Electronic structure investigation of spinel NiCo₂O₄ from quasi-particle self-consistent GW method

Quasi-particle self-consistent GW (QSGW) electronic structure calculation was performed in three spinel structures of ternary magnetic compound NiCo2O4, i.e. normal spinel and two of inverse spinel structures (Types A and B). The QSGW method is state-of-the-art on electronic structure investigation without using any empirical parameter. The results were also compared with those from the density functional theory (DFT) based on generalized gradient approximation (GGA). Half-metallicity was observed in all cases inves-tigated, where the band energy gap appears in the majority spin state. The QSGW indicated that the gap is larger than that of the corresponding GGA case. The QSGW may yield more localized 𝑑-orbitals, compared with those from the GGA. Based on the projected density of states (PDOS) analysis, Co atoms of the octahedral site [Co(oct)] were found to be a non-magnetic/weak magnetic configuration (low spin state) among them, resulting in a negligible contribution to the total magnetization. The largest contribution to the total magnetization is provided by Co atoms of tetrahedral [Co(tet)] site (high spin state). The QSGW indicated the difference of octahedral crystal fields on the different cation sites of Co(oct) and Ni(oct). The electronic states near the Fermi level consist mainly of Co(tet) 3d 𝑒g-orbitals. In the normal and inverse (type A) spinels, the Fermi level crosses the states with the DOS having a large sharp peak, implying instability of the electronic structure. The results of QSGW were discussed by comparing with those of one-shot GW and other methods.