Structural change of oxide-ion-conducting lanthanum silicate on heating from 295 to 1073 K

Structural change of La9.33(SiO4)6O2 on heating from 295 to 1073 K was investigated by means of high-temperature laboratory X-ray powder diffraction (CuKα1). The apatite-type crystal structures (space group P63/m) were refined by the Rietveld method, with the anisotropic displacement parameters being assigned for all atoms. The validity of the structural models was verified by the three-dimensional electron density distribution, the structural bias of which was reduced as much as possible using the maximum-entropy methods-based pattern fitting method. Each crystal structure was composed of the three types of polyhedra, LaO9, LaO7 and SiO4. With increasing temperature the La9.33(SiO4)6O2 crystal, together with the isotypic crystal of Nd9.33(SiO4)6O2, harmoniously and steadily expanded, with the mean linear thermal expansion coefficient of 9.4(1) × 10− 6 K− 1. On the other hand, the thermal expansion behaviors of the coordination polyhedra are different, in particular, between LaO9 and NdO9. The thermal motions of O2 atoms at high temperatures showed significant difference between the two materials, which would lead to the different thermal motions of the rigid SiO4 groups. This could correlate with the high oxide-ion conductivity of La9.33(SiO4)6O2.