The influence of defect structure changes at phase transition on electrical properties in the Bi0.75Pr0.25O1.5 oxide ion conductor

Abstract

The structural properties of Bi0.75Pr0.25O1.5 have been investigated by powder X-ray and neutron diffraction as a function of temperature up to around 800 °C. A layered rhombohedral structure is confirmed throughout the temperature range studied, with an order-disorder type phase transition (β2 ↔ β1) at ca. 730 °C. This transition is accompanied by partial migration of the oxide ions from the fluorite-like blocks of the layered structure to interstitial sites in the van der Waals gap between blocks. A reversible step change in total conductivity, associated with the phase transition, is preceded by non-linear behaviour observed in the Arrhenius plot of total conductivity at lower temperatures. A theoretical model based on a modification of the “cube-root” model successfully describes the conductivity data through the phase transition, showing the transition to involve a jump in inter-planar conductivity correlated with the appearance of interstitial oxide ions in the van der Waals gap. While the “cube root” model has been successfully used to explain phase transitions in classic 3-dimensional ionic conductors, the present study represents the first example of application of this model to a lower dimensional system.