Structure, phase transitions and ionic conductivity of K3NdSi6O15 center dot xH(2)O. II. Structure of beta-K3NdSi6O15

Hydrothermally grown crystals of beta-K3NdSi6O15, potassium neodymium silic ate, have been studied by single-crystal X-ray methods. Under appropriate c onditions, 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) Angstrom. Ther e are 30 atom sites in the asymmetric unit of the basic structure. With eig ht formula units per unit cell, the calculated density is 2.798 Mg m(-3). R efinement was carried out to a residual, wR(F-2), of 0.1177 [R(F) = 0.0416] using anisotropic temperature factors for all atoms. The structure is base d on (Si2O52-)(infinity) layers, connected by Nd polyhedra to form a three- dimensional framework. Potassium ion sites, some of which are only partiall y occupied, are located within channels that run between the silicate layer s. The silica-neodymia framework of beta-K3NdSi6O15, in particular the link ages formed between the silicate layers and Nd polyhedra, bears some simila rities to that of the essentially isocompositional phase alpha-K3NdSi6O15. 2H(2)O. In both, the silicate layers are corrugated so as to accommodate a simple cubic array of NdO6 octahedra with lattice constant similar to 7.5 A ngstrom. Furthermore, the Si2O5 layers in beta-K3NdSi6O15 are topologically identical to those of the mineral sazhinite, Na2HCeSi6O15. Although beta-K 3NdSi6O15 and sazhinite are not isostructural, the structures of each can b e described as slight distortions of a high-symmetry parent structure with space group Pbmm.