Electronic structure study of triangular lattices in FeGa2 S4, Fe2 Ga2 S5, and NiGa2 S4: Photoemission spectroscopy and Hartree-Fock calculations

We have studied the electronic structure of the Fe and Ni triangular lattices in FeGa2 S4, Fe2 Ga2 S5, and NiGa2 S4 using photoemission spectroscopy measurements, configuration-interaction calculations on FeS6 and NiS6 cluster models, and unrestricted Hartree-Fock calculations on FeS2 and NiS2 triangular lattices. The cluster-model analysis of the Fe2p core-level spectra shows that the S3p to Fe3d charge-transfer energy Δ is ∼2.5 eV in FeGa2 S4 and Fe2 Ga2 S5, in contrast to the small Δ (∼-1 eV) found in NiGa2 S4. The relationship between the Δ value and the superexchange pathway has been examined using the unrestricted Hartree-Fock calculations. In FeGa2 S4 and Fe2 Ga2 S5, the superexchange interaction between the nearest-neighbor sites is dominant while that between the third-nearest-neighbor sites is enhanced in NiGa2 S4.