Probing the local structure and phase transitions of Bi₄V₂O₁₁-based fast ionic conductors by combined Raman and XRD studies

Abstract

In this article, we report the structural phase transitions in Bi₄V₂O₁₁ as observed from temperature-dependent Raman scattering and X-ray diffraction measurements. Four different types of highly disordered coordination polyhedra around the vanadium atoms with large dispersion of V–O bond lengths are observed in Bi₄V₂O₁₁ at ambient temperature. The observed V–O bond lengths could be grouped into two categories, viz. shorter <1.7 Å and longer >1.7 Å. The Raman modes of Bi₄V₂O₁₁ could be assigned to vibration of these bonds and V–O–V linkages. We could correlate the difference in degree of anharmonicity of the phonon modes with temperature to differences in V–O bond strength. The local structure of vanadium–oxygen network in Bi₄V_1.8Cu_0.2O_10.7 was also obtained by similar studies. The effect of highly disordered anion sublattice in the doped compound is reflected in the broadening of the Raman modes.