RIETVELD REFINEMENT OF THE STRUCTURES OF DRY-SYNTHESIZED MFEIIISI2O6 LEUCITES (M=K, RB, CS) BY SYNCHROTRON X-RAY-POWDER DIFFRACTION

Analyses of high-resolution synchrotron X-ray powder diffraction patterns of dry-synthesized MFe(III)Si2O6 leucites (M = K, Rb, Cs) showed that the K- and Rb-containing leucites (iron potassium silicate, KFeSi2O6, and iron rubidium silicate, RbFeSi2O6) each have a tetragonal I4(1)/a structure and the Cs-containing leucite (caesium iron silicate, CsFeSi2O6) has a cubic Ia3BARd structure. The structures of these materials have been refined by the Rietveld method. In CsFeSi2O6 leucite, Fe(III) and Si are disordered on tetrahedral framework sites as required by space-group constraints. In KFeSi2O6 leucite, Fe shows significant ordering and is concentrated on the T3 tetrahedral site. RbFeSi2O6 leucite shows less pronounced T-site ordering reflecting the fact that it is closer to the tetragonal-cubic phase transition at which the T sites become identical.