Variable temperature neutron diffraction study of Bi3ReO8 oxide ion conductor

The crystal structure of fluorite-related Bi3ReO8 containing isolated ReO4 tetrahedra has been studied by powder X-ray and neutron diffraction at elevated temperatures. The phase is cubic and undergoes a phase transition between T ~ 150 °C and 250 °C passing from space group P213 at 25 °C to I213 at higher temperature. The material is an oxide ion conductor and presents a sharp increase in conductivity at T ~ 450 °C, with σ ~ 8 × 10− 3 S cm− 1 above 500 °C. Two different long range oxide ion diffusion mechanisms involving vacancy and interstitial defects are proposed. Rotation of isolated ReO4 tetrahedra seem to facilitate the oxide ion conduction.