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 d¹⁰-orbitals of the extra interstitial Ni_i in Ni-based half-Heusler XNi_1+xZ semiconductors. The orbitals from the interstitial Ni_i have the unique d¹⁰ configuration, split into high-energy e_g⁴ orbitals and low-energy t_2g⁶ orbitals under the octahedral crystal field. In X^IVNi_1+xZ^IV (X^IV=Ti, Zr, Hf; Z^IV=Sn, Pb), the localized Ni_i-e_g⁴ states fall within the intrinsic bandgap leading to a reduced bandgap. In X^IIINi_1+xZ^V (X^III=Sc, Y; Z^V=Sb, Bi), the Ni_i-e_g⁴ 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^IIINi_1+xZ^V than X^IVNi_1+xZ^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_g⁴ 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-d¹⁰ orbitals by solid solution of both X^IVNi_1+xZ^IV and X^IIINi_1+xZ^V is expected to further optimize the electrical transport.