High-temperature neutron scattering investigation of pure and doped lanthanum gallate

Pure LaGaO3 and the super ionic conductor La0.9Sr0.1Ga0.8Mg0.2O2.85 are structurally investigated by powder neutron diffraction at ambient temperature, 800, and 1000°C. At 25°C a distorted perovskite structure (orthorhombic, pure: Pnma, doped: Imma) is observed for both phases. The high temperature measurements result in a rhombohedral perovskite (R3c) for pure LaGaO3, whereas La0.9Sr0.1Ga0.8Mg0.2O2.85 exists in the cubic aristotype (Pm3m). The importance of neutron diffraction studies for the determination of the phase relationships in the La-Ga-Sr-Mg-O system is pointed out. From detailed analysis of the Debye-Waller factors including anharmonic contributions the diffusion pathways of the oxygen ions in La0.9Sr0.1Ga0.8Mg0.2O2.85 are obtained. The anions move near an arc of a circle above the edges of the vertex-shared GaO6-octahedra.