Effect of Dopant Polarizability on Oxygen Sublattice Order in Phase-Stabilized Cubic Bismuth Oxides

Bismuth oxide doped with isovalent rare earth cations retains the high temperature defective fluorite structure upon cooling down to room temperature. However, these doped materials undergo an order-disorder transition of the oxygen sublattice at about 600 degrees C. When annealed at temperatures less than the transition temperature the oxygen sublattice continues to order, and consequently oxygen ion conductivity undergoes a decay. Modeling of ordered structures based on TEM diffraction patterns indicates a < 111 > vacancy ordering in the anion sublattice. Neutron diffraction studies show additional structural changes in the oxygen sublattice due to ordering. These studies indicate that the ionic conductivity is dependent on the distribution of oxygen ions between the regular 8c sites and the interstitial 32f sites in the fluorite structure. Earlier neutron diffraction studies indicate that short range ordering of the anion sublattice is related to the polarizability of the cations. In this study we relate the stability of the disordered structure and the formation of long range order to the polarizability of the dopant cations, in terms of the time constant for conductivity decay and the dielectric constant.