Acta Crystallographica Section B

Structural Science

Volume 56, Part 3 (June 2000)

research papers

 

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

S. M. Haile and B. J. Wuensch

Abstract: Hydrothermally grown crystals of -K₃NdSi₆O₁₅, potassium neodymium silicate, have been studied by single-crystal X-ray methods. Under appropriate conditions, the compound crystallizes in space group Bb2₁m 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(F²), of 0.1177 [R(F) = 0.0416] using anisotropic temperature factors for all atoms. The structure is based on (Si₂O₅^2-) 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 -K₃NdSi₆O₁₅, in particular the linkages formed between the silicate layers and Nd polyhedra, bears some similarities to that of the essentially isocompositional phase -K₃NdSi₆O₁₅·2H₂O. In both, the silicate layers are corrugated so as to accommodate a simple cubic array of NdO₆ octahedra with lattice constant 7.5 Å. Furthermore, the Si₂O₅ layers in -K₃NdSi₆O₁₅ are topologically identical to those of the mineral sazhinite, Na₂HCeSi₆O₁₅. Although -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.

Keywords: Phase transitions; Ionic conductivity; Polymorphism; Silicate topology.

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