Structural Study of La0.8Sr0.2Ga0.85Mg0.15O2.825

The structure of La0.8Sr0.2Ga0.85Mg0.15O2.825was studied by powder neutron diffraction and transmission electron microscopy. Neutron powder diffraction revealed cubic symmetry but electron diffraction and imaging showed the presence of local superstructures with symmetry lower than the average cubic. The thermal stability of the superstructures was investigated using high-temperature electron diffraction. One of the superstructures was related to ordering of the oxygen vacancies into planes normal to <100>c. Increasing the temperature destroys this planar ordering and eliminates the associated streaking on the electron diffraction pattern. The temperature of the disappearance of the streaking correlates with the change in conduction activation energy above 500°C. After rapid cooling in the electron microscope, the ordering is fully restored to its original form, consistent with the hypothesis that the superstructure is formed by ordering of a very mobile ion, such as oxygen, in La0.8Sr0.2Ga0.85Mg0.15O2.825.