Abstract
Abstract The crystal structure of a sodium yttrium silicate with nominal composition Na9YSi6O18 was determined from laboratory X-ray powder diffraction. The actual composition of the crystals was found to be Y-enriched, and a corrected nominal composition of Na8.25Y1.25Si6O18 was established. The compound is cubic with space group Pa−3 and unit cell parameter of a = 15.120 Å. The structure consists of twelve-membered rings of corner-connected SiO4-tetrahedra, which are arranged around the 3-fold axes of the structure. It is isostructural with K4SrGe3O9 and Na4CaSi3O9. A profound disorder results from the absence of any stochiometric composition. The existence of a simultaneous occurrence of two different conformations of the rings relates to the various ordered and disordered cation positions in the structure. Micro-Raman spectra obtained from the polycrystalline material confirm the observed disorder. A model explaining the conformational variations appearing in this structure type was derived from a systematic comparison of isostructural compounds.
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