Probing the local structure and phase transitions of Bi<SUB>4</SUB>V<SUB>2</SUB>O<SUB>11</SUB>-based fast ionic conductors by combined Raman and XRD studies

Abstract

In this article, we report the structural phase transitions in Bi<SUB>4</SUB>V<SUB>2</SUB>O<SUB>11</SUB> 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 Bi<SUB>4</SUB>V<SUB>2</SUB>O<SUB>11</SUB> at ambient temperature. The observed V–O bond lengths could be grouped into two categories, viz. shorter &#60;1.7 Å and longer >1.7 Å. The Raman modes of Bi<SUB>4</SUB>V<SUB>2</SUB>O<SUB>11</SUB> 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 Bi<SUB>4</SUB>V<SUB>1.8</SUB>Cu<SUB>0.2</SUB>O<SUB>10.7</SUB> 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