Phase stability of Bi₂(V_1-xME_x)O _5.5+δ (ME = Li and Ag, x = 0.05 and 0.1)

The phase transition of high oxide-ion conductor Bi₂(V _1-xME_x)O_{5 5+δ} (ME = Li and Ag, x = 0.05 and 0.1) and its long-term phase stability against thermal decomposition have been studied. The transition behavior was determined by differential scanning calorimetry (DSC) as well as high-temperature X-ray diffraction (HT-XRD) analysis. The observed thermal and structural changes during temperature scans reveal the temperature dependence of electrical conductivity. Li- and Ag-doping do not sufficiently suppress thermal decomposition at intermediate temperatures between 400 and 600°C In particular, partial decomposition was detected in Bi₂(V_0.9Ag_{0 1})O_5.3 during a temperature scan by HT-XRD analysis, which explains the distinct change in electrical conductivity. We have generated pseudo-binary phase diagrams for Bi₂(V_i-xME_x)O_5.5+δ based on the X-ray diffraction analysis of powders annealed for 200h and differential thermal analysis. The thermodynamically stable region of Bi₂(V _1-xME_x)O_{5 5+δ} is not sufficiently expanded by Li- and Ag-doping. At around 500°C, Bi₂(V _1-xME_x)O_5.5+δ is metastable, although it shows the highest oxide-ion conductivity among solid oxides.