DETERMINATION OF THE CRYSTAL-STRUCTURE OF CASO4(H2O)0.5 BY X-RAY-DIFFRACTION METHODS AND POTENTIAL-PROFILE CALCULATIONS

Crystals of CaSO4(H2O)0.5 show a superstructure with high pseudo symmetry. They are pseudo-merohedrically twinned with a three-fold axis as twin element. The substructure is trigonal, space group P3(1)21, with a = 6.937(2), c = 6.345(1) angstrom and Z = 3. This average structure was refined (R = 0.055) with a statistical distribution of 1.5 H2O molecules over three positions in the tunnel of an AIII-CaSO4 like matrix. With potential profile calculations a model for an ordered distribution of H2O could be derived for the superstructure (a' = a square-root 3, b' = b, c' = 2c, all angles 90-degrees, Z = 12 and space group I2). This ordering is due to hydrogen bonding of H2O to the SO4(2-) groups which is responsible for the relative stability of the hemihydrate. Considering other subhydrates CaSO4(H2O)x (x = 0.62; x = 0.81) topological models for the (reversible) dehydration of these tunnel structures are given.