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

The new BIMEVOX series of Bi4HfxV2-xO11-(x/2)-delta; 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, alpha <-> beta <-> gamma, were well characterized for x = 0.05 composition. The existence of beta <-> gamma 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 gamma' <-> gamma 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 (sigma(340) = 7.31 x 10(-5) S cm(-1)) for x = 0.25 composition. Crown Copyright (c) 2009 Published by Elsevier B.V. All rights reserved.