Determinations of crystallographic space group and atomic arrangements in oxide-ion-conducting Nd_9.33(SiO₄)₆O₂

Abstract

In this paper we report single crystal X-ray diffraction study of an oxide-ion-conductor Nd_9.33(SiO₄)₆O₂ at 150 K. By detailed examinations of the atomic positions and an electron density distribution, it was clearly shown that Nd_9.33(SiO₄)₆O₂ has an apatite structure (space group P6₃/m) with no symmetry lowering or site-splitting. All oxygens in the hexagonal channel were located at their ideal positions (2a site at 0, 0, ¼) of the apatite structure, while their thermal displacements were highly anisotropic and that in the c-axis direction was more than four-times larger than those in other directions. Cation vacancies were located at the nine-coordinated 4f Nd site. The other Nd site (seven-coordinated 6h site) were basically fully occupied. This full occupation of the 6h Nd site makes potential waves inside the hexagonal channel smooth and transports of oxide-ions in the channel much easier. The distortion of SiO₄ tetrahedron was not apparent. Crystallographic data at 150 K: Cell dimensions a = 9.5556(6) Å and c = 7.019(2) Å; F₍₀₀₀₎ = 852; Z = 1; D_c = 5.779(2) g/cm³.