High-Temperature Powder Neutron Diffraction Study of the Oxide Ion Conductor La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85

Powder neutron diffraction data have been collected between room temperature and 1000°C for the oxide ion conductor La_0.9Sr_0.1Ga_0.8Mg_0.2O _2.85and 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°C. The structure of the doped system La_0.9Sr_0.1Ga_0.8Mg_0.2O _2.85is, 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°C: monoclinic (pseudo-orthorhombic)→monoclinic (pseudo-rhombohedral)→rhombohedral. Significant changes are also seen in the GaO₆octahedra 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.