Electronic structure of rare-earth nickel pnictides: Narrow-gap thermoelectric materials

We have studied the electronic structure of a class of half-Heusler compounds $M\mathrm{NiPn},$ where M is Y, La, Lu, Yb, and Pn is a pnicogen As, Sb, Bi. All these systems excepting Yb are narrow-gap semiconductors and are potential candidates for high-performance thermoelectric materials. The Yb system shows heavy fermion characteristics. Calculations were carried out within density-functional theory (generalized gradient approximation) using self-consistent full-potential linearized augmented plane-wave method. Comparison of the electronic structures of isoelectronic systems YNiSb and ZrNiSn, another narrow-gap semiconductor, brings out the role of hybridization on the energy gap formation. We also find that in YNiPn systems, the gap narrows as we go from As to Bi, a result of relativistic lowering of the Pn valence s band and its influence on the lowest conduction band. Our band-structure results for YbNiSb differs drastically from a previous calculation using a different method, but agrees closely with a similar mixed valence system YbPtBi.