Investigations of the electronic structure of d 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 d⁰ 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 π∗ 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⁺∼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.