External pressure and composition effects on the atomic and electronic structure of SnWO₄

The atomic and electronic structure of tin tungstates, α-SnWO₄, α-Sn_1.03W_0.99O₄ and β-SnWO₄, was studied by the W L₃-edge X-ray absorption spectroscopy and first-principles linear combination of atomic orbital (LCAO) calculations based on the hybrid exchange-correlation density functional (DFT)/Hartree-Fock (HF) scheme. It was found that the crystal structure of both α-phases is built up of strongly distorted WO₆ octahedra, whereas that of β-SnWO₄ is composed of nearly regular WO₄ tetrahedra. In addition, there are distorted SnO₆ octahedra in both α- and β-phases. The metal-oxygen octahedra distortion is explained by the second-order Jahn-Teller effect. The influence of pressure on the structure of α-SnWO₄ and β-SnWO₄ was studied in detail based on the calculated equations of state. The compressibility of β-SnWO₄ was found to be larger than that of α-SnWO₄. The existence of the insulator-to-metal transition was theoretically predicted in α-SnWO₄ at about 16 GPa and was explained by a symmetrization of metal-oxygen octahedra leading to a strong interaction of Sn 5s, W 5d and O 2p states and closing of band gap.