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

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 (Ti4+, Nb5+, Ta5+, Mo6+, and W6+) were examined primarily in perovskite and perovskite-related structures. In these compounds the top of the valence band is primarily oxygen 2p non-bonding in character, while the conduction band arises from the pi* interaction between the transition metal t(2g) 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: Mo6+ > W6+ > Nb5+ similar to Ti4+ > Ta5+. 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 K2NiF4 structure) does not significantly alter the band gap, whereas completely isolating the MO6 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. (C) 2003 Elsevier Inc. All rights reserved.