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

TL;DR

This study investigates the electronic structure and hybridization effects in V2O3 and Ti2O3, revealing how orbital hybridization influences their metal-insulator transitions and challenging simple molecular orbital models.

Contribution

It provides new insights into the role of a_{1g} and e_g^{pi} orbital hybridization in the electronic properties of corundum-type transition-metal oxides.

Findings

Pronounced bonding-antibonding splitting of a_{1g} states due to metal-metal overlap

Asymmetry and broadening of bonding a_{1g} states caused by hybridization with e_g^{pi} bands

Rejection of simple molecular orbital singlet state interpretation for metal-insulator transitions

Abstract

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^{pi} 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^{pi} hybridization, an interpretation in terms of molecular orbital singlet states on the metal-metal pairs along the c-axis is not an adequate description.