Na8.25Y1.25Si6O18 and its family of zwölfer ring silicates

Abstract The crystal structure of a sodium yttrium silicate with nominal composition Na 9 YSi 6 O 18 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 Na 8.25 Y 1.25 Si 6 O 18 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 SiO 4 -tetrahedra, which are arranged around the 3-fold axes of the structure. It is isostructural with K 4 SrGe 3 O 9 and Na 4 CaSi 3 O 9 . 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.