Structure of Na3 YSi60ts - a Unique Silicate Based on Discrete Si6015 Units, and a Possible Fast-Ion Conductor

Hydrothermal investigations in the high silica region of the Na20-Y 203-Si60 15 system, carried out in a search for novel fast-ion conductors (FIC's), yielded the new compound trisodium yttrium hexasilicate, Na3 YSi60 15 • Single-crystal X-ray methods revealed that Na3 YSi60 15 crystallizes in space group Ibmm, has lattice constants a = 10.468 (2), b = 15.2467 (13) and c = 8.3855 (6) A, 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 Si60 156double dreier-rings, rather than layers as might be expected from the Si to 0 ratio of 0.4. No isomorphs to Na3 YSi60 15 have been reported. * Author to whom correspondence should be addressed. Presently at: University of Washington, Department of Materials Science and Engineering, Seattle, WA 98102, USA. © 1995 International Union of Crystallography Printed in Great Britain all rights reserved Introduction High ionic conductivity has been observed in a number of alkali silicates. These include Na5 YS4012 (Shannon, Taylor, Gier, Chen & Berzins, 1978), Na2ZnSi04 (Frostang, Grins & Nygren, 1988), and the solid solution series Nat + xZr2SixP3 _ x012 [NASICON (Goodenough, Hong & Kafalas, 1976)]. In such materials, the alkali ion typically migrates via channels present in the silicate framework. The transport properties of alkali silicates are amenable to crystal-chemical tailoring because both the tunnel geometry and the size of the mobile ion can be altered via substitutions on the non-Si cation sites. Additionally, as with other ionic materials, the concentration of ionic defects can be modified via aliovalent substitutions, with the prospect of increasing the ionic conductivity. We were especially interested in Na3 YSi601s for a number of reasons. Firstly, we previously measured the conductivity of K3NdSi60 15 (Haile, Wuensch, Siegrist & Laudise, 1992) and found Acta Crystal/ographica Section B ISSN 0108-7681 © 1995