Structure and Bonding in SnWO4, PbWO4, and BiVO4: Lone Pairs vs Inert Pairs

Abstract

Three ternary oxides, SnWO₄, PbWO₄, and BiVO₄, containing p-block cations with ns²np⁰ electron configurations, so-called lone pair cations, have been studied theoretically using density functional theory and UV−visible diffuse reflectance spectroscopy. The computations reveal significant differences in the underlying electronic structures that are responsible for the varied crystal chemistry of the lone pair cations. The filled 5s orbitals of the Sn²⁺ ion interact strongly with the 2p orbitals of oxygen, which leads to a significant destabilization of symmetric structures (scheelite and zircon) favored by electrostatic forces. The destabilizing effect of this interaction can be significantly reduced by lowering the symmetry of the Sn²⁺ site to enable the antibonding Sn 5s−O 2p states to mix with the unfilled Sn 5p orbitals. This interaction produces a localized, nonbonding state at the top of the valence band that corresponds closely with the classical notion of a stereoactive electron lone pair. In compounds containing Pb²⁺ and Bi³⁺ the relativistic contraction of the 6s orbital reduces its interaction with oxygen, effectively diminishing its role in shaping the crystal chemical preferences of these ions. In PbWO₄ this leads to a stabilization of the symmetric scheelite structure. In the case of BiVO₄ the energy of the Bi 6s orbital is further stabilized. Despite this stabilization, the driving force for a stereoactive lone pair distortion appears to be enhanced. The energies of structures exhibiting distorted Bi³⁺ environments are competitive with structures that possess symmetric Bi³⁺ environments. Nevertheless, the “lone pair” that results associated with a distorted Bi³⁺ environment in BiVO₄ is more diffuse than the Sn²⁺ lone pair in β-SnWO₄. Furthermore, the distortion has a much smaller impact on the electronic structure near the Fermi level.