Structure of oxide ion-conducting lanthanum oxyapatite, La9.33(SiO4)6O2

Crystallographic space group and structural parameters of oxide ion conductor La9.33(SiO4)6O2 were investigated based on single-crystal X-ray diffraction data at room temperature (296 K). Results of structure refinements with various structure models indicated that the subjected compound has an apatite structure (space group P63/m) with no symmetry lowering and site splitting. The structure refinement was converged at R, wR, and S of 0.018, 0.026, and 0.90, respectively, with 41 parameters. Thermal displacements of the oxide ion at the hexagonal channel site (at 0,0,1/4) were extremely large along the c-axis direction. Seven-coordinated cation sites (6h La site) were basically fully occupied and cation vacancies were found at the nine-coordinated 4f La site. These results showed good coincidence with the compound's high oxide ion conductivity. Crystallographic data at 296 K: cell dimensions a = 9.714(1) Å and c = 7.183(2) Å; F(000) = 824; Z = 1; Dc = 5.464(2) g cm−3.