Unusual superexchange pathways in an NiS2 triangular lattice with negative charge-transfer energy

We have studied the electronic structure of the Ni triangular lattice in NiGa2S4 using photoemission spectroscopy and subsequent model calculations. The cluster-model analysis of the Ni 2p core-level spectrum shows that the S 3p to Ni 3d charge-transfer energy is ∼-1eV and the ground state is dominated by the d9L configuration (L is a S 3p hole). Cell perturbation analysis for the NiS2 triangular lattice indicates that the strong S 3p hole character of the ground state provides the enhanced superexchange interaction between the third-nearest-neighbor sites.