RIETVELD CRYSTAL-STRUCTURE REFINEMENTS, CRYSTAL-CHEMISTRY AND CALCULATED POWDER DIFFRACTION DATA FOR THE POLYMORPHS OF DICALCIUM SILICATE AND RELATED PHASES

The crystal structures of the gamma, beta, and alpha'(H) forms of dica lcium silicate (C2S) have been refined by Rietveld analysis of synchro tron X-ray data. The powder refinement of gamma-C2S, Bragg R = 4.01%, is of comparable quality to the reported single crystal refinement. Th e refinement of beta-C2S (stabilized with 0.5 wt % Cr2O3) gives a Brag g R of 3.84% and confirms the latest single crystal refinement obtaine d from twinned data. The alpha'(H)-C2S polymorph is produced when C2S is stabilized with 5 moi % Ca-3(PO4)(2), C3P; for this phase, a disord ered structure with all atoms except Si split into two half-occupied m irror-related positions refined to give a Bragg R of 3.58%. The struct ure of ''alpha-C2S'', prepared by stabilizing C2S with 10 mol% C3P, co rresponds to 6Ca(2)SiO(4) . 1Ca(3)(PO4)(2). Rietveld refinement using synchrotron X-ray powder data gives a Bragg R of 6.4% and confirms the single crystal structure which, due to twinning, was previously deter mined using superlattice reflections only. A structure for stoichiomet ric alpha-C2S is proposed, based on the high temperature form of K2SO4 . Powder diffraction patterns calculated from the crystal structure pa rameters are presented in graphical and numerical form for the C2S pha ses and related compounds. These patterns are free from the usual expe rimental uncertainties present in observed patterns and thus may be us ed to facilitate the identification and quantification of these closel y-related and complex phases in mixtures.