Charge order, orbital order, and electron localization in the Magnéli phase Ti₄O₇

The metal-insulator transition of the Magnéli phase Ti ₄O₇ is studied by means of augmented spherical wave (ASW) electronic structure calculations as based on density functional theory and the local density approximation. The results show that the metal-insulator transition arises from a complex interplay of charge order, orbital order, and singlet formation of those Ti 3d states which mediate metal-metal bonding inside the four-atom chains characteristic of the material. Ti₄O ₇ thus combines important aspects of Fe₃O₄ and VO₂. While the charge ordering closely resembles that observed at the Verwey transition, the orbital order and singlet formation appear to be identical to the mechanisms driving the metal-insulator transition of vanadium dioxide.