High Temperature Neutron Powder diffraction study of the Oxide Ion Conductor La_0.9Ga_0.1Mg_0.2O_2.85

Powder neutron diffraction data have been collected between room temperature and 1000 degrees C for the oxide ion conductor La0.9Sr0.1Ga0.8Mg0.2O2.85 and the undoped parent phase LaGaO3. In agreement with previous studies, refinement of the data for the undoped phase showed that the cell is orthorhombic (Pbnm) at room temperature and rhombohedral (R3c) between 250 and 1000 degrees C, The structure of the doped system La0.9Sr0.1Ga0.8 Mg0.2O2.85 is, however, significantly different from that of pure LaGaO3, The room temperature structure is monoclinic (I2/a), and there appear to be two phase transitions between 250 and 1000 degrees C: monoclinic (pseudo-orthorhombic) --> monoclinic (pseudo-rhombohedral) --> rhombohedral, Significant changes are also seen in the GaO6 octahedra on doping LaGaO3. In particular, the octahedra are substantially more distorted in the doped phase, The tilting of the octahedra is smaller for the doped phase and the degree of tilting is reduced as the temperature is raised. Possible correlation between the tilting of the octahedra and the activation energy for oxide ion conduction is presented, (C) 1998 Academic Press.