The electronic structures of vanadate salts: Cation substitution as a tool for band gap manipulation

The electronic structures of six ternary metal oxides containing isolated vanadate ions, Ba₃(VO₄)₂, Pb₃(VO₄)₂, YVO₄, BiVO₄, CeVO₄ and Ag₃VO₄ were studied using diffuse reflectance spectroscopy and electronic structure calculations. While the electronic structure near the Fermi level originates largely from the molecular orbitals of the vanadate ion, both experiment and theory show that the cation can strongly influence these electronic states. The observation that Ba₃(VO₄)₂ and YVO₄ have similar band gaps, both 3.8 eV, shows that cations with a noble gas configuration have little impact on the electronic structure. Band structure calculations support this hypothesis. In Pb₃(VO₄)₂ and BiVO₄ the band gap is reduced by 0.9-1.0 eV through interactions of (a) the filled cation 6s orbitals with nonbonding O 2p states at the top of the valence band, and (b) overlap of empty 6p orbitals with antibonding V 3d-O 2p states at the bottom of the conduction band. In Ag₃VO₄ mixing between filled Ag 4d and O 2p states destabilizes states at the top of the valence band leading to a large decrease in the band gap (E_g=2.2 eV). In CeVO₄ excitations from partially filled 4f orbitals into the conduction band lower the effective band gap to 1.8 eV. In the Ce_1−xBi_xVO₄ (0≤x≤0.5) and Ce_1−xY_xVO₄ (x=0.1, 0.2) solid solutions the band gap narrows slightly when Bi³⁺ or Y³⁺ are introduced. The nonlinear response of the band gap to changes in composition is a result of the localized nature of the Ce 4f orbitals.