Studies in composition and temperature dependence of phase stability in the Bi4AgxV2-xO11-(2x)-δ system and their influence on the oxide ion performance

Samples of the system Bi4AgxV2-xO11-(2x)-delta, 0.05 <= x <= 0.30 abbreviated by the acronym, BIAGVOX were readily synthesized by the standard solid state reaction. The composition and temperature dependence of polymorphism and of oxide ion performance were investigated using differential thermal analysis, and variable temperature X-ray powder diffraction and AC impedance spectroscopy. The partial substitution of larger Ag+ ion for V5+ in the parent compound has led to the occurrence of two phase transitions; beta -> gamma and gamma ' -> gamma for compositions x <= 0.15 and x >= 0.20, respectively. delta-Hexagonal phase was also formed within the stability region of gamma ' -> gamma transition at elevated temperatures. The temperature dependence of conductivity was typically influenced by these transitions. However, the oxide ion performance of gamma '-phase increased with increasing composition at lower temperatures, while that of gamma-phase showed a maximum at higher temperatures for x = 0.20.