Neutron diffraction study on lanthanum gallate perovskite compound series

The crystal structures of lanthanum gallate perovskite compounds, LaGaO₃, LaGa_0.8-Mg_0.2O_2.9, and La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8 which is well-known as a superior oxide ion conductor, have been analyzed by neutron powder diffraction to clarify the effect of the substitutions on the perovskite structures. The crystal structure of Mg²⁺-substituted LaGaO₃, LaGa_0.8-Mg_0.2O_2.9, belongs to the same orthorhombic space group, Pbnm, as that of LaGaO₃, and the oxygen vacancies were found to be created at the planar O2 atom sites of GaO₆ octahedron. On the other hand, the crystal structure of doubly Sr²⁺- and Mg²⁺-substituted LaGaO₃, La _0.8-Sr_0.2Ga_0.8Mg_0.2O_2.8, belongs to the cubic space group Pm3m and the oxygen vacancies were found to be in disorder. The distortion in the perovskite structures and the oxide ion conduction mechanism are discussed on the basis of the analytical results, together with the results of X-ray diffraction and total conductivity up to 800 °C.