers, TEGs should have a service life of at least 5000 hrs, with thousands cycles ON-OFF and, of course, be cheap as well.

Competition in the field of TEGs in the future may be as high as it currently is for thermo‐ electric coolers (TECs). However, nowadays, we see a lack of innovative and affordable products on the market.

Despite the long history of thermoelectric power generation, there are many pressing issues in the thermoelectric materials science and the manufacturing technology of TEGs. Unlike TECs, where the maximum temperature in module is typically less than 60 °C, in TEGs, due to high temperatures (hundreds of degrees Celsius) on the hot side and heavy thermo-me‐ chanical stresses in module, many processes become active, leading to a quick or gradual degradation in the performance of the thermoelectric materials and the TEG itself. These degradation processes are namely, interdiffusion, recrystallization, alloying, dissolution, phase transitions, phase separation, phase segregation, sublimation, oxidation, mechanical damage of legs, commutation and interconnections, and other phenomena.

This book is an attempt to arrange the interchange of research and development results con‐ cerned with hot topics in TEGs research, development and production, including:


We think, that the information presented in this book will be helpful to scientists and engi‐ neers involved in research and industry projects related to innovative thermoelectric power generation devices.

We are grateful to all the authors from the many countries of the World for their contribu‐ tions. We hope, that coordinating these efforts in thermoelectric power generation will result in enhancements to the human living standards on our planet.

> **Sergey Skipidarov** PhD, Member of International Academy of Refrigeration CEO, Ferrotec Nord Corporation Moscow, Russia

> > **Mikhail Nikitin** PhD, Science and Technology Adviser Ferrotec Nord Corporation Moscow, Russia

**Advanced Thermoelectric Materials**

Despite the long history of thermoelectric power generation, there are many pressing issues in the thermoelectric materials science and the manufacturing technology of TEGs. Unlike TECs, where the maximum temperature in module is typically less than 60 °C, in TEGs, due to high temperatures (hundreds of degrees Celsius) on the hot side and heavy thermo-me‐ chanical stresses in module, many processes become active, leading to a quick or gradual degradation in the performance of the thermoelectric materials and the TEG itself. These degradation processes are namely, interdiffusion, recrystallization, alloying, dissolution, phase transitions, phase separation, phase segregation, sublimation, oxidation, mechanical

This book is an attempt to arrange the interchange of research and development results con‐

1. Prospective thermoelectric materials for TEGs. The important theme here is ob‐ taining effective p-type materials for low-, mid-, and high temperature ranges of

2. Theoretical study and calculations of key parameters of inorganic and organic

4. Novel methods and apparatus for measuring performance of thermoelectric ma‐

5. Thermoelectric power generators simulation, modeling, design and practice.

We think, that the information presented in this book will be helpful to scientists and engi‐ neers involved in research and industry projects related to innovative thermoelectric power

We are grateful to all the authors from the many countries of the World for their contribu‐ tions. We hope, that coordinating these efforts in thermoelectric power generation will result

**Sergey Skipidarov**

Moscow, Russia

**Mikhail Nikitin**

Moscow, Russia

CEO, Ferrotec Nord Corporation

Ferrotec Nord Corporation

PhD, Science and Technology Adviser

PhD, Member of International Academy of Refrigeration

damage of legs, commutation and interconnections, and other phenomena.

3. Research results in innovative construction nanomaterials.

in enhancements to the human living standards on our planet.

TEGs operation.

terials and TEGs.

generation devices.

XII Preface

thermoelectric materials.

cerned with hot topics in TEGs research, development and production, including:

#### **Layered Cobaltites and Natural Chalcogenides for Thermoelectrics** Layered Cobaltites and Natural Chalcogenides for Thermoelectrics

Ran Ang Ran Ang

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65676

#### Abstract

We have systematically investigated thermoelectric properties by a series of doping in layered cobaltites Bi2Sr2Co2Oy, verifying the contribution of narrow band. In particular, Sommerfeld coefficient is dependent on charge carriers' density and as function of density of states (DOS) at Fermi level, which is responsible for the persistent enhancement of large thermoelectric power. Especially for Bi2Sr1.9Ca0.1Co2Oy, it may provide an excellent platform to be a promising candidate of thermoelectric materials. On the other hand, high-performance thermoelectric materials require elaborate doping and synthesis procedures, particularly the essential thermoelectric mechanism still remains extremely challenging to resolve. In this chapter, we show evidence that thermoelectricity can be directly generated by a natural chalcopyrite mineral Cu1+xFe1<sup>−</sup>xS2 from a deepsea hydrothermal vent, wherein the resistivity displays an excellent semiconducting character, while the large thermoelectric power and high power factor emerge in the low x region where the electron-magnon scattering and large effective mass manifest, indicative of the strong coupling between doped carriers and localized antiferromagnetic spins, adding a new dimension to realizing the charge dynamics. The present findings advance our understanding of basic behaviors of exotic states and demonstrate that low-cost thermoelectric energy generation and electron/hole carrier modulation in naturally abundant materials is feasible.

Keywords: layered cobalt oxides, narrow band contribution, natural chalcopyrite mineral, thermoelectricity generation, electron-magnon scattering

## 1. Introduction

Layered cobaltites with CdI2-type CoO2 block provide an excellent platform for investigating thermoelectric properties. A key to unveil mysterious thermoelectric properties lies in the twodimensional (2D) conducting CoO2 layer. For layered Bi-A-Co-O (A = Ca, Sr, and Ba), it also contains analogous conducting CoO2 layer [1]. In particular, layered Bi2Sr2Co2Oy (BSC)

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

exhibits a rather large thermoelectric power S (∼100 μV/K) at room temperature, which makes Bi2Sr2Co2Oy one of promising thermoelectric materials from the viewpoint of potential applications, analogous to other misfit-layered cobaltites, such as NaCo2O4 and Ca3Co4O9 [2–5]. However, most studies of Bi2Sr2Co2Oy system are mainly focused on the thermoelectric improvement [2, 3, 6]. The transport mechanism based on resistivity ρ and thermoelectric power S has not been clarified. Moreover, large S is totally different from conventional value (<10 μV/K) based on a broad band model [7]. In this chapter, we will show evidence on a narrow band contribution in doped Bi2Sr2Co2Oy [8]. And what's more, exotic enhancement of large S is related to local density of states (DOS) near Fermi level (EF) [9]. It could be effectively modulated thermoelectric performance by utilizing different doping. It is plausible to distinguish, which thermoelectric materials in doped Bi2Sr2Co2Oy could be regarded as potential candidates.

On the other hand, ternary chalcogenides serve as an ideal platform for investigating intricate physical and chemical characteristics controlling the efficiency of thermoelectric materials, and also are promising materials for potential applications in photovoltaics, luminescence, as well as thermoelectric and spintronic devices [10–13]. Ternary chalcopyrite-structured chalcogenides, such as CuFeS2, have attracted particular attention owing to their unique optical, electrical, magnetic, and thermal properties [14–28]. Studies on chalcopyrite (CuFeS2) have primarily focused on its electronic states [14, 15, 29–31]. However, the microscopic mechanism of electronic structure and thermoelectric character in CuFeS2, which presumably arises from some scenarios such as delocalization of the Fe 3d electrons, charge-transfer-driven hybridization between Fe 3d and S 3p orbitals, or density of the conduction band electron states, still remains highly controversial [17, 30, 32]. The intrinsic mechanism of good thermoelectric properties is still a vital question which needs to be clarified. Another important issue is that the fabrication of artificial chalcopyrite itself requires expensively complex synthesis procedures and relatively high cost of constituent precursors, thereby potentially limiting the large-scale applications in the thermoelectric field.

In this chapter, we confirm that an unexpected thermoelectricity can directly be generated in a natural chalcopyrite mineral Cu1+xFe1<sup>−</sup>xS2 from a deep-sea hydrothermal vent, and demonstrate that doped carriers have strong coupling with localized antiferromagnetic (AFM) spins, which greatly enhance the thermoelectric power S and power factor, revealing the significance of electron-magnon scattering and large effective mass [33]. This will open up another useful avenue in manipulating low-cost thermoelectricity or even electron/hole carriers via the natural energy materials abundantly deposited in the earth.

## 2. Thermoelectric properties and narrow band contribution of Bi2Sr1.9M0.1Co2O<sup>y</sup> and Bi2Sr2Co1.9X0.1O<sup>y</sup>

#### 2.1. Crystal structure and valence states of Co ions

The crystal structure of Bi2Sr2Co2Oy is shown in the inset in Figure 1, where conducting CoO2 layer with triangular lattice and insulating rocksalt Bi2Sr2O4 block layer are alternatively stacked along c-axis, similar to the case of high-temperature superconductors like Bi2Sr2CaCu2Oy. Scanning electron microscopy (SEM) characterization of Bi2Sr2Co2Oy indicates surface morphology of plate-like grains. Figure 1 shows X-ray diffraction (XRD) patterns of selected samples Bi2Sr2Co2Oy, Bi2Sr1.9Ca0.1Co2Oy, and Bi2Sr2Co1.9Mo0.1Oy with single phase, in agreement with XRD result of Bi1.4Pb0.6Sr2Co2Oy [34]. The average Co valence was determined based on energy dispersive spectroscopy (EDS) measurement for all samples. For Bi2Sr2Co2Oy, average Co valence is +3.330. For Bi2Sr1.9M0.1Co2Oy (M = Ag, Ca, and Y), average Co valence is +3.380, +3.330, and +3.280, respectively. For Bi2Sr2Co1.9X0.1Oy (X = Zr, Al, and Mo), average Co valence is +3.295, +3.347, and +3.189, respectively. X-ray photoemission spectroscopy (XPS) spectra (see Figure 4a) also show the valence states of Co 2p3/2 and 2p1/2 for selected Bi2Sr1.9Ca0.1Co2Oy sample. Photon energy of Co 2p3/2 and 2p1/2 is 779.4 and 794.2 eV, respectively, demonstrating mixed Co valence between +3 and +4.

Figure 1. Powder XRD patterns for Bi2Sr2Co2Oy, Bi2Sr1.9Ca0.1Co2Oy, and Bi2Sr2Co1.9Mo0.1Oy samples at room temperature. Inset: crystal structure and SEM image of Bi2Sr2Co2Oy.

#### 2.2. Resistivity and transport mechanism

exhibits a rather large thermoelectric power S (∼100 μV/K) at room temperature, which makes Bi2Sr2Co2Oy one of promising thermoelectric materials from the viewpoint of potential applications, analogous to other misfit-layered cobaltites, such as NaCo2O4 and Ca3Co4O9 [2–5]. However, most studies of Bi2Sr2Co2Oy system are mainly focused on the thermoelectric improvement [2, 3, 6]. The transport mechanism based on resistivity ρ and thermoelectric power S has not been clarified. Moreover, large S is totally different from conventional value (<10 μV/K) based on a broad band model [7]. In this chapter, we will show evidence on a narrow band contribution in doped Bi2Sr2Co2Oy [8]. And what's more, exotic enhancement of large S is related to local density of states (DOS) near Fermi level (EF) [9]. It could be effectively modulated thermoelectric performance by utilizing different doping. It is plausible to distinguish, which thermoelectric materials in doped Bi2Sr2Co2Oy could be regarded as potential

4 Thermoelectrics for Power Generation - A Look at Trends in the Technology

On the other hand, ternary chalcogenides serve as an ideal platform for investigating intricate physical and chemical characteristics controlling the efficiency of thermoelectric materials, and also are promising materials for potential applications in photovoltaics, luminescence, as well as thermoelectric and spintronic devices [10–13]. Ternary chalcopyrite-structured chalcogenides, such as CuFeS2, have attracted particular attention owing to their unique optical, electrical, magnetic, and thermal properties [14–28]. Studies on chalcopyrite (CuFeS2) have primarily focused on its electronic states [14, 15, 29–31]. However, the microscopic mechanism of electronic structure and thermoelectric character in CuFeS2, which presumably arises from some scenarios such as delocalization of the Fe 3d electrons, charge-transfer-driven hybridization between Fe 3d and S 3p orbitals, or density of the conduction band electron states, still remains highly controversial [17, 30, 32]. The intrinsic mechanism of good thermoelectric properties is still a vital question which needs to be clarified. Another important issue is that the fabrication of artificial chalcopyrite itself requires expensively complex synthesis procedures and relatively high cost of constituent precursors, thereby potentially limiting the large-scale applica-

In this chapter, we confirm that an unexpected thermoelectricity can directly be generated in a natural chalcopyrite mineral Cu1+xFe1<sup>−</sup>xS2 from a deep-sea hydrothermal vent, and demonstrate that doped carriers have strong coupling with localized antiferromagnetic (AFM) spins, which greatly enhance the thermoelectric power S and power factor, revealing the significance of electron-magnon scattering and large effective mass [33]. This will open up another useful avenue in manipulating low-cost thermoelectricity or even electron/hole carriers via the natu-

The crystal structure of Bi2Sr2Co2Oy is shown in the inset in Figure 1, where conducting CoO2 layer with triangular lattice and insulating rocksalt Bi2Sr2O4 block layer are alternatively stacked along c-axis, similar to the case of high-temperature superconductors like Bi2Sr2CaCu2Oy.

candidates.

tions in the thermoelectric field.

ral energy materials abundantly deposited in the earth.

Bi2Sr1.9M0.1Co2O<sup>y</sup> and Bi2Sr2Co1.9X0.1O<sup>y</sup>

2.1. Crystal structure and valence states of Co ions

2. Thermoelectric properties and narrow band contribution of

Figure 2a and d shows temperature dependence of resistivity ρ(T) of all samples. For parent Bi2Sr2Co2Oy sample, an upturning point at T<sup>p</sup> (∼75 K) is observed. Metallic behavior above T<sup>p</sup> appears, demonstrating existence of itinerant charge carriers. Compared with Bi2Sr2Co2Oy, ρ(T) of all doped samples (except Bi2Sr1.9Ca0.1Co2Oy) display total increase in view of the disorder effect. Furthermore, enhanced random Coulomb potential because of the doping induces the obvious shift of T<sup>p</sup> toward higher temperature. On the other hand, ρ(T) of Bi2Sr1.9Ca0.1Co2Oy presents an overall decrease due to introduction of hole–type charge carriers into conducting CoO2 layers.

Figure 2. (a) Temperature dependence of resistivity ρ(T) and inset: magnification plot of ρ(T) for Bi2Sr2Co2Oy (BSC) and Bi2Sr1.9M0.1Co2Oy (M = Ag, Ca, and Y) samples. (b) Plot of ln ρ against T−<sup>1</sup> for Bi2Sr2Co2Oy and Bi2Sr1.9M0.1Co2Oy samples. Solid lines stand for TAC fitting. Dashed curves express VRH fitting. (c) Bi2Sr2Co2Oy and Bi2Sr1.9M0.1Co2Oy dependence of activation energy ΔE, onset temperature T<sup>p</sup> of TAC, and onset temperature Thopping of VRH. The shadow in bold is guide to the eyes. (d)–(f) are similar to (a)–(c) but for Bi2Sr2Co1.9X0.1Oy (X = Zr, Al, and Mo) samples.

To get insight into the conduction mechanism below Tp, dependences of ln ρ on T−<sup>1</sup> are plotted in Figure 2b and e. At the beginning, it is found that thermally activated conduction (TAC) law matches ρ(T) data well below Tp, namely [35], ρðTÞ ¼ ρ0expðΔE=kBTÞ, where ΔE is activation energy. Interestingly, ρ(T) apparently deviates from the TAC behavior with decreasing temperature further, and it follows Mott's variable-range-hopping (VRH) model described by equation [35]: ρðTÞ ¼ ρ0exp½ðT0=TÞ n �. As seen from Figure 2c and f, obtained values of ΔE and onset temperature Thopping of Bi2Sr1.9Ca0.1Co2Oy (0.66 meV and 15.3 K) are the respective minimum, even smaller, than those of parent Bi2Sr2Co2Oy (0.70 meV and 16.2 K), while ΔE and Thopping of Bi2Sr2Co1.9Zr0.1Oy (2.75 meV and 63.6 K) are both maximum among all samples.

#### 2.3. Thermoelectric power and narrow band model

To get insight into the conduction mechanism below Tp, dependences of ln ρ on T−<sup>1</sup> are plotted in Figure 2b and e. At the beginning, it is found that thermally activated conduction (TAC) law matches ρ(T) data well below Tp, namely [35], ρðTÞ ¼ ρ0expðΔE=kBTÞ, where ΔE is activation energy. Interestingly, ρ(T) apparently deviates from the TAC behavior with decreasing temperature further, and it follows Mott's variable-range-hopping (VRH) model described by equation

bold is guide to the eyes. (d)–(f) are similar to (a)–(c) but for Bi2Sr2Co1.9X0.1Oy (X = Zr, Al, and Mo) samples.

6 Thermoelectrics for Power Generation - A Look at Trends in the Technology

Figure 2. (a) Temperature dependence of resistivity ρ(T) and inset: magnification plot of ρ(T) for Bi2Sr2Co2Oy (BSC) and Bi2Sr1.9M0.1Co2Oy (M = Ag, Ca, and Y) samples. (b) Plot of ln ρ against T−<sup>1</sup> for Bi2Sr2Co2Oy and Bi2Sr1.9M0.1Co2Oy samples. Solid lines stand for TAC fitting. Dashed curves express VRH fitting. (c) Bi2Sr2Co2Oy and Bi2Sr1.9M0.1Co2Oy dependence of activation energy ΔE, onset temperature T<sup>p</sup> of TAC, and onset temperature Thopping of VRH. The shadow in Figure 3a and b shows temperature dependence of thermoelectric power S(T) for all samples. Positive values of S reflect electrical transport feature dominated by holes. Values of S at room temperature for all doped samples produce a substantial increase, especially for Bi2Sr2Co1.9Mo0.1Oy (∼117 μV/K), compared with pristine Bi2Sr2Co2Oy (∼92 μV/K). Particularly, with decreasing the temperature until below Thopping, S(T) behavior follows with VRH model [36]: SVRH(T) ∼ aT1/2, where a is factor determined by density of localized states at Fermi level N(EF). The inset in Figure 3b reveals Anderson localization of Bi2Sr2Co1.9Mo0.1Oy, in correspondence with low-temperature resistivity.

Figure 3. Temperature dependence of thermoelectric power S(T) for Bi2Sr2Co2Oy, (a) Bi2Sr1.9M0.1Co2Oy (M = Ag, Ca, and Y), and (b) Bi2Sr2Co1.9X0.1Oy (X = Zr, Al, and Mo) samples. Inset: calculated and fitted results of (a) Boltzmann formula and (b) VRH model for Bi2Sr2Co1.9Mo0.1Oy sample, respectively. Schematic diagram of density of states in a narrow band with Anderson localization at (c) high temperatures (metallic or TAC region) and (d) low temperatures (VRH region).

In general, S is extremely small (<10 μV/K) and presents a metallic behavior in a broad band [7]. Taking into account the huge difference, large S at high temperatures (above 200 K) in a narrow band matches Heikes model [37]: S ¼ kB=e{ln½d=ð1−dÞ�}, where d is concentration of Co4+. The enhanced S at high temperatures is attributed to the competition between d and spin entropy. It is noted that S(T) is also described by narrow band model at intermediate temperatures. <sup>S</sup>(T) follows with Boltzmann formula [38]: <sup>S</sup>ðTÞ ¼ <sup>1</sup>=eT{∫ðE−EFÞE<sup>2</sup> <sup>d</sup>E=½eðE−EFÞ=2kB<sup>T</sup><sup>þ</sup> <sup>e</sup><sup>−</sup>ðE−EFÞ=2kB<sup>T</sup>� 2 }={∫E<sup>2</sup> <sup>d</sup>E=½eðE−EFÞ=2kB<sup>T</sup> <sup>þ</sup> <sup>e</sup><sup>−</sup>ðE−EFÞ=2kB<sup>T</sup>� 2 }. Calculated S(T) indicates monotonous increase with increasing T, as well as experimental result as plotted in the inset in Figure 3a, revealing the validity of narrow band model.

Actually, activation energy ΔE is equal to EF−EC, where E<sup>C</sup> is the upper mobility edge. As kBT/2 >ΔE, conduction mainly determined by contribution of excited holes in itinerant states as specified in Figure 3c. At high temperatures, the majority of acceptor-like states are fully ionized, that is, occurs complete excitation of holes, that resulting in metallic behavior of ρ(T)and diffused S(T) (Heikes formula). As kBT/2 is near to ΔE, TAC conduction forms (Boltzmann dispersion). As kBT/ 2 <ΔE, VRH conduction dominates the transport mechanism as shown in Figure 3d.

#### 2.4. X-ray photoemission spectroscopy and thermal conductivity

In order to further verify the narrow band model, we carried out XPS spectra for Bi2Sr1.9Ca0.1Co2Oy. As shown in Figure 4b, XPS spectra present an intense peak at ∼ 0.95 eV, in line with large S and metallic behavior. Between E<sup>F</sup> and ∼2.0 eV, Co 3d and O 2p orbitals play an important role, similar to pristine Bi2Sr2Co2Oy [39]. Moreover, strong hybridization between Co 3d and O 2p forms [39, 40]. Namely, antibonding t2g narrow bands contribute to intense peak at ∼0.95 eV, while bonding e<sup>g</sup> broad bands are responsible to peak within 3–8 eV. In addition, calculated S(T) is also consistent with experimental value based on magnitude and temperature dependence [39]. Therefore, the narrow band model is very suitable for explaining all experimental and theoretical results.

Figure 4. (a) Co 2p XPS spectra and (b) XPS spectra in wide binding-energy range for selected Bi2Sr1.9Ca0.1Co2Oy sample at room temperature.

Temperature dependence of total thermal conductivity κ(T) for all samples are shown in Figure 5a and d. κ(T) can be expressed by the sum of phononic component κph(T) and mobile charge carriers' component κe(T) as κ(T) = κph(T) + κe(T). Value of κe(T) can be estimated from the Wiedemann-Franz law, κe(T) = L0T/ρ, where L<sup>0</sup> ∼ 2.44 × 10−<sup>8</sup> V<sup>2</sup> /K<sup>2</sup> stands for Lorenz number. In Figure 5b and e, κph(T) dominates the thermal conductivity because CoO2 layer and Bi-Sr-O block layer induces the interface scattering. Dimension less figure of merit ZT = S2 T/ρκ reflects total thermoelectric performance (see Figure 5c and f). For pristine Bi2Sr2Co2Oy, ZT value reaches ∼ 0.007 at 300 K, while ZT value reaches 0.19 at 973 K, indicative of promising thermoelectric material for Bi2Sr2Co2Oy at high temperatures [2]. Especially for Bi2Sr1.9Ca0.1 Co2Oy, ZT value reaches maximum ∼ 0.012 at 137 K. Therefore, it is reasonable to predict that Bi2Sr1.9Ca0.1Co2Oy could be considered as one of potential ultra-high temperature thermoelectric materials, as well as pristine Bi2Sr2Co2Oy.

In general, S is extremely small (<10 μV/K) and presents a metallic behavior in a broad band [7]. Taking into account the huge difference, large S at high temperatures (above 200 K) in a narrow band matches Heikes model [37]: S ¼ kB=e{ln½d=ð1−dÞ�}, where d is concentration of Co4+. The enhanced S at high temperatures is attributed to the competition between d and spin entropy. It is noted that S(T) is also described by narrow band model at intermediate temper-

2

with increasing T, as well as experimental result as plotted in the inset in Figure 3a, revealing

Actually, activation energy ΔE is equal to EF−EC, where E<sup>C</sup> is the upper mobility edge. As kBT/2 >ΔE, conduction mainly determined by contribution of excited holes in itinerant states as specified in Figure 3c. At high temperatures, the majority of acceptor-like states are fully ionized, that is, occurs complete excitation of holes, that resulting in metallic behavior of ρ(T)and diffused S(T) (Heikes formula). As kBT/2 is near to ΔE, TAC conduction forms (Boltzmann dispersion). As kBT/

In order to further verify the narrow band model, we carried out XPS spectra for Bi2Sr1.9Ca0.1Co2Oy. As shown in Figure 4b, XPS spectra present an intense peak at ∼ 0.95 eV, in line with large S and metallic behavior. Between E<sup>F</sup> and ∼2.0 eV, Co 3d and O 2p orbitals play an important role, similar to pristine Bi2Sr2Co2Oy [39]. Moreover, strong hybridization between Co 3d and O 2p forms [39, 40]. Namely, antibonding t2g narrow bands contribute to intense peak at ∼0.95 eV, while bonding e<sup>g</sup> broad bands are responsible to peak within 3–8 eV. In addition, calculated S(T) is also consistent with experimental value based on magnitude and temperature dependence [39]. Therefore, the

narrow band model is very suitable for explaining all experimental and theoretical results.

Figure 4. (a) Co 2p XPS spectra and (b) XPS spectra in wide binding-energy range for selected Bi2Sr1.9Ca0.1Co2Oy sample

<sup>d</sup>E=½eðE−EFÞ=2kB<sup>T</sup><sup>þ</sup>

}. Calculated S(T) indicates monotonous increase

atures. <sup>S</sup>(T) follows with Boltzmann formula [38]: <sup>S</sup>ðTÞ ¼ <sup>1</sup>=eT{∫ðE−EFÞE<sup>2</sup>

2 <ΔE, VRH conduction dominates the transport mechanism as shown in Figure 3d.

<sup>d</sup>E=½eðE−EFÞ=2kB<sup>T</sup> <sup>þ</sup> <sup>e</sup><sup>−</sup>ðE−EFÞ=2kB<sup>T</sup>�

8 Thermoelectrics for Power Generation - A Look at Trends in the Technology

2.4. X-ray photoemission spectroscopy and thermal conductivity

<sup>e</sup><sup>−</sup>ðE−EFÞ=2kB<sup>T</sup>�

at room temperature.

2 }={∫E<sup>2</sup>

the validity of narrow band model.

Figure 5. Temperature dependence of (a) total thermal conductivity κ(T), (b) phononic component κph(T), and (c) dimensionless figure of merit ZT for BSC and Bi2Sr1.9M0.1Co2Oy (M = Ag, Ca, and Y) samples. (d)–(f) are similar to (a)– (c), but for Bi2Sr2Co1.9X0.1Oy (X = Zr, Al, and Mo) samples.

## 3. Exotic reinforcement of thermoelectric power in layered Bi2Sr2<sup>−</sup>xCaxCo2O<sup>y</sup>

#### 3.1. XRD patterns and electrical transport properties

The crystal structure of Bi2Sr2Co2Oy is shown in Figure 6a. Figure 6b shows XRD patterns of all Ca-doping samples with single phase in Bi2Sr2<sup>−</sup>xCaxCo2Oy (0.0 ≤ x ≤ 2.0). With increasing Ca content, diffraction peak along [003] direction distinctly shifts to higher angle as shown in the inset in Figure 6b, confirming the smaller ionic radius of Ca2+, than that of Sr2+. SEM characterization indicates surface morphology of plate-like grains and regular grain sizes for selected samples with x = 0.0 and 1.0, respectively.

Figure 6. (a) Crystal structure of Bi2Sr2Co2Oy. (b) Powder XRD patterns for Bi2Sr2-xCaxCo2Oy (0.0 ≤ x ≤ 2.0) samples at room temperature. Insets: magnified powder's XRD patterns along [003] direction for all samples and SEM images for selected samples with x = 0.0 and 1.0, respectively.

Figure 7a and b shows resistivity ρ(T) of all samples in Bi2Sr2<sup>−</sup>xCaxCo2Oy. For the present x = 0.0 polycrystalline sample, upturning point at T<sup>p</sup> (∼150 K) appears. Metallic behavior above T<sup>p</sup> is observed, demonstrating the existence of itinerant charge carriers. In comparison, for x = 0.0 single crystal [41], in-plane resistivity ρab also shows metallic behavior around room temperature, while it arises minimum near 80 K and diverges with further decreasing the temperature. Resistivity ρab value of single crystal for x = 0.0 at room temperature is ∼4 mOhm×cm and is smaller than that of our polycrystalline sample (∼15 mOhm×cm). On the other hand, compared with x = 0.0, ρ(T) of all Ca-doped samples produce total increase due to disorder effect. For the samples with x ≤ 0.5, enhanced random Coulomb potential because of Ca doping induces the shift of T<sup>p</sup> toward higher temperature. Interestingly, for the samples

with x ≥ 1.0, the signature of transition at T<sup>p</sup> completely vanishes and ρ(T) only presents an insulating-like behavior.

3. Exotic reinforcement of thermoelectric power in layered

The crystal structure of Bi2Sr2Co2Oy is shown in Figure 6a. Figure 6b shows XRD patterns of all Ca-doping samples with single phase in Bi2Sr2<sup>−</sup>xCaxCo2Oy (0.0 ≤ x ≤ 2.0). With increasing Ca content, diffraction peak along [003] direction distinctly shifts to higher angle as shown in the inset in Figure 6b, confirming the smaller ionic radius of Ca2+, than that of Sr2+. SEM characterization indicates surface morphology of plate-like grains and regular grain sizes for selected

Figure 7a and b shows resistivity ρ(T) of all samples in Bi2Sr2<sup>−</sup>xCaxCo2Oy. For the present x = 0.0 polycrystalline sample, upturning point at T<sup>p</sup> (∼150 K) appears. Metallic behavior above T<sup>p</sup> is observed, demonstrating the existence of itinerant charge carriers. In comparison, for x = 0.0 single crystal [41], in-plane resistivity ρab also shows metallic behavior around room temperature, while it arises minimum near 80 K and diverges with further decreasing the temperature. Resistivity ρab value of single crystal for x = 0.0 at room temperature is ∼4 mOhm×cm and is smaller than that of our polycrystalline sample (∼15 mOhm×cm). On the other hand, compared with x = 0.0, ρ(T) of all Ca-doped samples produce total increase due to disorder effect. For the samples with x ≤ 0.5, enhanced random Coulomb potential because of Ca doping induces the shift of T<sup>p</sup> toward higher temperature. Interestingly, for the samples

Figure 6. (a) Crystal structure of Bi2Sr2Co2Oy. (b) Powder XRD patterns for Bi2Sr2-xCaxCo2Oy (0.0 ≤ x ≤ 2.0) samples at room temperature. Insets: magnified powder's XRD patterns along [003] direction for all samples and SEM images for

3.1. XRD patterns and electrical transport properties

10 Thermoelectrics for Power Generation - A Look at Trends in the Technology

samples with x = 0.0 and 1.0, respectively.

selected samples with x = 0.0 and 1.0, respectively.

Bi2Sr2<sup>−</sup>xCaxCo2O<sup>y</sup>

Figure 7. (a) and (b) Temperature dependence of resistivity ρ(T). Insets: magnification plot of ρ(T) for Bi2Sr2-xCaxCo2Oy samples. (c) and (d) Plot of lnρ against 1/T. Solid lines present TAC fitting. Dashed curves stand for VRH fitting. (e) Ca concentration x dependence of activation energy ΔE, onset temperature T<sup>p</sup> of TAC, and onset temperature Thopping of VRH. (f) Ca concentration x dependence of resistivityρ300 K at room temperature.

To discern conduction mechanism below Tp, relationship of lnρ against 1/T is plotted in Figure 7c and d. As for x ≤ 0.5, at the beginning, it is found that TAC law matches ρ(T) data well below Tp, namely [35], ρðTÞ ¼ ρ0expðΔE=kBTÞ, where ΔE is activation energy. But ρ(T) apparently deviates from TAC behavior with decreasing the temperature further, and it follows Mott's VRH model described by equation [32]: ρðTÞ ¼ ρ0exp½ðT0=TÞ n �. However, as for x ≥ 1.0, ρ(T) meets VRH model only, in agreement with the insulating feature of x = 2.0 single crystal [1, 42, 43]. Obtained values of ΔE and onset temperature Thopping are plotted in Figure 7e. Basically, ΔE increases with Ca content, as well as T<sup>p</sup> for x ≤ 0.5. In comparison, the present value of ΔE based on sintering temperature 800°C is larger than the previous one of x = 0.0 at 900°C [8], revealing the difference of grain size effect. It is worth noting that values of Thopping and ρ300K at room temperature first increase and then decrease in whole Ca-doped range (see Figure 7e and f).

#### 3.2. Enhancement of thermoelectric power driven by Ca doping

Figure 8a shows thermoelectric power S(T) for all samples. Positive values of S demonstrate that majority of charge carriers are hole type. In addition, S exhibits a nearly Tindependent behavior above 200 K, while S strongly depends on T peculiarly below 150 K. Ca doping obviously boosts S300K at room temperature especially for heavy Ca contents (see Figure 8b). Large S300K value monotonously increases from 105 μV/K(x = 0.0) to 157 μV/K (x = 2.0). In general, the change of S should be related to variation of n. For x = 0 single crystal [38], Hall coefficient (RH) is positive and strongly dependent on the temperature in the range from 300 to 0 K. Increase of R<sup>H</sup> toward the lowest temperature is not simple due to the decrease of n, but rather due to anomalous Hall effect. It is noted that variation of R<sup>H</sup> with Pb doping is also similar to that of ρab. Pb doping slightly reduces the magnitude of RH, but the increase in number of charge carriers is much smaller than expected from chemical composition [41, 44].

As we know, S is rather low (<10 μV/K) with a metallic behavior in a broad band [7]. Taking into account the tremendous discrepancy, large S of Bi2Sr2<sup>−</sup>xCaxCo2Oy with a nearly T-independence at high temperatures in a narrow band should follow the so-called Heikes formula [37]: <sup>S</sup> <sup>¼</sup> kB=e{ln½ðg3=g4Þd=ð1−dÞ�}, where <sup>d</sup> is concentration of Co4+, and <sup>g</sup><sup>3</sup> and <sup>g</sup><sup>4</sup> are spin orbital degeneracies for Co3+ and Co4+ ions, respectively. Concentration d at room temperature can be deduced from charge carriers'density n. As visible in Figure 8c, as for x< 1.5, d decreases, while SHeikes (deriving from Heikes formula) increases, which is consistent with the trend of S300K. But for x ≥ 1.5, reduced SHeikes is reverse to persistent enhancement of S300K. Thus, we have to consider other possible reason of enhanced S for heavily doped samples.

#### 3.3. Specific heat and Sommerfeld coefficient

Next we will check whether the enhanced S originates from the increased effective masses through electronic correlation. To test this point, we performed measurement of specific heat C(T), which is plotted as C/T versus T<sup>2</sup> (see the inset in Figure 8d) for selected samples with x = 0.0, 0.5, 1.5, and 2.0. C(T) at low temperatures can be described as C(T) = γT + βT<sup>3</sup> [45], where γT and βT<sup>3</sup> denote electronic and lattice contribution to C(T), respectively. We can get electronic coefficient γ by the linear fitting according to C/T = γ + βT<sup>2</sup> [45]. Here, we need to explicitly interpret Sommerfeld coefficient γ. For the present system, unit formula should involve two cobalt atoms. For our polycrystalline sample with x = 0.0, a conventional way to get γ by extrapolating high-temperature linear part of C/T versus T = 0 gives very large value of ∼ 135 mJ mol−<sup>1</sup> K−<sup>2</sup> (see Figure 8d), comparable with that of x = 0.0 single crystal (∼140 mJ mol−<sup>1</sup> K−<sup>2</sup> ) [41]. However, it is observed that γ rapidly decreases with increasing Ca doping. For our sample with x = 2.0, value of γ is ∼85 mJ mol−<sup>1</sup> K−<sup>2</sup> . Differently, it is noted that value of γ is only 50 mJ mol−<sup>1</sup> K−<sup>2</sup> for Bi-Ca-Co-O system, while such a unit formula merely includes one cobalt atom [45].

To discern conduction mechanism below Tp, relationship of lnρ against 1/T is plotted in Figure 7c and d. As for x ≤ 0.5, at the beginning, it is found that TAC law matches ρ(T) data well below Tp, namely [35], ρðTÞ ¼ ρ0expðΔE=kBTÞ, where ΔE is activation energy. But ρ(T) apparently deviates from TAC behavior with decreasing the temperature further, and it follows

ρ(T) meets VRH model only, in agreement with the insulating feature of x = 2.0 single crystal [1, 42, 43]. Obtained values of ΔE and onset temperature Thopping are plotted in Figure 7e. Basically, ΔE increases with Ca content, as well as T<sup>p</sup> for x ≤ 0.5. In comparison, the present value of ΔE based on sintering temperature 800°C is larger than the previous one of x = 0.0 at 900°C [8], revealing the difference of grain size effect. It is worth noting that values of Thopping and ρ300K at room temperature first increase and then decrease in whole Ca-doped range (see

Figure 8a shows thermoelectric power S(T) for all samples. Positive values of S demonstrate that majority of charge carriers are hole type. In addition, S exhibits a nearly Tindependent behavior above 200 K, while S strongly depends on T peculiarly below 150 K. Ca doping obviously boosts S300K at room temperature especially for heavy Ca contents (see Figure 8b). Large S300K value monotonously increases from 105 μV/K(x = 0.0) to 157 μV/K (x = 2.0). In general, the change of S should be related to variation of n. For x = 0 single crystal [38], Hall coefficient (RH) is positive and strongly dependent on the temperature in the range from 300 to 0 K. Increase of R<sup>H</sup> toward the lowest temperature is not simple due to the decrease of n, but rather due to anomalous Hall effect. It is noted that variation of R<sup>H</sup> with Pb doping is also similar to that of ρab. Pb doping slightly reduces the magnitude of RH, but the increase in number of charge carriers is much smaller than expected from

As we know, S is rather low (<10 μV/K) with a metallic behavior in a broad band [7]. Taking into account the tremendous discrepancy, large S of Bi2Sr2<sup>−</sup>xCaxCo2Oy with a nearly T-independence at high temperatures in a narrow band should follow the so-called Heikes formula [37]: <sup>S</sup> <sup>¼</sup> kB=e{ln½ðg3=g4Þd=ð1−dÞ�}, where <sup>d</sup> is concentration of Co4+, and <sup>g</sup><sup>3</sup> and <sup>g</sup><sup>4</sup> are spin orbital degeneracies for Co3+ and Co4+ ions, respectively. Concentration d at room temperature can be deduced from charge carriers'density n. As visible in Figure 8c, as for x< 1.5, d decreases, while SHeikes (deriving from Heikes formula) increases, which is consistent with the trend of S300K. But for x ≥ 1.5, reduced SHeikes is reverse to persistent enhancement of S300K. Thus, we have to consider other possible reason of enhanced S for heavily doped

Next we will check whether the enhanced S originates from the increased effective masses through electronic correlation. To test this point, we performed measurement of specific heat C(T), which is plotted as C/T versus T<sup>2</sup> (see the inset in Figure 8d) for selected samples with x = 0.0, 0.5, 1.5, and 2.0. C(T) at low temperatures can be described as C(T) = γT + βT<sup>3</sup> [45], where γT and βT<sup>3</sup> denote electronic and lattice contribution to C(T), respectively. We can get

n

�. However, as for x ≥ 1.0,

Mott's VRH model described by equation [32]: ρðTÞ ¼ ρ0exp½ðT0=TÞ

12 Thermoelectrics for Power Generation - A Look at Trends in the Technology

3.2. Enhancement of thermoelectric power driven by Ca doping

Figure 7e and f).

chemical composition [41, 44].

3.3. Specific heat and Sommerfeld coefficient

samples.

Figure 8. (a) Temperature dependence of thermoelectric power S(T) for Bi2Sr2-xCaxCo2Oy samples. (b) Ca concentration x dependence of S and charge carriers' density n at room temperature, respectively. (c) Ca concentration x dependence of Co4+ ion (deduced from charge carriers' density n) and corresponding SHeikes (originating from Heikes formula) at room temperature, respectively. (d) Ca concentration x dependence of electronic coefficient γ deriving from specific heat C(T). Inset: temperature dependence of C(T) plotted as C/T versus T<sup>2</sup> based on fitting lines for x = 0.0, 0.5, 1.5, and 2.0, respectively.

Now we discuss the underlying implications of enhanced S with Ca doping. As mentioned above, as for x < 1.5, decreased d based on Heikes formula should be responsible for the enhanced S. But for x ≥ 1.5, local modification of DOS and band structure near E<sup>F</sup> could play crucial role. <sup>S</sup>(T) can be defined by Mott formula [39]: <sup>S</sup>ðTÞ¼ðπ<sup>2</sup>kBTÞ=ð3eÞ½dlnσðEÞ=dE� <sup>E</sup>¼E<sup>F</sup> , where σ(E) is electrical conductivity with σ(E) = n(E)eυ(E), υ(E) is mobility, n(E) is charge carriers' density with n(E) = D(E)f(E), D(E) is DOS, and f(E) is Fermi function. Apparently, in terms of Mott formula, the enhancement of S for x ≥ 1.5 should be attributed to the increase of local DOS near EF. In details, with decreasing A-site ionic radius (i.e., with increasing Ca content), tolerance factor decreases (not shown here), which leads to changes of lattice distortion in CoO2 layer and local band structure near EF, reminiscent of layered perovskite cobaltite SrLnCoO4 (Ln stands for different rare earth elements) [46]. Ultimately, value of S for x ≥ 1.5 would be enhanced. Based on all of above results, one should emphasize that Sommerfeld coefficient γ is dependent on n, and also as function of DOS at EF, which leads to continuous enhancement of large S.

## 4. Thermoelectricity generation and electron-magnon scattering in a natural chalcopyrite mineral

#### 4.1. Crystal structure and SEM characterization

A series of natural chalcopyrite minerals, Cu1+xFe1<sup>−</sup>xS2 (x = 0.17, 0.08, and 0.02), were obtained from a hydrothermal vent site named Snow Chimney in the Mariner field of Lau Basin [47]. Basically, mineral composition obtained from intact natural sulfide chimneys has no variation. Subsamples with x = 0.02 and 0.08 were obtained from the most interior chimney part, whereas subsample with x = 0.17 was obtained from the middle chimney wall region. The highly fluctuated and variable physicochemical conditions lead to obvious differences in mineral composition [48]. Figure 9 shows sketches of its crystal structure and atomic planes, in which chalcopyrite crystallizes in a tetragonal lattice with space group of I-42d and produces honeycomb structure characteristic [49]. Each Fe and Cu atom is encircled by tetrahedron of S atom. The highlighted planes indicateatomic zig-zag pattern, which is likely responsible to phonon scattering. XRD Rietveld refinement of power pattern indicates that three natural samples are single phase with standard chalcopyrite structure. For x = 0.08, refined lattice parameters a and c are 5.278 and 10.402 Å, respectively (see Figure 10).

Figure 9. Crystal structure of Cu1+xFe1-xS2. Ball-and-stick model of the crystal structure (left) viewed along a-axis with black lines indicating unit cell. Stick model (right) showing characteristic honeycomb structure of chalcopyrite. Identical atomic arrangement is highlighted in gray in both structures, but projection is along different axes.

local DOS near EF. In details, with decreasing A-site ionic radius (i.e., with increasing Ca content), tolerance factor decreases (not shown here), which leads to changes of lattice distortion in CoO2 layer and local band structure near EF, reminiscent of layered perovskite cobaltite SrLnCoO4 (Ln stands for different rare earth elements) [46]. Ultimately, value of S for x ≥ 1.5 would be enhanced. Based on all of above results, one should emphasize that Sommerfeld coefficient γ is dependent on n, and also as function of DOS at EF, which leads to

4. Thermoelectricity generation and electron-magnon scattering in a

A series of natural chalcopyrite minerals, Cu1+xFe1<sup>−</sup>xS2 (x = 0.17, 0.08, and 0.02), were obtained from a hydrothermal vent site named Snow Chimney in the Mariner field of Lau Basin [47]. Basically, mineral composition obtained from intact natural sulfide chimneys has no variation. Subsamples with x = 0.02 and 0.08 were obtained from the most interior chimney part, whereas subsample with x = 0.17 was obtained from the middle chimney wall region. The highly fluctuated and variable physicochemical conditions lead to obvious differences in mineral composition [48]. Figure 9 shows sketches of its crystal structure and atomic planes, in which chalcopyrite crystallizes in a tetragonal lattice with space group of I-42d and produces honeycomb structure characteristic [49]. Each Fe and Cu atom is encircled by tetrahedron of S atom. The highlighted planes indicateatomic zig-zag pattern, which is likely responsible to phonon scattering. XRD Rietveld refinement of power pattern indicates that three natural samples are single phase with standard chalcopyrite structure. For x = 0.08, refined lattice parameters a and

Figure 9. Crystal structure of Cu1+xFe1-xS2. Ball-and-stick model of the crystal structure (left) viewed along a-axis with black lines indicating unit cell. Stick model (right) showing characteristic honeycomb structure of chalcopyrite. Identical

atomic arrangement is highlighted in gray in both structures, but projection is along different axes.

continuous enhancement of large S.

natural chalcopyrite mineral

4.1. Crystal structure and SEM characterization

14 Thermoelectrics for Power Generation - A Look at Trends in the Technology

c are 5.278 and 10.402 Å, respectively (see Figure 10).

Figure 10. Powder XRD patterns with Rietveld refinement for natural sample of Cu1+xFe1-xS2 (x = 0.08). Red line indicates experimentally observed data, and black line overlapping them refers to calculated data. Vertical tick is related to the Bragg angles positions in space group I-42d. The lowest profile shows the difference between observed and calculated patterns. Rietveld refinement indicates that it is standard chalcopyrite structure.

To probe the microstructures of natural Cu1+xFe1<sup>−</sup>xS2, we performed SEM characterization (Figure 11). SEM analysis revealed that natural chalcopyrite with x = 0.08 had layered structure. Three examined natural samples were found to contain morphological diversity, which is characteristic of chalcopyrite minerals, and suggest different physical and chemical behaviors of various microstructures. The SEM observation may provide important insights of the relevance between physical and chemical functions and behaviors of chalcopyrite minerals.

Figure 11. Surface morphology of natural sample of Cu1+xFe1-xS2 (x = 0.08) showing characteristic layered structure. (a) Areas showing cracked layered structure in natural sample Cu1+xFe1<sup>−</sup>xS2 (x = 0.08), scale bar: 10 μm. (b) Densely layered structure, scale bar: 5 μm. (c) Triangular pattern surrounded by layered structure, scale bar: 1 μm.

#### 4.2. Thermoelectricity generation and electronic states

To examine the functional properties of natural Cu1+xFe1<sup>−</sup>xS2 samples, we first measured resistivity (ρ) as function of temperature (T). Three examined natural samples exhibited excellent conductive behavior with semiconductive characteristics (Figure 12a). With the reduction of x, the overall resistivity decreased due to the emergence of doped charge carriers. Value of ρ300K for x = 0.17, 0.08, and 0.02 was 4.97, 0.11, and 1.01 Ohm×cm, respectively. Compared with x = 0.08, the increase of resistivity for x = 0.02 stems from the enhanced random Coulomb potential owing to the natural doping.

Figure 12. Formation of thermoelectricity by Cu1+xFe1-xS2. (a) Temperature dependence of resistivity ρ in three natural samples of Cu1+xFe1-xS2. (b) Temperature dependence of thermoelectric power S for three samples.

In order to track the evolution of electronic states, we carried out thermoelectric power (S) measurement (Figure 12b), where the sign of S changes. For x = 0.17, the sign of S switches from negative to positive around 235 K with decreasing temperature (Figure 12b). It is amazing to observe two unusual peaks: a broad peak (Tm; 32 μV/K, 186 K) and a sharper peak (Tp; 215 μV/K, 11 K), indicating the majority of hole carriers (p-type). It is of particular interest that, for x = 0.08 and 0.02, T<sup>p</sup> peak utterly disappears, while T<sup>m</sup> peak becomes wider and rapidly shifts to a lower temperature, where S presents very large negative values, demonstrating the majority of electron carriers (n-type), in line with negative Hall coefficient R<sup>H</sup> (Figure 13). Large S300K reached a remarkable value of −713 and −457 μV/K for x = 0.08 and 0.02, respectively. Namely, more electrons are activated at room temperature with increasing Fe concentration. For x = 0.08, charge carriers' mobility μ300K and density n300K are 1.8 cm<sup>2</sup> V−<sup>1</sup> s <sup>−</sup><sup>1</sup> and 3.5 × 10<sup>19</sup> cm−<sup>3</sup> , obtained from R<sup>H</sup> = 1/ne and μ= 1/neρ. In addition, Fe magnetic moment may also play an key role to induce large S, indicative of strong coupling between magnetic ions and doped charge carriers because synthetic CuFeS2 presents AFM ordering at 823 K [15].

Figure 13. Hall effect of natural sample of Cu1+xFe1-xS2 (x = 0.08). (a) Temperature dependence of Hall coefficient RH. (b) Temperature dependence of charge carriers' density n. Value of R<sup>H</sup> (cm<sup>3</sup> C−<sup>1</sup> ) is determined by n (cm−<sup>3</sup> ) and electron charge e, that is, R<sup>H</sup> = 1/ne, where e = 1.602176 × 10−<sup>19</sup> C. The shadow in bold is guide to the eyes.

#### 4.3. Electron-magnon scattering and large effective mass

4.2. Thermoelectricity generation and electronic states

16 Thermoelectrics for Power Generation - A Look at Trends in the Technology

to the natural doping.

are 1.8 cm<sup>2</sup> V−<sup>1</sup> s

ordering at 823 K [15].

<sup>−</sup><sup>1</sup> and 3.5 × 10<sup>19</sup> cm−<sup>3</sup>

To examine the functional properties of natural Cu1+xFe1<sup>−</sup>xS2 samples, we first measured resistivity (ρ) as function of temperature (T). Three examined natural samples exhibited excellent conductive behavior with semiconductive characteristics (Figure 12a). With the reduction of x, the overall resistivity decreased due to the emergence of doped charge carriers. Value of ρ300K for x = 0.17, 0.08, and 0.02 was 4.97, 0.11, and 1.01 Ohm×cm, respectively. Compared with x = 0.08, the increase of resistivity for x = 0.02 stems from the enhanced random Coulomb potential owing

In order to track the evolution of electronic states, we carried out thermoelectric power (S) measurement (Figure 12b), where the sign of S changes. For x = 0.17, the sign of S switches from negative to positive around 235 K with decreasing temperature (Figure 12b). It is amazing to observe two unusual peaks: a broad peak (Tm; 32 μV/K, 186 K) and a sharper peak (Tp; 215 μV/K, 11 K), indicating the majority of hole carriers (p-type). It is of particular interest that, for x = 0.08 and 0.02, T<sup>p</sup> peak utterly disappears, while T<sup>m</sup> peak becomes wider and rapidly shifts to a lower temperature, where S presents very large negative values, demonstrating the majority of electron carriers (n-type), in line with negative Hall coefficient R<sup>H</sup> (Figure 13). Large S300K reached a remarkable value of −713 and −457 μV/K for x = 0.08 and 0.02, respectively. Namely, more electrons are activated at room temperature with increasing Fe concentration. For x = 0.08, charge carriers' mobility μ300K and density n300K

Figure 12. Formation of thermoelectricity by Cu1+xFe1-xS2. (a) Temperature dependence of resistivity ρ in three natural

samples of Cu1+xFe1-xS2. (b) Temperature dependence of thermoelectric power S for three samples.

magnetic moment may also play an key role to induce large S, indicative of strong coupling between magnetic ions and doped charge carriers because synthetic CuFeS2 presents AFM

, obtained from R<sup>H</sup> = 1/ne and μ= 1/neρ. In addition, Fe

The matter of imperative concern is how to understand the origin of T<sup>m</sup> peak and conduction mechanism. According to Mott's formula, <sup>S</sup> can be qualitatively expressed as <sup>S</sup> <sup>¼</sup> <sup>−</sup>π<sup>2</sup>k<sup>2</sup> <sup>B</sup>T=3e ½σ'ðEFÞ=σðEFÞ�, where k<sup>B</sup> is Boltzmann constant, σðEFÞ is electrical conductivity at Fermi level EF, and σ' denotes d[σðEÞ]/dE [35]. If one assumes σ' is a constant accompanied by isotropic electrical transport properties, namely, σ−<sup>1</sup> = ρ, then ΔS/S<sup>0</sup> Δρ/ρ<sup>0</sup> can be derived. Plot of ΔS/S<sup>0</sup> versus Δρ/ρ<sup>0</sup> for x = 0.17 (Figure 14) shows that all experimental data near T<sup>m</sup> at T<sup>0</sup> from 155 to 300 K deviate from the theoretical calculation, the linearity. These results verify that exotic mechanism of S(T) in natural sample is beyond the framework of conventional thermoelectric picture [50].

To better discern intrinsic transport mechanism of Cu1+xFe1−xS2, we incorporate spin-wave theory to analyze temperature dependence of S. For x = 0.08 and 0.02, field-cooling magnetization and loop hysteresis indicate the localized ferromagnetism (FM) at low temperatures because of additional Fe moments (Figure 15). However, strong AFM interaction at high temperatures dominates for three natural samples. Generally speaking, spin waves can scatter electrons for AFM or FM materials, resulting in magnon-drag effect [12]. To check this issue, we developed magnon-drag model, S=S0+S3/2T3/2+S4T<sup>4</sup> , where S<sup>0</sup> is value of S at T = 0, S3/2T3/2 term stems from electron-magnon scattering, and S4T<sup>4</sup> term is related to spin-wave fluctuation in AFM phase. Using this model of magnon drag, the predicted values for three samples closely matched S(T) data (Figure 16a and b). As the absolute value of S3/2 is nearly six orders of magnitude larger than that of S<sup>4</sup> (Table 1), electron-magnon scattering dominates S(T) curve. Thus, T<sup>m</sup> peak is predicted to originate from magnon drag due to the strong electron-magnon interaction.

Figure 14. Correlation between thermoelectric power S(T) and resistivity ρ(T). Relative changes of ΔS/S<sup>0</sup> versus Δρ/ρ<sup>0</sup> in natural sample with x = 0.17 at various temperatures (T<sup>0</sup> = 155, 185, 200, 215, 230, 240, 250, 270, and 300 K). The present experimental data substantially deviates from the linear relationship predicted by Mott's formula, which is indicated by dotted line.

Figure 15. Magnetic properties of natural Cu1+xFe1<sup>−</sup>xS2. (a, b) Temperature dependence of field-cooling (FC) magnetization, M, in three natural samples of Cu1+xFe1<sup>−</sup>xS2, measured in applied magnetic field of H = 0.1 T (a) and H =1T(b). (c) Magnetic field dependence of magnetization, M, for three samples, measured at 40 K.

To gain more insight into the correlation between magnon drag, doped carriers, and S, we plotted parameters S0, S3/2, and S<sup>4</sup> as a function of x (Table 1). S0, S3/2, and S<sup>4</sup> for x = 0.08 has largest absolute values among three natural samples, in agreement with the largest S, smallest ρ, and highest power factor. Unlike S<sup>0</sup> and S4, dependence of S3/2 is quite unique (Figure 16c). The sign of S3/2 varies from positive to negative with increasing Fe concentration, suggesting the alternation of p-type and n-type charge carriers and orbital degree of freedom of Fe 3d band with AFM ordering. Additionally, electron-magnon scattering occupies thermoelectric properties, indicating strong coupling between doped charge carriers and AFM spins. Furthermore, ρ(T) follows TAC model ρðTÞ ¼ ρ0expðΔE=kBTÞ, where ΔE is activation energy [35]. Notably, the fitted energy gap of ΔE (60.1, 4.9, and 11.8 meV for x = 0.17, 0.08, and 0.02, respectively), which verifies the existence of localized Fe spins, is markedly smaller than that of artificial chalcopyrite [21, 29–31]. It is noted that experimental S(T) result is well described by electronmagnon scattering up to ∼200 K, while it deviates from theoretical lines for higher temperatures. In particular, power factor S<sup>2</sup> /ρ shows an abrupt enhancement above 200 K for x = 0.08 (Figure 16d), in agreement with that of R<sup>H</sup> and n (Figure 13). Above 200 K, large effective mass (m\* ) leads to high power factor and large S due to low μ and high n. For x = 0.08, it exhibits the largest m\* value (1.6 m0) at room temperature, where m<sup>0</sup> is free electron mass. Therefore, we can conclude that robust electron-magnon scattering and large m\* induce unexpected thermoelectricity generation in natural chalcopyrite mineral.

Figure 16. Temperature dependence of S for Cu1+xFe1<sup>−</sup>xS2 samples with x = 0.17 (a) and x = 0.08 and 0.02 (b). Symbols represent experimental data and solid lines correspond to theoretical simulation based on the model of magnon drag, S = S<sup>0</sup> + S3/2T3/2 + S4T<sup>4</sup> . (c) Obtained parameters S3/2 and ΔE are plotted as function of Fe content, where S3/2 represents the electronmagnon scattering process and ΔE is activation energy. (d) Temperature dependence of power factor, S<sup>2</sup> /ρ, for three samples.

To gain more insight into the correlation between magnon drag, doped carriers, and S, we plotted parameters S0, S3/2, and S<sup>4</sup> as a function of x (Table 1). S0, S3/2, and S<sup>4</sup> for x = 0.08 has largest absolute values among three natural samples, in agreement with the largest S, smallest ρ, and highest power factor. Unlike S<sup>0</sup> and S4, dependence of S3/2 is quite unique (Figure 16c). The sign of S3/2 varies from positive to negative with increasing Fe concentration, suggesting

Figure 15. Magnetic properties of natural Cu1+xFe1<sup>−</sup>xS2. (a, b) Temperature dependence of field-cooling (FC) magnetization, M, in three natural samples of Cu1+xFe1<sup>−</sup>xS2, measured in applied magnetic field of H = 0.1 T (a) and H =1T(b). (c)

Figure 14. Correlation between thermoelectric power S(T) and resistivity ρ(T). Relative changes of ΔS/S<sup>0</sup> versus Δρ/ρ<sup>0</sup> in natural sample with x = 0.17 at various temperatures (T<sup>0</sup> = 155, 185, 200, 215, 230, 240, 250, 270, and 300 K). The present experimental data substantially deviates from the linear relationship predicted by Mott's formula, which is indicated by

18 Thermoelectrics for Power Generation - A Look at Trends in the Technology

Magnetic field dependence of magnetization, M, for three samples, measured at 40 K.

dotted line.


The parameter T<sup>m</sup> represents the peak of magnon drag, which stems from the experimental S(T) curve. The parameters S0, S3/2, and S<sup>4</sup> stem from the model of magnon drag, S= S<sup>0</sup> + S3/2T3/2 + S4T<sup>4</sup> . The parameter ΔE is the activation energy, which stems from the TAC model, ρ(T) = ρ<sup>0</sup> exp (ΔE/kBT).

Table 1. Obtained parameters based on theoretical simulation.

In terms of thermal conductivity κ, phononic component κph dominates for three natural samples owing to negligible electronic component κ<sup>e</sup> (Figure 17). For the optimal sample with x = 0.08, value of ZT can reach 0.03 at room temperature (Figure 17), thus indicating that natural chalcopyrite semiconductor is a promising candidate for thermoelectric energy materials. It is quite striking that the spontaneous doping process during deep-sea hydrothermal vent mineral precipitations led to natural thermoelectric improvement, which is similar to natural mineral tetrahedrites [51].

Figure 17. Thermal conductivity and phonon scattering of natural Cu1+xFe1<sup>−</sup>xS2. (a) Temperature dependence of total thermal conductivity κ. (b) Temperature dependence of electronic component κe. (c) Temperature dependence of phononic component κph. (d) Temperature dependence of dimensionless figure of merit ZT.

## 5. Conclusions

In terms of thermal conductivity κ, phononic component κph dominates for three natural samples owing to negligible electronic component κ<sup>e</sup> (Figure 17). For the optimal sample with x = 0.08, value of ZT can reach 0.03 at room temperature (Figure 17), thus indicating that natural chalcopyrite semiconductor is a promising candidate for thermoelectric energy materials. It is quite striking that the spontaneous doping process during deep-sea hydrothermal vent mineral precipitations led to

Figure 17. Thermal conductivity and phonon scattering of natural Cu1+xFe1<sup>−</sup>xS2. (a) Temperature dependence of total thermal conductivity κ. (b) Temperature dependence of electronic component κe. (c) Temperature dependence of

phononic component κph. (d) Temperature dependence of dimensionless figure of merit ZT.

The parameter T<sup>m</sup> represents the peak of magnon drag, which stems from the experimental S(T) curve. The parameters S0,

) S3/2(μVK<sup>−</sup>5/2) S<sup>4</sup> (μVK−<sup>5</sup>

) ΔE(meV)

. The parameter ΔE is the activation energy, which

natural thermoelectric improvement, which is similar to natural mineral tetrahedrites [51].

x = 0.17 186 −6.21 0.03 −3.84×10−<sup>8</sup> 60.1 x = 0.08 68 −75.45 −0.08 −5.47×10−<sup>8</sup> 4.9 x = 0.02 38 −10.61 −0.04 −3.95×10−<sup>8</sup> 11.8

Parameter Tm(K) S0(μVK−<sup>1</sup>

stems from the TAC model, ρ(T) = ρ<sup>0</sup> exp (ΔE/kBT).

S3/2, and S<sup>4</sup> stem from the model of magnon drag, S= S<sup>0</sup> + S3/2T3/2 + S4T<sup>4</sup>

20 Thermoelectrics for Power Generation - A Look at Trends in the Technology

Table 1. Obtained parameters based on theoretical simulation.

Our results of layered cobaltites Bi2Sr2Co2Oy system based on narrow band model are not only helpful to understand large S and transport mechanism but also differentiate other systems based on a broad band model. In particular, we give the experimental evidence by Hall effect and C(T) measurements, demonstrating that Sommerfeld coefficient γ is dependent on charge carriers' density n, and also as a function of DOS at EF, which induces exotic enhancement of large S in Bi2Sr2<sup>−</sup>xCaxCo2Oy. Especially for Bi2Sr1.9Ca0.1Co2Oy, it may provide an excellent platform to be regarded as potential candidates for thermoelectric materials.

In addition, we demonstrated direct thermoelectricity generation in natural chalcogenides, Cu1+xFe1<sup>−</sup>xS2, which was shown to have large S value and high power factor in the low x region, in which electron-magnon scattering and large m\* values were detected. Since doped charge carriers exist in strong coupling with localized spins, the unusual alternation of p- and n-type carriers should be of paramount importance in understanding charge dynamics arising from 3d orbital degrees of freedom. Such a finding of exotic thermoelectric properties in natural but not synthetic chalcopyrite opens a novel research field for manipulating low-cost thermoelectricity or even electron/hole carriers, providing therefore a new perspective on technical feasibility for designing and pinpointing the surface-morphology-engineered devices via the naturally abundant materials.

## Acknowledgements

The author gratefully thanks L. H. Yin, W. H. Song, Y. P. Sun, A. U. Khan, N. Tsujii, K. Takai, R. Nakamura, and T. Mori for their fruitful collaboration in the study of layered cobaltites and natural chalcogenides for thermoelectrics. This work was supported by the National Natural Science Foundation of China under Contract No. 10904151, the Fund of Chinese Academy of Sciences for Excellent Graduates, and the NIMS Open Innovation Center (NOIC) of Japan. The author thanks the Sichuan University Talent Introduction Research Funding (grant No. YJ201537) and Sichuan University Outstanding Young Scholars Research Funding (grant No. 2015SCU04A20) of China for financial support.

## Author details

Ran Ang

Address all correspondence to: rang@scu.edu.cn

1 Key Laboratory of Radiation Physics and Technology, Ministry of Education, Institute of Nuclear Science and Technology, Sichuan University, Chengdu, China

2 Institute of New Energy and Low-Carbon Technology, Sichuan University, Chengdu, China

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#### **Electrical Conductivity, Thermoelectric Power and Crystal and Band Structures of EDOB-EDT-TTF Salts Composed of PF6 <sup>−</sup>, AsF6 <sup>−</sup> and SbF6 <sup>−</sup> Electrical Conductivity, Thermoelectric Power and Crystal and Band Structures of EDOB-EDT-TTF Salts Composed of PF6 <sup>−</sup>, AsF6 <sup>−</sup> and SbF6 −**

Tomoko Inayoshi Tomoko Inayoshi

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65561

#### **Abstract**

Novel 2:1 EDOB-EDT-TTF radical salts with different octahedral PF6 − , AsF6 − , and SbF6 − anions were prepared by electrochemical oxidation. AsF6 salt was found to be isostructural to PF6 salt and had a triclinic crystal structure, while SbF6 salt was not isostructural with PF6 salt and had monoclinic crystal structure. PF6 salt had higher metal-to-semiconductor (MS) transition temperature, than that of AsF6 salt, while SbF6 salt exhibited semiconductive behavior throughout the temperature range of electrical conductivity measurements. To clarify MS transition of these salts, thermoelectric power measurements were also carried out. Thus, thermoelectric power apparatus was constructed and measurements were performed simultaneously with thermoelectric power and electrical resistivity measurements. Crystal structural features for EDOB-EDT-TTF salts at 90, 293, 330 and 350 K, as well as conductivity, thermoelectric power measurements and band structures before and after MS transition are described.

**Keywords:** EDOB-EDT-TTF radical salts, conductivity, thermoelectric power

## **1. Introduction**

Highly conducting organic TTF·TCNQ complex composed of tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) was reported by Heeger et al. in 1973 [1] and has been studied by many physicists and chemists as a one-dimensional conducting system [2, 3]. TTF·TCNQ provides highly anisotropic charge-transfer (CT) complex with metallic properties down to 58 K along *b*-axis, in which crystal structure [4] has two columns of TTF and TCNQ. Thermoelectric power of TTF·TCNQ along *b*-axis is negative and proportional to absolute

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

temperature down to 140 K, but not along *a*-axis [5]. This is consistent with electrical conductivity measured along different axes. Apparatus for thermoelectric power measurement on organic single crystals has been reported by Chaikin and Kwak [6]. It is designed specifically for small fragile anisotropic samples, such as TCNQ salts. Measurements can be taken with a small (0.5 K) temperature gradient for good temperature resolution.

Bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) is a good electron donor, and CT complexes and radical salts composed of BEDT-TTF also grow excellent crystals. BEDT-TTF radical salts afford many superconductors as two-dimensional conducting system. In the case of such twodimensional system, thermoelectric power often exhibits complicated temperature dependence and anisotropy. Mori and Inokuchi have found an agreement between thermoelectric power and calculations for *β*-(BEDT-TTF)2I3 and *κ*-(BEDT-TTF)2Cu(NCS)2 [7].

Bis(ethylenedioxy)dibenzotetrathiafulvalene (BEDO-DBTTF) modified with strong electrondonating groups containing ethylenedioxy groups has been synthesized [8]. BEDO-DBTTF CT complexes and salts afforded no metallic compounds. Therefore, we have synthesized a new unsymmetrical EDOB-EDT-TTF donor, which consists of parts of BEDO-DBTTF and BEDT-TTF, and EDOB-EDT-TTF radical salts with octahedral PF6 − , AsF6 − , and SbF6 − anions [9, 10]. Based on electrical resistivity measurements, PF6 and AsF6 salts underwent a metal-tosemiconductor (MS) transition. X-ray analyses of these salts elucidated their crystal characteristics. Simultaneous measurements of thermoelectric power and electrical resistivity on a single sample were performed for these salts. MS transition temperatures of these PF6 and AsF6 salts were also determined from their thermoelectric power.

## **2. Preparation of organic conductors**

#### **2.1. Synthesis of unsymmetrical EDOB-EDT-TTF donor**

As shown in **Figure 1**, synthesis of EDOB-EDT-TTF was carried out using two synthetic methods: cross-coupling (I) and thermal decomposition (II), resulting in 30 and 27% yields, respectively.

#### *2.1.1. Cross-coupling method*

To the suspension of **1** (3.0 g, 12 mmol) and **2** (2.5 g, 12 mmol) in dry benzene (40 mL) was added triethyl phosphate (30 mL, 130 mmol). The mixture was refluxed for 5 h at 80°C. The resulting orange precipitate was removed by filtration, and the filtrate was purified by silica gel column chromatography using CHCl3/hexane (5:1) as the eluent. The second fraction was collected, and the resulting product was recrystallized using ethyl acetate to afford EDOB-EDT-TTF as orange crystals (30% yield): mp 260—262. 1 H NMR (CDCl3): *δ*3.30 (4H, s, −SCH2), 4.23 (4H, s, −OCH2), 6.77 (2H, s, ArH). MS (EI) *m/z*: 402 (M+ ). IR (KBr, *ν*max cm−1): 1575 (w), 1540 (w), 1478 (s), 1456 (s), 1300 (s), 1376 (m), 1360 (m), 1100 (m), 1063 (s), 908 (m), 895 (m), 854 (m), 772 (w). UV-vis (CHCl3) *λ*max, nm: 454, 344, 313, > 260. Anal. calcd for C14H10O2S6: C, 41.77; H, 2.50; S, 47.78. Found: C, 41.77; H, 2.47; S, 47.74. The product was subsequently subjected to Xray crystal structure analysis.

**Figure 1.** Synthetic routes of EDOB-EDT-TTF.

temperature down to 140 K, but not along *a*-axis [5]. This is consistent with electrical conductivity measured along different axes. Apparatus for thermoelectric power measurement on organic single crystals has been reported by Chaikin and Kwak [6]. It is designed specifically for small fragile anisotropic samples, such as TCNQ salts. Measurements can be taken with a

Bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) is a good electron donor, and CT complexes and radical salts composed of BEDT-TTF also grow excellent crystals. BEDT-TTF radical salts afford many superconductors as two-dimensional conducting system. In the case of such twodimensional system, thermoelectric power often exhibits complicated temperature dependence and anisotropy. Mori and Inokuchi have found an agreement between thermoelectric

Bis(ethylenedioxy)dibenzotetrathiafulvalene (BEDO-DBTTF) modified with strong electrondonating groups containing ethylenedioxy groups has been synthesized [8]. BEDO-DBTTF CT complexes and salts afforded no metallic compounds. Therefore, we have synthesized a new unsymmetrical EDOB-EDT-TTF donor, which consists of parts of BEDO-DBTTF and BEDT-

Based on electrical resistivity measurements, PF6 and AsF6 salts underwent a metal-tosemiconductor (MS) transition. X-ray analyses of these salts elucidated their crystal characteristics. Simultaneous measurements of thermoelectric power and electrical resistivity on a single sample were performed for these salts. MS transition temperatures of these PF6 and

As shown in **Figure 1**, synthesis of EDOB-EDT-TTF was carried out using two synthetic methods: cross-coupling (I) and thermal decomposition (II), resulting in 30 and 27% yields,

To the suspension of **1** (3.0 g, 12 mmol) and **2** (2.5 g, 12 mmol) in dry benzene (40 mL) was added triethyl phosphate (30 mL, 130 mmol). The mixture was refluxed for 5 h at 80°C. The resulting orange precipitate was removed by filtration, and the filtrate was purified by silica gel column chromatography using CHCl3/hexane (5:1) as the eluent. The second fraction was collected, and the resulting product was recrystallized using ethyl acetate to afford EDOB-

(w), 1478 (s), 1456 (s), 1300 (s), 1376 (m), 1360 (m), 1100 (m), 1063 (s), 908 (m), 895 (m), 854 (m), 772 (w). UV-vis (CHCl3) *λ*max, nm: 454, 344, 313, > 260. Anal. calcd for C14H10O2S6: C, 41.77; H,

− , AsF6 −

, and SbF6

H NMR (CDCl3): *δ*3.30 (4H, s, −SCH2),

). IR (KBr, *ν*max cm−1): 1575 (w), 1540

−

anions [9, 10].

small (0.5 K) temperature gradient for good temperature resolution.

28 Thermoelectrics for Power Generation - A Look at Trends in the Technology

TTF, and EDOB-EDT-TTF radical salts with octahedral PF6

AsF6 salts were also determined from their thermoelectric power.

**2. Preparation of organic conductors**

respectively.

*2.1.1. Cross-coupling method*

**2.1. Synthesis of unsymmetrical EDOB-EDT-TTF donor**

EDT-TTF as orange crystals (30% yield): mp 260—262. 1

4.23 (4H, s, −OCH2), 6.77 (2H, s, ArH). MS (EI) *m/z*: 402 (M+

power and calculations for *β*-(BEDT-TTF)2I3 and *κ*-(BEDT-TTF)2Cu(NCS)2 [7].

#### *2.1.2. Thermal decomposition method*

*2-(Methylthio)-5,6-ethylenedioxy-1,3-benzodithiole* (**3**). To a solution of **1** (1.03 g, 4.25 mmol) in dry THF (150 mL) at −78°C under nitrogen was added solution of MeLi (5.1 mL, 6 mmol) dropwise via a syringe. After stirring for 5 h, the mixture was treated with acetic acid (2 mL) and allowed to warm to room temperature. The mixture was combined with water, and the desired compound was extracted using CH3Cl. The organic layer was dried using MgSO4 and evaporated *in vacuo* to afford the crude product, which was purified using silica gel chromatography with tetrahydrofuran THF/*n*-hexane (1:3) as the eluent to afford **3** as a colorless oil (0.74 g, 66%). 1 H NMR (CDCl3): *δ*2.22 (3H, s, −SCH3), 4.21 (4H, s, −OCH2), 5.93 (1H, s, −CH), 6.79 (2H, s, ArH). HRMS (EI) (*m/z*): calcd for C10H10O2S2, 257.9843; found, 257.9718.

*Hexathioorthooxalate* **(4)**. To a stirred solution of **3** (2.60 g, 10 mmol) in dry THF (120 mL) at −78°C under nitrogen was added *n*-BuLi (6.35 mL, 9.5 mmol) dropwise using a syringe. After stirring for 1 h, 4,5-ethylenedithio-1,3-dithiol-2-thione (2.24 g, 10 mmol) in THF was added dropwise to the mixture, followed by addition of an excess of MeI (1.91 mL, 30 mmol) via a syringe to the mixture after 1 h. After further stirring for 1 h, the solution was treated with portions of 1 mol/dm3 aq. NH4Cl. The mixture was allowed to warm to room temperature, and the aqueous layer was extracted with CH2Cl2. The organic phase was concentrated and purified using silica gel column chromatography with CH3Cl/*n*-hexane (1:2) as the eluent to afford **4** as a yellow solid (2.40 g, 45%): mp 207-213 (dec. 150°C);1 H NMR (CDCl3): *δ*2.46 (3H, s, −SCH3), 2.53 (3H, s, −SCH3), 3.25 (4H, m, −SCH2), 4.21 (4H, s, −OCH2), 6.62 (2H, s, ArH). Anal. calcd for C16H16O2S8: C, 38.68; H, 3.25. Found: C, 38.56; H, 3.17.

*EDOB-EDT-TTF*. After refluxing **4** in 1,1,2-trichloroethane (TCE) for 12 h, the crude product was purified by column chromatography with CH2Cl2/*n*-hexane (1:1) as the eluent followed by recrystallization from ethyl acetate to yield EDOB-EDT-TTF (27%). 1 H NMR and MS data were comparable to those obtained by the cross-coupling method.

The redox potentials of unsymmetrical EDOB-EDT-TTF appeared middle between that of BEDT-TTF and BEDO-DBTTF, as similar to other unsymmetrical TTF derivatives reported in [11]. The difference potential (Δ*E*) between the first redox potential (*E*1/2(1)) and the second redox potential (*E*1/2(2)) is related to intramolecular on-site Coulomb repulsion energy, *U*. The Δ*E* of EDOB-EDT-TTF also showed middle between that of BEDT-TTF and BEDO-BDTTF. The *U* of EDOB-EDT-TTF decreased compared to that of BEDO-BDTTF.

## **2.2. Preparation of CT complexes and radical salts**

Hot solutions of each donor and acceptor in acetonitrile were mixed. After the reaction mixture was cooled to room temperature (RT), the resulting precipitate was collected by filtration. Complexes were washed with the same organic solvent and dried *in vacuo*. Black octahedral PF6, AsF6 and SbF6 salts were obtained by electrochemical oxidation in distilled 1,2-dichloroethane or TCE under constant current of 1 μA in a mixture of the donor and the tetra-*n*butylammonium salts of the corresponding anions at RT using H-shaped cell with Pt electrodes for 2 weeks. Stoichiometry of CT complexes and radical salts was determined using elemental analysis or X-ray crystallographic analyses.

## **3. Measurements**

## **3.1. Electrical conductivity measurement**

DC conductivities were measured with standard four- or two-probe techniques, using Keithley 220 current source, Keithley 199 voltage/scanner, Keithley 195A voltmeter, and Scientific Instruments 9650 temperature controller. For powder samples, measurements were performed on compressed pellets, which were cut to form orthorhombic shape. Gold wires were glued to the samples with gold paint (Tokuriki Chemical, no. 8560).

#### **3.2. Apparatus for simultaneous measurements of thermoelectric power and resistivity on organic conductors**

Simultaneous measurements of thermoelectric power and resistivity by a two-probe method of PF6, AsF6, and SbF6 salts were performed using computer-interfaced system, which schematic diagram is shown in **Figure 2**.

Software program for controlling the system was created in LabVIEW by Computer Automation Co. and System Approach Co. By applying different digital signals using relay and counter timer, we could perform simultaneous measurements of thermoelectric power and two-probe electrical resistivity on a single sample over the entire temperature range [12]. First, by opening the circuit through Keithley 2002 multimeter attached 2001-Scan scanner card as the relay, sample voltage (∆*V*) between V+ and V− electrodes was measured by Keithley 2182 nanovoltmeter. Temperature gradient, ∆*T*, between two copper plates was measured by another Keithley 2182 nanovoltmeter. Thermoelectric power was determined. Second, after thermal equilibrium was sufficiently reached between two copper plates, by connecting the circuit through the relay, sample voltage was measured by Keithley 2182 nanovoltmeter, while constant current was supplied to the sample reversing leads to cancel thermal EMFs by Keithley 220 current source. Two-probe resistance measurement was determined by subtracting these two voltages and averaging. Using thermoelectric power stage made by MMR Technologies Inc. radical salts and the reference Cu-constantan or Au/Fe-Chromel thermocouple wire were glued onto copper plates using gold paint. Thermal gradient across the sample was applied by heating chip resistor (110 Ohm), which was applied from loop 2 of LakeShore 331S temperature controller. Thermoelectric power was determined from the slope of the line and was calculated as follows: *S* = Δ*V*/Δ*T* [13], where Δ*T* of 15 total points was typically < 0.5 K. Changing voltage value was measured at specific fixed time intervals by 15 points simultaneously using different nanovoltmeters. Therefore, it is necessary to apply synchronously digital signals to two Keithley 2182 nanovoltmeters. By using National Instruments PCI-6602 counter timer, we could measure 15 points synchronously to determine Δ*V* and Δ*T*. It was necessary to subtract the offset drift in order to obtain absolute thermoelectric power of the samples. At each measurement, temperature of the sample holder in the cryostat was controlled using loop 1 of LakeShore 331S temperature controller with sensor (DT-470). System was checked by measurements with Pt standard [14] and TTF·TCNQ complex [5, 15] in temperature range 4–350 K.

potential (*E*1/2(2)) is related to intramolecular on-site Coulomb repulsion energy, *U*. The Δ*E* of EDOB-EDT-TTF also showed middle between that of BEDT-TTF and BEDO-BDTTF. The *U* of

Hot solutions of each donor and acceptor in acetonitrile were mixed. After the reaction mixture was cooled to room temperature (RT), the resulting precipitate was collected by filtration. Complexes were washed with the same organic solvent and dried *in vacuo*. Black octahedral PF6, AsF6 and SbF6 salts were obtained by electrochemical oxidation in distilled 1,2-dichloroethane or TCE under constant current of 1 μA in a mixture of the donor and the tetra-*n*butylammonium salts of the corresponding anions at RT using H-shaped cell with Pt electrodes for 2 weeks. Stoichiometry of CT complexes and radical salts was determined using elemental

DC conductivities were measured with standard four- or two-probe techniques, using Keithley 220 current source, Keithley 199 voltage/scanner, Keithley 195A voltmeter, and Scientific Instruments 9650 temperature controller. For powder samples, measurements were performed on compressed pellets, which were cut to form orthorhombic shape. Gold wires were glued to

**3.2. Apparatus for simultaneous measurements of thermoelectric power and resistivity on**

Simultaneous measurements of thermoelectric power and resistivity by a two-probe method of PF6, AsF6, and SbF6 salts were performed using computer-interfaced system, which sche-

Software program for controlling the system was created in LabVIEW by Computer Automation Co. and System Approach Co. By applying different digital signals using relay and counter timer, we could perform simultaneous measurements of thermoelectric power and two-probe electrical resistivity on a single sample over the entire temperature range [12]. First, by opening the circuit through Keithley 2002 multimeter attached 2001-Scan scanner card as the relay,

meter. Temperature gradient, ∆*T*, between two copper plates was measured by another Keithley 2182 nanovoltmeter. Thermoelectric power was determined. Second, after thermal equilibrium was sufficiently reached between two copper plates, by connecting the circuit through the relay, sample voltage was measured by Keithley 2182 nanovoltmeter, while constant current was supplied to the sample reversing leads to cancel thermal EMFs by Keithley 220 current source. Two-probe resistance measurement was determined by subtract-

electrodes was measured by Keithley 2182 nanovolt-

and V−

EDOB-EDT-TTF decreased compared to that of BEDO-BDTTF.

**2.2. Preparation of CT complexes and radical salts**

30 Thermoelectrics for Power Generation - A Look at Trends in the Technology

analysis or X-ray crystallographic analyses.

**3.1. Electrical conductivity measurement**

matic diagram is shown in **Figure 2**.

sample voltage (∆*V*) between V+

the samples with gold paint (Tokuriki Chemical, no. 8560).

**3. Measurements**

**organic conductors**

**Figure 2.** Schematic diagram for simultaneous measurements of thermoelectric power and two-probe electrical resistivity. CTM: counter timer 6602; TC: temperature controller 331S; V: nanovoltmeter 2182; CS: current source 220; R: relay 2002; H: heater; S: sample; T: thermocouple wire; CR: chip resistor.

Simultaneous measurements of thermoelectric power and resistivity of (EDOB-EDT-TTF)2PF6 were also performed on Quantum Design PPMS Model P670 Thermal Transport System (TTO) in temperature range 232-327 K. The crystal, which was glued to two-probe using bar-shaped copper leads, was mounted on a TTO sample puck.

## **4. Crystal structure**

Single crystal structure analyses have been carried out for PF6 salt at 298 K, AsF6 salt at 90, 293, 330, and 350 K, SbF6 salt at 90 and 293 K. Crystallographic data are listed in **Table 1**.


**Table 1.** Crystallographic data for EDOB-EDT-TTF salts.

## **4.1. Crystal structure of (EDOB-EDT-TTF)2PF6**

Crystal structure of 2:1 (EDOB-EDT-TTF)2PF6 at 298 K is depicted in **Figure 3** and belongs to triclinic *P*−1 space group. Cation layers of EDOB-EDT-TTF molecules and anion layers of PF6 − anions are arranged alternately along the direction of *a*-axis. Donor molecules in the crystal are stacked in alternating orientations along the stacking axis. Average interplanar donordonor distances in the columns equal to 3.635 and 3.641 Å, respectively. This donor packing arrangement is so-called β-type structure [16] as in β-(BEDT-TTF)2I3 [17]. Intermolecular sideby-side short contacts, less than van der Waals (vdW) [18] sum, are observed at S(6)…S(6) (3.55 Å: vdW sum = 3.60 Å) and O(1)…H(11A) (2.63 Å: vdW sum = 2.72 Å). Intermolecular intrastack short contacts O(1)…H(5A) (2.693 Å) and C(8)…H(4A) (2.756 Å: vdW sum = 2.90 Å) are alternatively found along *b*-axis. Intermolecular S…F short contacts less, than vdW sum (3.27 Å for S…F) between EDOB-EDT-TTF cations and hexafluorophosphate anions, are not observed. Octahedral anion does not show rotational disorder.

Simultaneous measurements of thermoelectric power and resistivity of (EDOB-EDT-TTF)2PF6 were also performed on Quantum Design PPMS Model P670 Thermal Transport System (TTO) in temperature range 232-327 K. The crystal, which was glued to two-probe using bar-shaped

Single crystal structure analyses have been carried out for PF6 salt at 298 K, AsF6 salt at 90, 293,

**AsF6 salt at 330 K** 

**AsF6 salt at 350 K** 

**SbF6 salt at 90 K** 

**SbF6 salt at 293 K**

−

330, and 350 K, SbF6 salt at 90 and 293 K. Crystallographic data are listed in **Table 1**.

Chemical formula C28H20F6O4PS12 C28H20AsF6O4S12 C28H20F6O4S12Sb

Formula weight 950.23 994.15 1040.99 Crystal system Triclinic Triclinic Monoclinic Space group *P*–1 *P*–1 *C*2/c

**AsF6 salt at 293 K** 

*a*/Å 7.003 6.875 7.000 7.036 7.057 37.805 38.105 *b*/Å 8.074 7.914 8.061 8.092 8.112 8.204 8.340 *c*/Å 16.326 16.411 16.424 16.411 16.404 11.371 11.429

*β*/° 78.07 98.34 78.41 78.46 78.49 103.23 102.77

*V*/Å3 871.7 847.8 876.2 883.4 888.0 3433.0 3542.4 *Z* 1 1 1 1 1 4 4 *T*/K 298 90 293 330 350 90 293 CCDC no. 819768 809939 809703 802121 799928 799214

Crystal structure of 2:1 (EDOB-EDT-TTF)2PF6 at 298 K is depicted in **Figure 3** and belongs to triclinic *P*−1 space group. Cation layers of EDOB-EDT-TTF molecules and anion layers of PF6

anions are arranged alternately along the direction of *a*-axis. Donor molecules in the crystal are stacked in alternating orientations along the stacking axis. Average interplanar donordonor distances in the columns equal to 3.635 and 3.641 Å, respectively. This donor packing arrangement is so-called β-type structure [16] as in β-(BEDT-TTF)2I3 [17]. Intermolecular sideby-side short contacts, less than van der Waals (vdW) [18] sum, are observed at S(6)…S(6) (3.55 Å: vdW sum = 3.60 Å) and O(1)…H(11A) (2.63 Å: vdW sum = 2.72 Å). Intermolecular intrastack

copper leads, was mounted on a TTO sample puck.

32 Thermoelectrics for Power Generation - A Look at Trends in the Technology

**PF6 salt at 298 K** 

**Table 1.** Crystallographic data for EDOB-EDT-TTF salts.

**4.1. Crystal structure of (EDOB-EDT-TTF)2PF6**

**AsF6 salt at 90 K** 

*α*/° 76.02 103.40 76.07 76.23 76.34

*γ*/° 81.50 97.11 81.45 81.07 80.85

**4. Crystal structure**

**Figure 3.** Crystal structure of (EDOB-EDT-TTF)2PF6: (a) viewed along *b*-axis and (b) viewed along molecular long axis. Broken lines represent intermolecular short contacts.

#### **4.2. Crystal structure of (EDOB-EDT-TTF)2AsF6**

Crystal structure of (EDOB-EDT-TTF)2AsF6 is isostructural to PF6 salt and was elucidated at various temperatures (90, 293, 330 and 350 K). Cation layers of donor molecules and anion layers of AsF6 − anions are arranged alternately along the direction of *c*-axis as shown in **Figure 4**. EDOB-EDT-TTF molecules are packed head-to-tail in face-to-face overlapping manner and alternately stacked with different interplanar along *b*-axis. There are intermolecular short S…S, O…H and S…F contacts, less than vdW sum. Interstack S(1)…S(1) short contacts less, than 3.60 Å, are observed over the entire temperature range (3.530 Å at 90 K, 3.548 Å at 293 K, 3.555 Å at 330 K, and 3.563 Å at 350 K), and S(3)…S(6) short contacts are found at 90 K (3.536 Å), but not at 293 K (3.629 Å), 330 K (3.654 Å), and 350 K (3.669 Å). S…S short contacts are found only between the stacks and not within the stacks. Side-by-side O(2)… H(6B)–C(6) short contacts (2.673 Å at 90 K, 2.647 Å at 293 K, 2.637 Å at 330 K, and 2.634 Å at 350 K), less than vdW sum in intermolecular interstack along *c*-axis, are observed over the entire temperature range as shown in **Figure 4**. O(2)…H(12A)−C(12) short contacts in intermolecular intrastack along *b*-axis (**Figure 4a**) can be seen at 90 K (2.440 Å), 293 K (2.610 Å) and

**Figure 4.** Crystal structure of (EDOB-EDT-TTF)2AsF6 at 293 K showing short contacts as dashed lines: (a) projection in *bc* plane and (b) projection in *ac* plane.

330 K (2.721 Å), but not at 350 K (2.757 Å). Only O(2)…H(12A)−C(12) short contacts exist within the intermolecular intrastack dimer. The other O(2)…H(6B), S…S, and S…F short contacts exist in the intermolecular interstack along the transverse direction. Owing to the effect of dimerization, distances of O(2)…H(12A)−C(12) at 90, 293, and 330 K are shorter than those at 350 K. Intermolecular S(5)…F(2) short contacts, less than vdW sum between EDOB-EDT-TTF cations and hexafluoroarsenate anions, are observed at 90 K (3.080 Å), 293 K (3.208 Å), and 330 K (3.255 Å), but not at 350 K (3.279 Å). Octahedral anion does not show rotational disorder.

## **4.3. Crystal structure of (EDOB-EDT-TTF)2SbF6**

**Figure 4**. EDOB-EDT-TTF molecules are packed head-to-tail in face-to-face overlapping manner and alternately stacked with different interplanar along *b*-axis. There are intermolecular short S…S, O…H and S…F contacts, less than vdW sum. Interstack S(1)…S(1) short contacts less, than 3.60 Å, are observed over the entire temperature range (3.530 Å at 90 K, 3.548 Å at 293 K, 3.555 Å at 330 K, and 3.563 Å at 350 K), and S(3)…S(6) short contacts are found at 90 K (3.536 Å), but not at 293 K (3.629 Å), 330 K (3.654 Å), and 350 K (3.669 Å). S…S short contacts are found only between the stacks and not within the stacks. Side-by-side O(2)… H(6B)–C(6) short contacts (2.673 Å at 90 K, 2.647 Å at 293 K, 2.637 Å at 330 K, and 2.634 Å at 350 K), less than vdW sum in intermolecular interstack along *c*-axis, are observed over the entire temperature range as shown in **Figure 4**. O(2)…H(12A)−C(12) short contacts in intermolecular intrastack along *b*-axis (**Figure 4a**) can be seen at 90 K (2.440 Å), 293 K (2.610 Å) and

34 Thermoelectrics for Power Generation - A Look at Trends in the Technology

**Figure 4.** Crystal structure of (EDOB-EDT-TTF)2AsF6 at 293 K showing short contacts as dashed lines: (a) projection in

*bc* plane and (b) projection in *ac* plane.

(EDOB-EDT-TTF)2SbF6 crystallizes in two forms, plate and needle. Needle form is too much small for X-ray crystal structure analysis. Crystal structure of plate (EDOB-EDT-TTF)2SbF6 is not isostructural to AsF6 and PF6 salts and belongs to monoclinic *C*2/*c* space group, which was elucidated at 90 and 293 K. Crystal structure of plate form at 293 K is shown in **Figure 5**, and octahedral SbF6 − anion does not show a disorder. Molecular arrangement of EDOB-EDT-TTF molecules is quite different from that in (EDOB-EDT-TTF)2AsF6. Two EDOB-EDT-TTF molecules are paired with molecular planes almost parallel, and adjacent pairs are nearly perpendicular to each other. This type of molecular arrangement tends to create two-dimensional networks [19]. However, SbF6 salt is semiconductor. Intermolecular short S…S and O…H contacts are observed, but no S…F contacts are found at 293 and 90 K. S…S short contacts at 293 K are between S(4)…S(5) (3.502 Å) and S(5)…S(6) (3.519 Å) as shown in **Figure 5**. S…S short contacts at 90 K are seen at S(3)…S(6) (3.462 Å) and S(5)…S(6) (3.467 Å) and at S(2)…S(3) (3.563 Å) as shown in **Figure 6**. Dimerization of two EDOB-EDT-TTF molecules becomes stronger at 90 K. O…H short contacts are observed at O(2)…H(13B) (2.607 Å) (along *b*-axis) and O(2)… H(3) (2.470 Å) (along *c*-axis) at 293 K and O(1)…H(12A) (2.503 Å) (along *b*-axis) and O(1)… H(8) (2.425 Å) (along *c*-axis) at 90 K as shown in **Figures 5** and **6**.

**Figure 5.** Crystal structure of plate (EDOB-EDT-TTF)2SbF6 at 293 K showing intermolecular short O…H and S…S contacts as dotted lines.

**Figure 6.** Crystal structure of plate (EDOB-EDT-TTF)2SbF6 at 90 K showing intermolecular short H…H and S…S contacts as dotted lines.

## **5. Electrical conductivity**

**Table 2** summarizes appearances, component ratios, metal-to-semiconductor (MS) transition, room-temperature electrical conductivity, and activation energies of EDOB-EDT-TTF complexes and salts. A newly [10] and the previously [9] reported (EDOB-EDT-TTF)2PF6 salts exhibited electrical resistivity decrease with heating (**Figure 7**) and showed resistivity minimum at 340 K, then gradual increase up to 350 K. As shown in **Figure 8**, new black plates (EDOB-EDT-TTF)2AsF6 (*σ*RT = 2.6 S cm−1) exhibited distinct minimum in resistivity at 315 K confirming MS transition at this temperature. The semiconductive region (< 315 K) showed the activation energy of *E*a = 0.13 eV. It was found that *T*MS decreased with increase in anion size, as well as β-(BEDT-TTF)2X (X = PF6 and AsF6) [20, 21]. Electrical conductivity of new black plate and fine needle SbF6 salts at room temperature was 4.4 × 10−2 S cm−1 (*E*a = 0.13 eV) and 2.9 × 10−3 S cm−1 (*E*a = 0.13 eV), respectively. SbF6 salts behaved as semiconductors from room temperature to 100 K.


a Measured on a compressed pellet by a four-probe method.

b by a four-probe method.

c by a two-probe method.

**Table 2.** Electric conductivities of EDOB-EDT-TTF complexes and salts.

Electrical Conductivity, Thermoelectric Power and Crystal and Band Structures of EDOB-EDT-TTF Salts... http://dx.doi.org/10.5772/65561 37

**Figure 6.** Crystal structure of plate (EDOB-EDT-TTF)2SbF6 at 90 K showing intermolecular short H…H and S…S con-

**Table 2** summarizes appearances, component ratios, metal-to-semiconductor (MS) transition, room-temperature electrical conductivity, and activation energies of EDOB-EDT-TTF complexes and salts. A newly [10] and the previously [9] reported (EDOB-EDT-TTF)2PF6 salts exhibited electrical resistivity decrease with heating (**Figure 7**) and showed resistivity minimum at 340 K, then gradual increase up to 350 K. As shown in **Figure 8**, new black plates (EDOB-EDT-TTF)2AsF6 (*σ*RT = 2.6 S cm−1) exhibited distinct minimum in resistivity at 315 K confirming MS transition at this temperature. The semiconductive region (< 315 K) showed the activation energy of *E*a = 0.13 eV. It was found that *T*MS decreased with increase in anion size, as well as β-(BEDT-TTF)2X (X = PF6 and AsF6) [20, 21]. Electrical conductivity of new black plate and fine needle SbF6 salts at room temperature was 4.4 × 10−2 S cm−1 (*E*a = 0.13 eV) and 2.9 × 10−3 S cm−1 (*E*a = 0.13 eV), respectively. SbF6 salts behaved as semiconductors from room

**Acceptor or anion Appearance Stoichiometry D:A** *T***MS/K** *σ***RT/S cm−1** *E***a/eV References** M2TCNQ Black powder 1:1 Insulatora [9] TCNQ Black power 1:1 1.9a 0.085 [9] FTCNQ Dark green powder 9.6 × 10−2a 0.13 [9] F2TCNQ Dark green powder 4:1 7.1 × 10−2a 0.13 [9]

<sup>−</sup> Black plate 2:1 340 1.7 × 10b 0.23 [9]

<sup>−</sup> Black plate 2:1 337 1.0c 0.17 [10]

<sup>−</sup> Black plate 2:1 315 2.6c 0.13 [10]

<sup>−</sup> Black plate 2:1 4.4 × 10−2c 0.13 [10]

<sup>−</sup> Black fine needle 2:1 2.9 × 10−3c 0.13 [10]

tacts as dotted lines.

**5. Electrical conductivity**

36 Thermoelectrics for Power Generation - A Look at Trends in the Technology

temperature to 100 K.

by a four-probe method.

by a two-probe method.

Measured on a compressed pellet by a four-probe method.

**Table 2.** Electric conductivities of EDOB-EDT-TTF complexes and salts.

PF6

PF6

AsF6

SbF6

SbF6

a

b

c

**Figure 7.** Temperature dependence of resistance of single crystals (EDOB-EDT-TTF)2PF6 in the heating run. Data for two crystals are plotted.

**Figure 8.** Temperature dependence of resistance of single crystals (EDOB-EDT-TTF)2AsF6 in the heating run. Data for two crystals are plotted.

## **6. Thermoelectric power**

Chaikin et al. have described in references [15, 22, 23], that thermoelectric power coefficient in metallic state for a single one-dimensional band is given by:

$$S = \frac{-\pi^2 k\_B^2 T}{3|e|} \left( \frac{\cos\left(\frac{\pi\rho}{2}\right)}{2|t|\sin^2(\frac{\pi\rho}{2})} + \frac{\pi'(\varepsilon)}{\pi(\varepsilon)\_{\varepsilon F}} \right),\tag{1}$$

where *τ*(*ε*) is energy-dependent electron scattering time, *ρ* is amount of charge transfer, *t* is transfer integral (4*t* is the bandwidth), *kB* is Boltzmann constant, *EF* is Fermi energy, and *T* denotes temperature. When band structure contribution to thermoelectric power dominates in Eq. (1), the sign of thermoelectric power is negative for *ρ* < 1 and positive for *ρ* > 1. shows linear temperature dependence in metallic region.

Thermoelectric power coefficient for semiconductor is given by:

$$LS = \frac{-k\_{\rm \\_}}{|e|} \left( \frac{b-1}{b+1} \frac{E\_a}{k\_B T} + \ln \frac{m\_h}{m\_e} \right),\tag{2}$$

where *b* is the ratio of electron-to-hole mobility, and *mh* and *me* are, respectively, effective mass of hole and electron. shows *T*−1 temperature dependence for semiconductor.

Conwell has shown that near-constant value close to −60 μV/K over wide temperature range is obtained with model, in which there are strong on-site correlations, [20, 24].

In this study, thermoelectric power measurements were carried out to clarify MS transition of these salts. Thermoelectric powers of PF6, AsF6, and SbF6 salts were measured in temperature range 220–360 K.

#### **6.1. Thermoelectric power of (EDOB-EDT-TTF)2PF6**

Positive value of thermoelectric power implies hole-like character of conduction charge carriers. The sign (•) of thermoelectric power of PF6 salt along the crystal growth axis was positive above 235 K as shown in **Figure 9**. Thermoelectric power value of PF6 salt jumps around 305 and 340 K. Inflection of thermoelectric power curve occurs around 270 K. These jumps and inflection were reproduced for different three samples. Thermoelectric power data (▲) measured by Quantum Design PPMS also shows jumps around 272 and 305 K. Thermoelectric power jumps of PF6 salt shown in **Figure 9** is not clear, however, that of PF6 salt was proportional to absolute temperature down to 320 K, which is characteristic of metallic conduction. Below 320 K, value of thermoelectric power dropped gradually, indicating MS transition around 320 K. These transition temperatures of PF6 salt corresponded with the results of electrical resistivity (▪). Thermoelectric power of PF6 salt dropped below 305 K, decreasing rapidly below 270 K. Metallic properties denoted both by thermoelectric power and by electrical resistivity measurements.

**Figure 9.** Simultaneous measurements of temperature dependence of thermoelectric power (•) and electrical resistivity (▪) of (EDOB-EDT-TTF)2PF6 salt. Data of thermoelectric power (▴) and resistivity (▾) were measured by Quantum Design PPMS in temperature range 232–327 K. Solid line extrapolates to zero at *T* = 0 K.

#### **6.2. Thermoelectric power of (EDOB-EDT-TTF)2AsF6**

**6. Thermoelectric power**

range 220–360 K.

metallic state for a single one-dimensional band is given by:

38 Thermoelectrics for Power Generation - A Look at Trends in the Technology

2 2

p

Thermoelectric power coefficient for semiconductor is given by:

linear temperature dependence in metallic region.

**6.1. Thermoelectric power of (EDOB-EDT-TTF)2PF6**

*B*

*cos k T <sup>S</sup> <sup>e</sup> t sin*

Chaikin et al. have described in references [15, 22, 23], that thermoelectric power coefficient in

2 <sup>2</sup> ' , <sup>3</sup> 2 () <sup>2</sup>

æ ö æ ö ç ÷ ç ÷ - è ø = +

pr

where *τ*(*ε*) is energy-dependent electron scattering time, *ρ* is amount of charge transfer, *t* is transfer integral (4*t* is the bandwidth), *kB* is Boltzmann constant, *EF* is Fermi energy, and *T* denotes temperature. When band structure contribution to thermoelectric power dominates in Eq. (1), the sign of thermoelectric power is negative for *ρ* < 1 and positive for *ρ* > 1. shows

> <sup>2</sup> <sup>1</sup> , <sup>1</sup> *B ah*

where *b* is the ratio of electron-to-hole mobility, and *mh* and *me* are, respectively, effective mass

Conwell has shown that near-constant value close to −60 μV/K over wide temperature range

In this study, thermoelectric power measurements were carried out to clarify MS transition of these salts. Thermoelectric powers of PF6, AsF6, and SbF6 salts were measured in temperature

Positive value of thermoelectric power implies hole-like character of conduction charge carriers. The sign (•) of thermoelectric power of PF6 salt along the crystal growth axis was positive above 235 K as shown in **Figure 9**. Thermoelectric power value of PF6 salt jumps around 305 and 340 K. Inflection of thermoelectric power curve occurs around 270 K. These jumps and inflection were reproduced for different three samples. Thermoelectric power data (▲) measured by Quantum Design PPMS also shows jumps around 272 and 305 K. Thermoelectric power jumps of PF6 salt shown in **Figure 9** is not clear, however, that of PF6 salt was proportional to absolute temperature down to 320 K, which is characteristic of metallic conduction. Below 320 K, value of thermoelectric power dropped gradually, indicating MS transition around 320 K. These transition temperatures of PF6 salt corresponded with the

*kbE m <sup>S</sup> ln e b kT m* - - æ ö = + ç ÷

of hole and electron. shows *T*−1 temperature dependence for semiconductor.

is obtained with model, in which there are strong on-site correlations, [20, 24].

*B e*

è ø

pr

( ) ( )

t e

t e

*EF*

è ø <sup>+</sup> (2)

(1)

**Figure 10** shows simultaneous measurements of temperature dependence of thermoelectric power and electrical resistivity of (EDOB-EDT-TTF)2AsF6 salt. Thermoelectric power of AsF6 salt along the crystal growth axis was linear with temperature down to 310 K, which is characteristic of a metal. Thermoelectric power value of AsF6 salt seems to jump around 305 K and then drops below 300 K, indicating MS transition around 310 K. MS transition temperature observed in thermoelectric power seems to be slightly lower than that of electrical conductivity [25]. Thermoelectric power of AsF6 salt again showed rapid decrease below 260 K. Inflection of thermoelectric power curve was detected around 260 K. These jumps and inflection were reproduced for different two samples. The sign of thermoelectric power of AsF6 salt was positive above 220 K. Thermoelectric power of AsF6 salt became negative below 220 K and

decreased with decreasing temperature. MS transition temperatures of PF6 and AsF6 salts observed in thermoelectric power decreased in order of 320 and 310 K.

**Figure 10.** Simultaneous measurements of temperature dependence of thermoelectric power (•) and electrical resistivity (▪) of (EDOB-EDT-TTF)2AsF6 salt. Solid line extrapolates to zero at *T* = 0 K.

#### **6.3. Thermoelectric power of (EDOB-EDT-TTF)2SbF6**

Simultaneous measurements of temperature dependence of thermoelectric power and electrical resistivity of (EDOB-EDT-TTF)2SbF6 salt are shown in **Figure 11**. Thermoelectric power value of plate SbF6 salt was negative below 335 K and decreased with decreasing temperature. Negative value of thermoelectric power implies electron-like character of conduction charge carriers. Thermoelectric power exhibits *T*−1-temperature dependence, which is a characteristic of semiconductor.

**Figure 11.** Simultaneous measurements of temperature dependence of thermoelectric power (•) and resistivity (▪) of (EDOB-EDT-TTF)2SbF6 salt.
