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 P3121, with a = 6.937(2), c = 6.345(1) Å 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|[unk], b′ = b, c′ = 2c, all angles 90°, Z = 12 and space group I2). This ordering is due to hydrogen bonding of H2O to the SO[unk] 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.
Zeitschrift für Kristallographie – Crystalline Materials offers a place for researchers to present results of their crystallographic studies. The journal includes theoretical as well as experimental research. It publishes Original Papers, Letters and Review Articles in manifold areas of crystallography.