Refinement of the structure of synthetic syngenite K2Ca(SO4)2 H2O from X-ray powder diffraction data

The refinement of the crystal structure of synthetic syngenite K2Ca(SO4) center dot H2O has been carried out through the Rietveld method on X-ray powder diffraction data. Space group is P2(1)/m, a = 9.7710 (2) angstrom, b = 7.1449 (2) angstrom, c = 6.2469 (1) angstrom, beta = 103.992 (2). Conventional agreement indices are Rp = 5.26 %, wRp = 7.02 %, R-F = 3.91 %, reduced chi(2) = 1.185. The structure consists of an alternation of sheets made of columns of KO8 polyhedra linked by zig-zag chains of CaO9 polyhedra and by two independent sulfate groups S1 and S2. Both Ca and K cations coordinate the water molecule and each H forms a hydrogen-bond with O4. The Ow...O4 distance of 2.69 (1) angstrom is in good agreement with 2.75 angstrom derived from IR measurements.