Structural studies on a high-pressure polymorph of NaYSi2O6

High-pressure synthesis experiments in the system Na2O-Y2O3-SiO2 revealed the existence of a previously unknown polymorph of NaYSi2O6 or Na3Y3[Si3O9](2) which was quenched from 3.0 GPa and 1000 degrees C. Structural investigations on this modification have been performed using single-crystal X-ray diffraction data collected at ambient conditions. Furthermore, unpolarized micro-Raman spectra have been obtained from single-crystal material. The high-P modification of NaYSi2O6 crystallizes in the centrosymmetric space group C2/c with 12 formula units per cell (a = 8.2131(9) angstrom, b = 10.3983(14) angstrom, c = 17.6542(21) angstrom, beta = 100.804(9)degrees, V = 1481.0(3) angstrom(3), R(vertical bar F-i) = 0.033 for 1142 independent observed reflections) and belongs to the group of cyclo-silicates. Basic building units are isolated three-membered [Si3O9] rings located in layers parallel to (010). Within a single layer the rings are concentrated in strings parallel to [100]. The sequence of directedness of up (0 or down (D) pointing tetrahedra of a single ring is UUU or DDD, respectively. Stacking of the layers parallel to b results in the formation of a three-dimensional structure in which yttrium and sodium cations are incorporated for charge compensation. In more detail, four non-tetrahedral cation positions can be differentiated which are coordinated by 6 and 8 oxygen ligands. Refinements of the site occupancies did not reveal any indication for mixed Na-Y populations on these positions. Finally, several geometrical parameters of rings occurring in cyclo-trisilicate structures have been compiled and are discussed. (c) 2007 Elsevier Inc. All rights reserved.