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

Influence of dopant concentration on thermal behaviour of Bi4HfxV2-xO11-(x/2)-delta 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,alpha <-> beta <-> gamma, with slightly different onset temperatures compared to that of parent, Whereas the existence of beta <-> gamma transition was well confirmed in composition range 0.15 <= x <= 0.20 and typified in Arihenius plots to two line regions of different activation energy For composition range 0.25 <= x <= 0.40, no significant structural changes associated with -gamma' <-> gamma transition is visible in X-ray powder diffraction data with variable temperature. Despite this, the existence of gamma' <-> gamma 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 gamma <-> beta' transition, resulting from distortion of the tetragonal phase structure at higher temperatures The relationship between phase stability and ionic conductivity was also rationalized Crown Copyright (c) 2009 Published by Elsevier B.V. All rights reserved