A detailed description of the charge density difference of BiSb(Se0.92Br0.08)3.

Lately, the nominal 19-electron half-Heusler compound NbCoSb, historically viewed as a metal, has attracted reacquaintance and widespread attention because of its unexpected high thermoelectric (TE) performance. Here, the electronic structures of Nb x Co 1−y Ni y Sb (x = 0.8, 1; y = 0, 0.1) have been systematically investigated by using the first-principles method and semiclassical Boltzmann transport theory. We demonstrate that Ni doping at Co sites in NbCoSb with 20% intrinsic Nb vacancies (Nb 0.8 CoSb) leads to a large band degeneracy, and the conduction band edge are mainly provided by the d-orbitals of the Ni, Co, and Nb atoms. The relative localization of the d orbitals not only remarkably increased the density-of-states effective mass near the Fermi energy but also retained a high mobility, which resulted in an optimal electrical conductivity without significant reduction of the Seebeck coefficient (∼−411.08 μV K −1 at 800 K) and thereby a large power factor. Meanwhile, Ni doping still preserved a low lattice thermal conductivity. As a result, a peak zT of 1.18 was achieved at 1100 K in the compound Nb 0.8 Co 0.9 Ni 0.1 Sb with a carrier concentration of 4.35 × 10 20 cm −3 . The present work identifies that Ni doping is an effective method to improve the TE properties of nominally 19-electron NbCoSb compounds with Nb vacancies and demonstrates the great potential for searching of nonstoichiometric 19electron half-Heusler compounds with vacancies.