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 Tm³⁺ 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 Cu₂Te, thus making it a promising candidate for mid‐temperature TE applications. Theoretical studies uncover the electronic structure modifications from the metallic Cu₂Te to the narrow gap semiconductor TmCuTe₂ that lead to such a remarkable performance enhancement.