  The metal-insulator transition of the Magneli phase Ti4O7 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.
Ti4O7 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.
