*3.3.1. Sn0.9Y0.1Se*

**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.

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

**Figure 5.** (a) XRD pattern and its Rietveld-refinement of Sn0.9Mo0.1Se, showing an excellent agreement between observed

Se crystal structure, highlighting the layers of

(crosses) and calculated (black line) profiles. (b) View of the Sn1−xMo<sup>x</sup>

polyhedra perpendicular to **a** axis.

12 Bringing Thermoelectricity into Reality

Sn(Mo)Se3

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

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 near its bottom.

The electrical conductivity of Sn0.9Y0.1Se rises exponentially with temperature, corresponding to a thermally activated semiconductor with an activation energy of E<sup>g</sup> ~0.2 eV, obtained 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 intergrain hopping with activation energy of around 0.2 eV.

since its power factor is rather small, probably because of a lack of effective charge transfer

Nanostructured Thermoelectric Chalcogenides http://dx.doi.org/10.5772/intechopen.75442 15

We have prepared a series of molybdenum-alloyed SnSe samples at various concentrations, from 1% up to 30% Mo. According to the change of lattice parameters, Mo clearly enters the SnSe structure. However, the dependence of any of the properties studied (Seebeck coefficient, electrical conductivity, unit-cell volume) vary non-monotonously, thus the use of Mo as an alloying element is questionable. Without more detailed structural studies it is impossible to assess whether Mo indeed incorporates at the nominal compositions into SnSe. For 5% Mo-doped SnSe, the temperature-dependent Seebeck coefficient is shown in **Figure 9a**. The Seebeck coefficient is positive and reaches a maximum of 400 μV/K around 700 K. While the samples are rather resistive at room temperature, we observed an abrupt increment of the electrical conductivity at 800 K, reaching values up to perhaps 10,000 S/m

For SnSe doped with 1% manganese (**Figure 9b**), we also find an abrupt change of sign of the Seebeck coefficient, as described above for Sn0.9Y0.1Se. However, for Mn-doped SnSe the thermoelectric power is positive at low temperatures, and inverts above 630 K. A similar explanation considering two types of charge carriers may be invoked here. Importantly, this behavior is intrinsic to the material, not an effect of thermally induced chemical changes, as it is reversible, and reproducible in several thermal cycles. Therefore, Mn-doping provides a

The Ag-doped SnSe system is an example where the interplay of the Seebeck coefficient and electrical conductivity follows the Pisarenko relation, in **Table 1**. For low Ag-doping the Seebeck coefficient is around 200 μV/K at room temperature with an electrical conductivity

(i.e. doping).

(not shown).

*3.3.4. Sn0.99Mn0.01Se*

*3.3.5. Sn0.9Ag0.1Se*

high temperature n-type SnSe thermoelectric element.

**Figure 9.** Seebeck coefficient of (a) Sn0.95Mo0.05Se, (b) Sn0.99Mn0.01Se, and (c) Sn0.9Ag0.1Se.

*3.3.3. Sn0.95Mo0.05Se*

**Figure 8.** (a) Seebeck coefficient, (c) electrical conductivity, (e) power factor of Sn0.9Y0.1Se, and (b) Seebeck coefficient, (d) electrical conductivity, (f) power factor of Sn0.97Cd0.03Se and insert in (d) inverse semilog-plot of electrical conductivity indicating a gap.

The power factor reaches a maximum of 0.5 mW/mK<sup>2</sup> at around 800 K. As we will show below and have published before [36–39], the thermal conductivity for SnSe alloys produced by arc-melting is less than 0.5 W/mK, indicating a possible figure of merit zT > 0.8 for Sn0.9Y0.1Se.
