**Acknowledgements**

of the Wiedemann-Franz law [7], we can estimate charge contribution to the thermal conduc-

cal conductivity. For the highly resistive SnSe and its alloys, total thermal conductivity is dominated by the lattice thermal conductivity by a factor of 10,000:1. The temperature dependence of the lattice thermal conductivity follows the T−1 trend (black-dashed line in **Figure 11**),

The extremely low thermal conductivities observed in all the materials of the different doped

We propose arc-melting technique as a fast one-step process to elaborate nanostructured

peculiar microstructure consisting of stacks of nanosheets. The presence of many extended grain boundaries perturbs the propagation of phonons and brings about extremely low thermal conductivities, around 0.5 W/mK. Using as quick screening tool the RT Seebeck coefficient and electrical conductivity, our preliminary survey of different SnSe-related transition metal alloys shows very distinct behaviors: certain doping elements such as Mo or Cd are able to enhance the RT Seebeck coefficient to values as high as 640 μV/K (3% Cd), while other elements such as Cr practically kill the thermoelectric performance (−2 μV/K for 5% Cr). The microscopic origin of this diverse behavior is far from being understood. Crucially, we find that some dopants, such as Y and Mn, can provide n-type alloys with a change of sign of the Seebeck coefficient. Sn0.90Y0.10Se is n-type at low temperature and changes to p-type around 600 K. This sign change is reversible, and this behavior was reproduced in several specimens. The opposite behavior is observed for Sn0.99Mn0.01Se, which changes from p-type to n-type above 630 K. Many materials showed evidence of the Pisarenko relationship at work: dopants that, at some concentration, can yield highly conducting samples, usually with almost zero Seebeck coefficient, whereas for pure or lightly doped SnSe alloys bad conduction with large Seebeck effects are observed. This rich phenomenology can be useful as a guide for further, deeper research. We also describe an instrument to characterize the high-temperature Seebeck coefficient and electrical conductivity in disk-shaped pellets directly obtained from the intermetallic ingots. Problems with alloying of the samples at high temperature are avoided by the use of Nb pistons and Nb-based thermocouples that are chemically inert to reactive p-block elements. This is shown to be essential for the accuracy and reproducibility of the measurements, avoiding the degradation of the materials

In the context of thermoelectric devices, our finding of stable n-type doped SnSe at elevated temperatures prepared by a straightforward method would allow the use of SnSe derivatives for high-temperature applications. Thermoelectric devices require both n- and p-type semiconductors with matching thermoelectric, electronic, and mechanical properties, and this material system based on SnSe alloys could find application, for example, in the exhaust heat

Se family (M: transition metals). All the specimens show a

showing that the phonon-phonon interaction dominates in this temperature range.

series reported here afford considerably high figures of merit, zT.

*Tσ*, with *L0*

the Lorentz number and σ the electri-

tivity as a function of temperature, *κch = L0*

18 Bringing Thermoelectricity into Reality

**4. Conclusion**

intermetallic alloys of the Sn1−xM<sup>x</sup>

after thermally cycling up to 950 K.

recovery or in solar-hybrid TEG applications.

We thank the financial support of the Spanish Ministry of Science and Innovation to the project MAT2017–84496-R.
