Structure of Na₃NdSi₆O₁₅.2H₂O - a Layered Silicate with Paths for Possible Fast-Ion Conduction

Hydrothermal investigations of the high silica region of the Na₂O-Nd₂O₃-SiO₂ system, carried out in the search for new fast-ion conductors (FIC's), yielded the compound Na₃NdSi₆O₁₅·2H₂O (sodium neodymium silicate). Single-crystal X-ray methods provided lattice constants of a = 7.385 (2), b = 30.831 (7) and c = 7.1168 (13) Å, space group Cmm₂, and 22 atoms in the asymmetric unit. With four formula units per unit cell, the calculated density is 2.68 g cm^-3. Refinement was carried out with 1113 independent structure factors to a weighted residual wR(F) of 2.63% [8.09% for wR(F²)] using anisotropic temperature factors for all atoms. The structure, based on corrugated Si₆O₁₅ layers containing four-, five-, six- and eight- membered rings, is related to that of a model previously reported for a compound assigned the composition NaNdSi₆O₁₃(OH)₂·nH₂O. Our structure differs in the placement of sodium ions and water molecules, and contains no hydroxyl groups. We believe that both studies examined the same phase.

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