X-ray diffraction study of K3NdSi7O17: a new framework silicate with a linear Si-O-Si bond.
Hydrothermal investigations in the high-silica region of the K(2)O-Nd(2)O(3)-SiO(2) system, carried out in a search for novel fast-ion conductors (FICs), yielded the new compound tripotassium neodymium heptasilicate, K(3)NdSi(7)O(17). Single-crystal X-ray methods revealed that K(3)NdSi(7)O(17) crystallizes in space group P3, has lattice constants a = 16.131 (2) and c = 7.7146 (19) A, Z = 4, and 22 atoms in the asymmetric unit. Refinement was carried out to a residual, R(F), of 0.0253 and a weighted residual, wR(F(2)), of 0.0702 using anisotropic displacement parameters for all atoms. The silicate anion forms an interrupted framework, within which both Nd octahedra and K polyhedra are situated. The structure is unusual in that it contains a symmetry-constrained Si-O-Si bond angle of 180 degrees. No isomorphs to K(3)NdSi(7)O(17) are known.
Comparison of the crystal chemistry of selected MSi6O15-based silicates
Abstract The structures of four A3MSi6O15(·nH2O) silicates (A = Na or K, M = Nd or Y) recently determined by the authors are compared with one another and with the structures of related silicates. The 2:5 Si:O ratio in these compounds (silicate tetrahedra linked by sharing three O atoms per tetrahedron plus one unshared O atom) permits layer, double-chain, or doublering configurations. In α-K3NdSi6O15·2H2O, β-K3NdSi6O15, and Na3NdSi6O15·2H2O, the linkage is found to result in a corrugated layered structure, in β-Na3YSi6O15 a double-chain structure, and in α-Na3YSi6O15 a unique double-ring structure. Although the factors that govern the stabilities of ring vs. chain vs. layered structures remain to be completely elucidated, it is apparent in the layered structures that larger M cations (such as Nd and Ce) produce a greater degree of corrugation than do Zr and Ti cations. The more open structures of neodymium and cerium silicates contain large channels that may serve as pathways for fast alkali ion transport.
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