Covalent bonding and hybridization effects in the corundum-type transition-metal oxides V₂O₃ and Ti₂O₃

The electronic structure of the corundum-type transition-metal oxides V₂O₃ and Ti₂O₃ 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 3d a_1g density of states, which is present in both compounds. The a_1g 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_1g states, which is caused by hybridization with the e_g^π bands. In contrast, the antibonding a_1g states display no such hybridization and form a sharp peak. Our results shed new light on the role of the a_1g orbitals for the metal-insulator transitions of V₂O₃. In particular, due to a_1g-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.