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

Abstract

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 Cmm 2, 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(F 2)] 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.