Structure of Na_3NdSi_6O_(15)._2H_2O - A Layered Silicate with Paths for Possible Fast Ion Conductor

Hydrothermal investigations of the high silica region of the Na₂0-Nd₂0₃-SiO₂ system, carried out in the search for new fast-ion conductors (FIC's), yielded the compound Na₃NdSi₆O₁₅.2H₂0 (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 2.68gcm⁻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.nH₂0. 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.