Characterization and ab Initio XRPD Structure Determination of a Novel Silicate with Vierer Single Chains:  The Crystal Structure of NaYSi₂O₆

The crystal structure of a sodium yttrium silicate with composition NaYSi₂O₆ has been determined from laboratory X-ray powder diffraction data by simulated annealing, and has been subsequently refined with the Rietveld technique. The compound is monoclinic with space group P2₁/c and unit cell parameters of a = 5.40787(2) Å, b = 13.69784(5) Å, c = 7.58431(3) Å, and β = 109.9140(3)° at 23.5 °C (Z = 4). The structure was found to be a single-chain silicate with a chain periodicity of four. The two symmetry dependent [Si₄O₁₂] chains in the unit cell are parallel to c. A prominent feature is the strong folding of the crankshaft-like chains within the b,c-plane resulting in intrachain Si−Si−Si angles close to 90°. The coordination of the Y³⁺ ions by O^2- is 7-fold in the form of slightly irregular pentagonal bipyramids, with oxygen atoms from four different chains contributing to the coordination polyhedron. Na⁺ ions are irregularly coordinated by 10 oxygens from two neighboring chains. No disorder of Na⁺ and Y³⁺ between the two nontetrahedral cation sites could be observed. Furthermore, micro-Raman spectra have been obtained from the polycrystalline material.