Study on phase stability and ionic conductivity in HfIV-substituted bismuth vanadate

Influence of dopant concentration on thermal behaviour of Bi4HfxV2−xO11−(x/2)−δ was studied over composition range 0 ≤ x ≤ 0.40 by combination of data obtained from X-ray powder diffraction, differential scanning calorimetric and conductivity measurements. For very low dopant concentrations, the system was found to mimic the parent compound in exhibiting two consecutive transitions,α ↔ β ↔ γ, with slightly different onset temperatures compared to that of parent, whereas the existence of β ↔ γ transition was well confirmed in composition range 0.15 ≤ x ≤ 0.20 and typified in Arrhenius plots to two line regions of different activation energy. For composition range 0.25 ≤ x ≤ 0.40, no significant structural changes associated with γ′ ↔ γ transition is visible in X-ray powder diffraction data with variable temperature. Despite this, the existence of γ′ ↔ γ transition was evident by some complex incommensurate modulations observed in Arrhenius plots and DSC/DTA thermograms. It was surprising to note the appearance of a new destructive γ ↔ β′ transition, resulting from distortion of the tetragonal phase structure at higher temperatures. The relationship between phase stability and ionic conductivity was also rationalized.