Electronic structure of and covalency driven metal-insulator transition in BaCo1-xNixS2

We investigate the evolution of the electronic structure across the metal-to-insulator transition in the isostructural series, BaCo_1-xNi_xS₂, using electron spectroscopic studies. Our experimental results show a progressive depletion of the spectral intensity at and near the Fermi level across the series, with decreasing x. Analysis of the core-level and valence-band photoemission spectra in conjunction with detailed many-body calculations show that the ground state of the system is highly covalent. We estimate the electronic structure parameters of the system and deduce that the hopping interaction strength t_pd decreases in the solid solution with a decrease of the Ni content, while U_dd and Δ do not change appreciably. Thus, the metal-to-insulator transition in this system is found to be driven primarily by covalency effects, though the static disorder in the system is expected to play an important role in determining the details of physical properties.