Structure, phase transitions and ionic conductivity of K_3NdSi_6O_(15)·xH_2O. II. Structure of β-K_3NdSi_6O_(15)

Abstract

Hydrothermally grown crystals of β-K_3NdSi_6O_(15), potassium neodymium silicate, have been studied by single-crystal X-ray methods. Under appropriate conditions, the compound crystallizes in space group Bb2_1m and has lattice constants a = 14.370 (2), b = 15.518 (2) and c = 14.265 (2) Å. 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(F^2), of 0.1177 [R(F) = 0.0416] using anisotropic temperature factors for all atoms. The structure is based on (Si_2O_5^(2-))∞ 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 β-K_3NdSi_6O_(15), in particular the linkages formed between the silicate layers and Nd polyhedra, bears some similarities to that of the essentially isocompositional phase α-K_3NdSi_6O_(15)·2H_2O. In both, the silicate layers are corrugated so as to accommodate a simple cubic array of NdO_6 octahedra with lattice constant 7.5 Å. Furthermore, the Si_2O_5 layers in β-K_3NdSi_6O_(15) are topologically identical to those of the mineral sazhinite, Na_2HCeSi_6O_(15). Although β-K_3NdSi_6O_(15) 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.

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