**3. Results and discussion**

#### **3.1. XRD characterization**

Materials of the series Sn1−xM<sup>x</sup> Se (M = Ti, Cr, Mn, Zn, Mo, Ag, Cd, Au) were prepared by a simple and straightforward arc-melting technique, yielding highly textured SnSe-type samples. About 2 g ingots were obtained in each case; a part was ground to powder for structural characterization, the remaining pellet was used for transport measurements. XRD patterns are all characteristic of the orthorhombic GeSe structural type, and can be Rietveld-refined in the orthorhombic *Pnma* space group. Both Sn(M) and Se atoms occupy 4*c* (x, ¼, z) positions. Dopant M atoms are distributed at random at Sn positions: different occupation tests were performed in the refinement, verifying M inclusion in the Sn sublattice. **Table 1** includes the unit-cell parameter and volume for each sample; the left panel of **Figure 5a** illustrates the quality of the fit for Sn0.9Mo0.1Se, with good discrepancy factors (R<sup>p</sup> = 2.29%, Rwp = 3.07%, Rexp = 1.73%, χ<sup>2</sup> = 3.26, RBragg = 4.42%). The structure consists of trigonal pyramids Sn(M)Se3 forming double layers perpendicular to the [100] direction, as shown in **Figure 5b**.

**Figure 6** illustrates, as an example, the unit cell-volume variation for some selected M dopant atoms. A contraction of the cell is observed when Ag is introduced at Sn positions (**Figure 6a**), whereas the cell volume expands as Ti is introduced in the crystal structure (**Figure 6b**), as a consequence of the different ionic radii of the concerned atoms. In either case, the unit-cell variation assesses the incorporation of the dopant atoms in the crystal structure, discarding the inclusion of spurious phases in the grain boundaries or as secondary phases.

#### **3.2. Scanning electron microscopy (SEM)**

The layered crystal structure of SnSe-type compounds, containing strong covalent bonds within the layers, and much weaker interactions between adjacent layers, promotes the easy cleavage of materials. The arc-furnace procedure, involving a fast quenching of the samples from the molten state, drives a peculiar microstructure consisting of piles of stacking sheets, as shown in **Figure 7**.

Illustrated for M = Mn and M = Cd, the same typical microstructure is observed for all the samples. We find individual sheets with thickness below 0.1 μm (typically 20 to 40 nm). This micro- or nanostructuration has strong influence on the thermoelectric properties, especially the many surface boundaries that are responsible for scattering of both charge carriers and phonons (i.e. the electrical and the thermal conductivity).

#### **3.3. Transport measurements**

The Seebeck coefficient and electrical conductivity were measured in coin-shaped pellets of 12 mm diameter in the home-made device described before. **Table 1** includes the thermopower and electrical conductivity at room temperature (RT) for all the studied samples, which is useful as a preliminary screening in order to avoid particularly time-consuming high-temperature measurements. For the sake of comparison, some samples were also prepared by ball milling followed by SPS processes; inferior (lower) Seebeck coefficients and (considerably lower) electrical conductivities were invariably obtained, proving the excellence of the arc-melting procedure

**Table 1.** Seebeck coefficient, electrical conductivity, and unit cell parameters of the different Sn1−xM<sup>x</sup>

**Material Seebeck** 

at room temperature.

**coefficient (μV/K)**

**Electrical conductivity (S/m)**

**Lattice constant a (Å)**

SnSe 403 13 11.500 (2) 4.151 (1) 4.443 (1) 212.04 (9) SnSe (SPS) 316 0.7 11.502 (2) 4.1539 (8) 4.4441 (9) 212.33 (7) Sn0.9Y0.1Se −81 6 11.503 (3) 4.157 (1) 4.436 (2) 212.1 (1) Sn0.95Ti0.05Se 12 111 11.497 (2) 4.1531 (8) 4.443 (1) 212.14 (7) Sn0.90Ti0.10Se 11.523 (7) 4.165 (3) 4.430 (3) 212.6 (3) Sn0.85Ti0.15Se 7 2841 11.508 (4) 4.155 (1) 4.448 (2) 212.7 (1) Sn0.95Cr0.05Se −2 174 11.494 (3) 4.151 (1) 4.440 (1) 211.8 (1) Sn0.9Cr0.1Se −5 251 11.495 (3) 4.151 (1) 4.441 (1) 211.9 (1) Sn0.99Mo0.01Se 480 4 11.492 (2) 4.1516 (7) 4.4424 (9) 211.94 (6) Sn0.97Mo0.03Se 500 8 11.502 (2) 4.1560 (8) 4.446 (1) 212.53 (7) Sn0.95Mo0.05Se 280 15 11.499 (2) 4.1529 (8) 4.445 (1) 212.29 (7) Sn0.9Mo0.1Se 442 3 11.502 (2) 4.1523(8) 4.445 (1) 212.29 (7) Sn0.8Mo0.2Se 404 8 11.504 (2) 4.1553 (9) 4.445 (1) 212.51 (8) Sn0.7Mo0.3Se 357 47 11.504 (2) 4.155 (1) 4.444 (1) 212.41 (8) Sn0.99Mn0.01Se 453 42 11.509 (2) 4.1558 (8) 4.447 (1) 212.72 (8) Sn0.99Ag0.01Se 220 330 11.505 (2) 4.155 (1) 4.446 (1) 212.57 (9) Sn0.97Ag0.03Se 161 111 11.500 (2) 4.1553 (9) 4.445 (1) 212.42 (8) Sn0.95Ag0.05Se 112 276 11.502 (2) 4.155(1) 4.445 (1) 212.40 (9) Sn0.9Ag0.1Se 123 3411 11.496 (4) 4.152 (2) 4.441 (2) 212.0 (2) Sn0.99Au0.01Se 368 224 11.506 (3) 4.155 (1) 4.446 (1) 212.6 (1) Sn0.9Zn0.1Se 432 18 11.500 (4) 4.154 (1) 4.444 (2) 212.3 (1) Sn0.99Cd0.01Se 590 2 11.502 (3) 4.153 (1) 4.443 (2) 212.2 (1) Sn0.97Cd0.03Se 640 0.7 11.497 (3) 4.152 (1) 4.440 (2) 211.9 (1) Sn0.95Cd0.05Se 549 4 11.503 (3) 4.154 (1) 4.441 (1) 212.2 (1) Sn0.9Cd0.1Se 482 23 11.513 (3) 4.161 (1) 4.444 (2) 212.9 (1) Sn0.8Cd0.2Se 508 6 11.506 (3) 4.156 (1) 4.441 (2) 212.4 (1)

**Lattice constant b (Å)**

**Lattice constant c (Å)**

Nanostructured Thermoelectric Chalcogenides http://dx.doi.org/10.5772/intechopen.75442

**Volume (Å3**

Se alloys measured

**)**

11


**3. Results and discussion**

Materials of the series Sn1−xM<sup>x</sup>

trigonal pyramids Sn(M)Se3

**3.2. Scanning electron microscopy (SEM)**

phonons (i.e. the electrical and the thermal conductivity).

shown in **Figure 5b**.

ary phases.

as shown in **Figure 7**.

**3.3. Transport measurements**

Se (M = Ti, Cr, Mn, Zn, Mo, Ag, Cd, Au) were prepared by

forming double layers perpendicular to the [100] direction, as

a simple and straightforward arc-melting technique, yielding highly textured SnSe-type samples. About 2 g ingots were obtained in each case; a part was ground to powder for structural characterization, the remaining pellet was used for transport measurements. XRD patterns are all characteristic of the orthorhombic GeSe structural type, and can be Rietveld-refined in the orthorhombic *Pnma* space group. Both Sn(M) and Se atoms occupy 4*c* (x, ¼, z) positions. Dopant M atoms are distributed at random at Sn positions: different occupation tests were performed in the refinement, verifying M inclusion in the Sn sublattice. **Table 1** includes the unit-cell parameter and volume for each sample; the left panel of **Figure 5a** illustrates the quality of the fit for Sn0.9Mo0.1Se, with good discrepancy factors (R<sup>p</sup> = 2.29%, Rwp = 3.07%, Rexp = 1.73%, χ<sup>2</sup> = 3.26, RBragg = 4.42%). The structure consists of

**Figure 6** illustrates, as an example, the unit cell-volume variation for some selected M dopant atoms. A contraction of the cell is observed when Ag is introduced at Sn positions (**Figure 6a**), whereas the cell volume expands as Ti is introduced in the crystal structure (**Figure 6b**), as a consequence of the different ionic radii of the concerned atoms. In either case, the unit-cell variation assesses the incorporation of the dopant atoms in the crystal structure, discarding the inclusion of spurious phases in the grain boundaries or as second-

The layered crystal structure of SnSe-type compounds, containing strong covalent bonds within the layers, and much weaker interactions between adjacent layers, promotes the easy cleavage of materials. The arc-furnace procedure, involving a fast quenching of the samples from the molten state, drives a peculiar microstructure consisting of piles of stacking sheets,

Illustrated for M = Mn and M = Cd, the same typical microstructure is observed for all the samples. We find individual sheets with thickness below 0.1 μm (typically 20 to 40 nm). This micro- or nanostructuration has strong influence on the thermoelectric properties, especially the many surface boundaries that are responsible for scattering of both charge carriers and

The Seebeck coefficient and electrical conductivity were measured in coin-shaped pellets of 12 mm diameter in the home-made device described before. **Table 1** includes the thermopower and electrical conductivity at room temperature (RT) for all the studied samples, which is useful

**3.1. XRD characterization**

10 Bringing Thermoelectricity into Reality

**Table 1.** Seebeck coefficient, electrical conductivity, and unit cell parameters of the different Sn1−xM<sup>x</sup> Se alloys measured at room temperature.

as a preliminary screening in order to avoid particularly time-consuming high-temperature measurements. For the sake of comparison, some samples were also prepared by ball milling followed by SPS processes; inferior (lower) Seebeck coefficients and (considerably lower) electrical conductivities were invariably obtained, proving the excellence of the arc-melting procedure

reported here. Some samples were subsequently chosen for a more detailed high temperature

Nanostructured Thermoelectric Chalcogenides http://dx.doi.org/10.5772/intechopen.75442 13

As mentioned in the Introduction, pure tin selenide has good thermoelectric properties at high temperature [18]. However, in our screening we focus on the room temperature properties, and although we observed a high Seebeck coefficient, its electrical conductivity turned out to be much lower, by one or two orders of magnitude, than in single crystals. A good reference value for the electrical conductivity would be above 1000 S/m. Our approach has been to enhance this state of affairs by doping SnSe with adequate elements to boost the RT electrical conductivity, while keeping a good Seebeck coefficient. The thermoelectric performance is certainly enhanced at elevated temperature, as we indeed observe here. In addition, doping elements that yield almost zero Seebeck coefficient but good electrical conductivity are very promising for further optimization at much lower doping concentrations, such as

From **Table 1**, it is noteworthy that certain doping elements such as Mo or Cd are able to enhance the RT Seebeck coefficient to as high as 480 μV/K (Mo0.01), 500 μV/K (Mo0.03), 590 μV/K (Cd0.01) or even 640 μV/K (Cd0.03), while other elements such as Cr kill the thermoelectric performance (−2 μV/K for Cr0.05). The microscopic origin of this behavior is beyond this study, since our aim was the preliminary identification of those doping elements that induce a better performance. Some selected samples with promising properties were studied above room

Starting from the left of the periodic table, we have alloyed SnSe with 10% yttrium, to obtain Sn0.9Y0.1Se. Interestingly, it turns out to be an n-type semiconductor, with negative Seebeck coefficient, at room temperature. The thermal evolution of the Seebeck coefficient and the

The Seebeck coefficient changes sign around 600 K. This sign change is reversible, reproducible in the same sample, and was observed in several specimens. The explanation has to do with a scenario where negative and positive carriers coexist in the material. At lower temperatures electrons are the majority carriers and holes the minority carriers. At higher temperatures, more holes are excited, in such a way that holes become the majority carriers, dominating their contribution to the Seebeck effect, turning it positive. This might happen, for example, if a narrow band of defects lies near the top of the valence band with the Fermi-level trapped

The electrical conductivity of Sn0.9Y0.1Se rises exponentially with temperature, correspond-

from the inset of **Figure 8c**. The excellent fit shows that, despite the geometry utilized in the home-made apparatus, it provides highly reliable temperature-dependent relative conductivity values. Such a small band-gap is not intrinsic to pure SnSe. It may correspond to the abovementioned defect band. Another possibility is that the electrical conductivity, invariably poor in arc-melting produced SnSe, and its alloys, at room temperature [36–39], is limited by inter-

~0.2 eV, obtained

ing to a thermally activated semiconductor with an activation energy of E<sup>g</sup>

grain hopping with activation energy of around 0.2 eV.

temperature and are described in the following sections.

electrical conductivity are shown in **Figure 8a**.

study, up to 950 K.

the case of Ti, or Cr.

*3.3.1. Sn0.9Y0.1Se*

near its bottom.

**Figure 5.** (a) XRD pattern and its Rietveld-refinement of Sn0.9Mo0.1Se, showing an excellent agreement between observed (crosses) and calculated (black line) profiles. (b) View of the Sn1−xMo<sup>x</sup> Se crystal structure, highlighting the layers of Sn(Mo)Se3 polyhedra perpendicular to **a** axis.

**Figure 6.** Unit-cell volume variation with the concentration of dopant element for (a) Ag and (b) Ti.

**Figure 7.** SEM image of as-grown Sn1−xM<sup>x</sup> Se, exhibiting nanostructuration, consisting of piles of nanometric platelets (perpendicular to [100] direction) taken with (a) 2000 magnification for M = Mn and (b) 2500 magnification for M = Cd.

reported here. Some samples were subsequently chosen for a more detailed high temperature study, up to 950 K.

As mentioned in the Introduction, pure tin selenide has good thermoelectric properties at high temperature [18]. However, in our screening we focus on the room temperature properties, and although we observed a high Seebeck coefficient, its electrical conductivity turned out to be much lower, by one or two orders of magnitude, than in single crystals. A good reference value for the electrical conductivity would be above 1000 S/m. Our approach has been to enhance this state of affairs by doping SnSe with adequate elements to boost the RT electrical conductivity, while keeping a good Seebeck coefficient. The thermoelectric performance is certainly enhanced at elevated temperature, as we indeed observe here. In addition, doping elements that yield almost zero Seebeck coefficient but good electrical conductivity are very promising for further optimization at much lower doping concentrations, such as the case of Ti, or Cr.

From **Table 1**, it is noteworthy that certain doping elements such as Mo or Cd are able to enhance the RT Seebeck coefficient to as high as 480 μV/K (Mo0.01), 500 μV/K (Mo0.03), 590 μV/K (Cd0.01) or even 640 μV/K (Cd0.03), while other elements such as Cr kill the thermoelectric performance (−2 μV/K for Cr0.05). The microscopic origin of this behavior is beyond this study, since our aim was the preliminary identification of those doping elements that induce a better performance. Some selected samples with promising properties were studied above room temperature and are described in the following sections.
