Full-shell d-orbitals of interstitial Ni and anomalous electrical transport in Ni-based half-Heusler thermoelectric semiconductors

We systematically investigate the anomalous electronic properties and electrical transport induced by full-shell d 10 -orbitals of the extra interstitial Ni i in Ni-based half-Heusler XNi 1+ x Z semiconductors. The orbitals from the interstitial Ni i have the unique d 10 configuration, split into high-energy e g 4 orbitals and low-energy t 2g 6 orbitals under the octahedral crystal field . In X IV Ni 1+ x Z IV (X IV =Ti, Zr, Hf; Z IV =Sn, Pb), the localized Ni i - e g 4 states fall within the intrinsic bandgap leading to a reduced bandgap. In X III Ni 1+ x Z V (X III =Sc, Y; Z V =Sb, Bi), the Ni i - e g 4 states overlap with the intrinsic valence bands. Additionally, the interstitial Ni i perturb the nearest neighbor X atomic coordination, leading to the splitting of degenerate conduction band minimum, which is stronger in X III Ni 1+ x Z V than X IV Ni 1+ x Z IV . Trace amounts of interstitial Ni i significantly impact the electrical transport properties. The introduction of the extra interstitial Ni i reduces the density of states effective mass, the electron group velocity , and the relaxation time, leading to a decrease of the Seebeck coefficient and electrical conductivity at low and medium temperatures. Nevertheless, the introduction of localized Ni i - e g 4 states within the bandgap as new valence band maximum attenuate the high-temperature bipolar effect at low carrier concentration intervals, thus maintaining a high thermopower at elevated temperatures. Furthermore, the tuning of the Ni i - d 10 orbitals by solid solution of both X IV Ni 1+ x Z IV and X III Ni 1+ x Z V is expected to further optimize the electrical transport.