Investigations of the electronic structure of d0 transition metal oxides belonging to the perovskite family

Abstract

Computational and experimental studies using linear muffin tin orbital methods and UV-visible diffuse reflectance spectroscopy, respectively, were performed to quantitatively probe the relationships between composition, crystal structure and the electronic structure of oxides containing octahedrally coordinated d 0 transition metal ions. The ions investigated in this study (Ti 4+, Nb 5+, Ta 5+, Mo 6+, and W 6+) were examined primarily in perovskite and perovskite-related structures. In these compounds the top of the valence band is primarily oxygen 2 p non-bonding in character, while the conduction band arises from the π ∗ interaction between the transition metal t 2 g orbitals and oxygen. For isostructural compounds the band gap increases as the effective electronegativity of the transition metal ion decreases. The effective electronegativity decreases in the following order: Mo 6+>W 6+>Nb 5+∼Ti 4+>Ta 5+. The band gap is also sensitive to changes in the conduction band width, which is maximized for structures possessing linear M–O– M bonds, such as the cubic perovskite structure. As this bond angle decreases (e.g., via octahedral tilting distortions) the conduction band narrows and the band gap increases. Decreasing the dimensionality from 3-D (e.g., the cubic perovskite structure) to 2-D (e.g., the K 2NiF 4 structure) does not significantly alter the band gap, whereas completely isolating the MO 6 octahedra (e.g., the ordered double perovskite structure) narrows the conduction band width dramatically and leads to a significant increase in the band gap. Inductive effects due to the presence of electropositive “spectator” cations (alkali, alkaline earth, and rare-earth cations) tend to be small and can generally be neglected.