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.