Covalent bonding and hybridization effects in the corundum-type transition-metal oxides V 2 O 3 and Ti 2 O 3

The electronic structure of the corundum-type transition-metal oxides V 2 O 3 and Ti 2 O 3 is studied by means of the augmented spherical wave method, based on density-functional theory and the local density approximation. Comparing the results for the vanadate and the titanate allows us to understand the peculiar shape of the metal 3 d a 1 g density of states, which is present in both compounds. The a 1 g states are subject to pronounced bonding-antibonding splitting due to metal-metal overlap along the c -axis of the corundum structure. However, the corresponding partial density of states is strongly asymmetric with considerably more weight on the high-energy branch. We argue that this asymmetry is due to an unexpected broadening of the bonding a 1 g states, which is caused by hybridization with the e g π bands. In contrast, the antibonding a 1 g states display no such hybridization and form a sharp peak. Our results shed new light on the role of the a 1 g orbitals for the metal-insulator transitions of V 2 O 3 . In particular, due to a 1 g - e g π hybridization, an interpretation in terms of molecular orbital singlet states on the metal-metal pairs along the c -axis is not an adequate description.