Quantum-mechanical oxidation states of metal ions in the solid-state binary sulfides

Abstract The solid-state metal sulfides have versatile technological and industrial applications; however, a comprehensive understanding of their chemical states remains in lack mainly due to the complicated bonding behavior of sulfur. Herein I conduct the systematic first-principles DFT + U calculations on the solid-state metal (all transition metals and the early five actinide metals Th, Pa, U, Np, Pu) binary sulfides, focusing on quantitative determination of the quantum-mechanical oxidation states (OSqm) of metal ions by counting d or f orbital occupation numbers. The results show the variation of OSqm of a specific metal ion with sulfur stoichiometry and the trend of OSqm of all considered metal ions. The most remarkable aspect is that OSqm in many sulfides are not consistent with formal OS (OSf) assigned by ionic approximation, especially for the sulfides with the highest sulfur composition. After detailed analysis of OSqm in actinide sulfides, I conclude that the relatively weak bonding between metal and sulfur, the relatively strong S-S bonding interaction, and the localization/itinerancy dual nature of d and f electrons can rationally elucidate the deviation of OSqm from OSf. Such quantitative determination of OSqm in the binary sulfides is expected to offer an alternative for the further exploration of more complicated sulfides and the theoretical design of novel sulfides.