Lanthanum- and Oxygen-Deficient Crystal Structures of Oxide–Ion Conducting Apatite-Type Silicates

International audienceThe structural change of La9.332x(SiO4)6O23x with increasing x value was investigated by means of laboratory X-ray powder diffraction (CuKa1). Crystal structures of the three types of specimens with x50, 0.06, and 0.085 were refined by the Rietveld method, with the anisotropic displacement (U) parameters being assigned for all atoms. Each crystal structure was composed of three types of polyhedra: La1O9, La2O7, and SiO4. With increasing x value, the occupancies (g) of both La1 and La2 sites decreased steadily. The La vacancies were charge balanced by the deficiency of the mobile O atoms that were not covalently bonded to Si atoms. The polyhedral volume of La2O7 decreased steadily about 0.7% with a decrease of the g(La2) value from 0.995 to 0.982. On the other hand, the polyhedral volume of La1O9 showed an increase of about 1.0%with a decrease of the g(La1) value from 0.841 to 0.818. The marked difference in deformation behavior of the two types of polyhedra could be attributable to the distinction in site occupancies between La1 [g(La1)o0.85] and La2 [g(La2)40.98]. For the mobile O atoms, the displacement magnitudes, being evaluated from the U parameters, decreased steadily with increasing x value. The positional disordering of these atom