Structure, phase transitions and ionic conductivity of K3NdSi6O15 x xH2O. II. Structure of beta-K3NdSi6O15.

Hydrothermally grown crystals of beta-K3NdSi6O15, potassium neodymium silicate, have been studied by single-crystal X-ray methods. Under appropriate conditions, the compound crystallizes in space group Bb2(1)m and has lattice constants a = 14.370 (2), b = 15.518 (2) and c = 14.265 (2) A. There are 30 atom sites in the asymmetric unit of the basic structure. With eight formula units per unit cell, the calculated density is 2.798 Mg m(-3). Refinement was carried out to a residual, wR(F2), of 0.1177 [R(F) = 0.0416] using anisotropic temperature factors for all atoms. The structure is based on (Si2O5(2-))infinity layers, connected by Nd polyhedra to form a three-dimensional framework. Potassium ion sites, some of which are only partially occupied, are located within channels that run between the silicate layers. The silica-neodymia framework of beta-K3NdSi6O15, in particular the linkages formed between the silicate layers and Nd polyhedra, bears some similarities to that of the essentially isocompositional phase alpha-K3NdSi6O15 x 2H2O. In both, the silicate layers are corrugated so as to accommodate a simple cubic array of NdO6 octahedra with lattice constant approximately 7.5 A. Furthermore, the Si2O5 layers in beta-K3NdSi6O15 are topologically identical to those of the mineral sazhinite, Na2HCeSi6O15. Although beta-K3NdSi6O15 and sazhinite are not isostructural, the structures of each can be described as slight distortions of a high-symmetry parent structure with space group Pbmm.