It was noted that samples which were doped with isovalent rare-earth cations retained the high-temperature defective fluorite structure after cooling to room temperature. However, the doped material underwent an order-disorder transition of the O sub-lattice at about 600C. When annealed at temperatures below the transition temperature, the O sub-lattice continued to order and the O-ion conductivity consequently decayed. The conductivity activation energies of the ordered structures, after lengthy aging at 500C, were lower than those of the structures before aging. Modelling of ordered structures, on the basis of transmission electron microscopic diffraction patterns, indicated a (111) vacancy ordering in the anion sub-lattice. Neutron diffraction studies revealed additional structural changes, in the O sub-lattice, due to ordering. These studies indicated that the ionic conductivity depended upon the distribution of O ions between the regular 8c sites and the interstitial 32f sites in the fluorite structure. A transport mechanism for O ions through interstitial positions was proposed which was based upon transmission electron microscopic and neutron diffraction studies, and the conductivity activation energies of the ordered and disordered structures.

Effect of Oxygen Sublattice Ordering on Interstitial Transport Mechanism and Conductivity Activation Energies in Phase-Stabilized Cubic Bismuth Oxides. S.Boyapati, E.D.Wachsman, N.Jiang: Solid State Ionics, 2001, 140[1-2], 149-60