抄録
Structural change of La9.33-2x(SiO4)6O2-3x with increasing x-value was investigated by means of XRPD. Crystal structures of the three-types of specimens with x = 0, 0.06 and 0.085 were refined by the Rietveld method, with the anisotropic displacement 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 steadily decreased. 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 steadily decreased about 0.7 % with the decrease of g(La2)-value. On the other hand, the polyhedral volume of La1O9 showed an increase of about 1.0 % with the decrease of the g(La1)-value. The marked difference in deformation behavior of the two-types of polyhedra could be attributable to the distinction in site ccupancies between La1 and La2. For the mobile O atoms, the displacement magnitudes steadily decreased with increasing x-value. The positional disordering of these atoms might be related to their mobilities at high temperatures.
