Enhanced Thermoelectric Performance in 18-Electron Nb_0.8CoSb Half-Heusler Compound with Intrinsic Nb Vacancies

Typical 18-electron half-Heusler compounds, ZrNiSn and NbFeSb, are identified as promising high-temperature thermoelectric materials. NbCoSb with nominal 19 valence electrons, which is supposed to be metallic, is recently reported to also exhibit thermoelectric properties of a heavily doped n-type semiconductor. Here for the first time, it is experimentally demonstrated that the nominal 19-electron NbCoSb is actually the composite of 18-electron Nb_{0.8+ δ}CoSb (0 ≤ δ < 0.05) and impurity phases. Single-phase Nb_{0.8+ δ}CoSb with intrinsic Nb vacancies, following the 18-electron rule, possesses improved thermoelectric performance, and the slight change in the content of Nb vacancies has a profound effect on the thermoelectric properties. The carrier concentration can be controlled by varying the Nb deficiency, and the optimization of the thermoelectric properties can be realized within the narrow pure phase region. Benefiting from the elimination of impurity phases and the optimization of carrier concentration, thermoelectric performance is remarkably enhanced by ≈100% and a maximum zT of 0.9 is achieved in Nb_0.83CoSb at 1123 K. This work expands the family of half-Heusler thermoelectric materials and opens a new avenue for searching for nominal 19-electron half-Heusler compounds with intrinsic vacancies as promising thermoelectric materials.