Thermal Evolution of the Crystal Structure of the Rhombohedral Bi0.75Sr0.25O1.375Phase: A Single Crystal Neutron Diffraction Study

The crystal structure of the rhombohedral (R3̄m) Bi0.75Sr0.25O1.375phase has been determined at room and high temperature using single crystal neutron diffraction. It can be described at all temperatures as the regular repetition of three fluorite-like sheets shifted with respect to one another by a vector μ = 2 (a/3 + b/3 + c/3) and thus separated by an intersheet space free of anions. Interstitial anions partially occupy delocalized sites in the vicinity of the unsubstituted interface Bi-layers and are responsible for the high ionic conductivity of this phase. They are mainly located on the fluorite-like sheet side of this Bi-layer at room temperature. At high temperature, they partly migrate to the opposite side, i.e., the intersheet side, a phenomenon which results in the β2⇒ β1transition and a sudden increase of the conductivity. In light of these and previous results the different behavior of various alkaline earth-doped rhombehedral phases (Ca, Sr, Ba) and of the La-doped homologue is analyzed and rationalized.