Composition dependence of the phase transition and ionic conductivity in BIHFVOX system

The new BIMEVOX series of Bi 4Hf x V 2− x O 11−( x/2)− δ ; 0 ≤ x ≤ 0.40 was synthesized by solid state reactions. Structural phase transitions as a function of composition were investigated by FT-IR, X-ray powder diffraction, differential scanning calorimetry and Arrhenius plots of conductivity. Two consecutive phase transitions, α ↔ β ↔ γ, were well characterized for x = 0.05 composition. The existence of β ↔ γ transition was clearly evident for relatively higher compositions 0.15 ≤ x ≤ 0.20 which is typified to two line regions of different activation energy on the Arrhenius plots. However, the existence of only γ′ ↔ γ transition was clearly confirmed for high compositions 0.25 ≤ x ≤ 0.40. AC impedance analysis also showed that the ionic conductivity is mainly due to the grain contribution which is clearly pronounced by the short-range diffusibility of oxide ion vacancies within the grains. The composition dependence of conductivity exhibited significant maximum in low temperature region ( σ 340 = 7.31 × 10 −5 S cm −1) for x = 0.25 composition.