CRYSTAL-STRUCTURE REFINEMENT OF SYNTHETIC OSUMILITE-TYPE PHASES - BAMG2AL6SI9O30, SRMG2AL6SI9O30 AND MG2AL4SI11O30

Three synthetic osumilite-type double-ring silicates of technical inte rest as crystalline basis of glass-ceramic materials with compositions BaMg2Al6Si9O30, SrMg2Al6Si9O30 and Mg2Al4Si11O(30) (MAS) were synthes ized by crystallization of glasses and their crystal structures were s ubsequently refined. The structure of BaMg2Al6Si9O30 (P6/mcc, a = 10.1 29(2), c = 14.340(2) Angstrom) was refined to R(w) = 2.46% for 1020 si ngle-crystal reflections with MoK alpha X-radiation. The structures of SrMg2Al6Si9O30 (a = 10.1273(5), c = 14.2274(8) Angstrom) and Mg2Al4Si 11O30 (a = 10.058(2), c = 14.336(3) Angstrom) were refined in space gr oup P6/mcc from CuK alpha X-ray powder data using multiphase Rietveld methods. The C site between consecutive double-ring units is occupied by Ba2+ and Sr2+ in BaMg2Al6Si9O30 and SrMg2Al6Si9O30, respectively, w hile it is empty in MAS osumilite. This leads to increased C-O2 distan ces in MAS osumilite (3.18(1) Angstrom) compared to 3.041(1) and 3.025 (9) Angstrom for the Ba- and Sr-bearing compounds, respectively. The l onger C-O2 distance is compensated by a stronger distortion of the nei ghbouring T1 double-rings and T2 tetrahedra. In BaMg2Al6Si9O30 and SrM g2Al6Si9O30, Mg is constrained to the octahedral A site whereas in MAS osumilite the A octahedron and the T2 tetrahedron have a mixed Mg, Al occupation. Osumilite-type compounds with partial Al in the six-membe red double-rings show a different T1-O-T1 angular distortion compared to double-ring silicates with pure Si12O30 units. The reason for this difference is not primarily the Al concentration on T1 but the higher ionic valences on A and T2 which become necessary for charge balance.