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

The electronic structures of six tertiary metal oxides containing isolated vanadate ions, Ba-3(VO4)(2), Pb-3(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 Ba-3(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 Pb-3(VO4)(2) and BiVO4 the band gal) 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 0 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 xBixVO4 (0 <= x <= 0.5) and Ce1-xYxVO4 (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. (C) 2009 Elsevier Inc. All rights reserved.