Chemical modification and energetically favorable atomic disorder of a layered thermoelectric material TmCuTe2 leading to high performance.

Thermoelectric (TE) materials have continuously attracted interest worldwide owing to their capability of converting heat into electricity. However, discovery and design of new TE material system remains one of the greatest difficulties. A TE material, TmCuTe2 , has been designed by a substructure approach and successfully synthesized. The structure mainly features CuTe4 -based layers stacking along the c axis that are separated by Tm(3+) cations. Such an intrinsic Cu site vacancy structure undergoes a first-order phase transition at around 606 K driven by the energetically favorable uniform Cu atom re-distribution on the covalent CuTe4 -based layer substructure, as shown by crystal structure simulations and variable-temperature XRD data. Featured with very low thermal conductivity (ca. 0.6 W m(-1)  K(-1) ), large Seebeck coefficient (+185 μV K(-1) ), and moderate electrical conductivity (220 S cm(-1) ), TmCuTe2 has a maximum ZT of 0.81 at 745 K, which is nine times higher than the value of 0.09 for binary Cu2 Te, thus making it a promising candidate for mid-temperature TE applications. Theoretical studies uncover the electronic structure modifications from the metallic Cu2 Te to the narrow gap semiconductor TmCuTe2 that lead to such a remarkable performance enhancement.