STRUCTURE OF NA3YSI6O15 - A UNIQUE SILICATE BASED ON DISCRETE SI6O15 UNITS, AND A POSSIBLE FAST-ION CONDUCTOR

Hydrothermal investigations in the high silica region of the Na2O-Y2O3-Si6O15 system, carried out in a search for novel fast-ion conductors (FIC's), yielded the new compound trisodium yttrium hexasilicate, Na3YSi6O15. Single-crystal X-ray methods revealed that Na3YSi6O15 crystallizes in space group Ibmm, has lattice constants a= 10.468(2), b = 15.2467(13) and c = 8.3855(6) Angstrom, Z = 4, and 11 atoms in the asymmetric unit. Refinement was carried out to a weighted residual of 3.53% using anisotropic temperature factors for all atoms. The structure is unique in that the silica tetrahedra form isolated Si6O156- double dreier-rings, rather than layers as might be expected from the Si to O ratio of 0.4. No isomorphs to Na3YSi6015 have been reported.