A new calcium sulfate hemi-hydrate

Calcium sulfate hydrates receive significant attention due to numerous large scale industrial applications. There has been a long debate on the possible existence of two gypsum hemi-hydrate polymorphs, denoted alpha- and beta-CaSO4 center dot 0.5H(2)O. In this work, a new crystal structure of calcium sulfate hemi-hydrates is presented, denoted beta-CaSO4 center dot 0.5H(2)O. The structure was solved using powder neutron diffraction data, the space group is P3(1) and the unit cell in a hexagonal setting a = 6.9268(1), c = 12.7565(3) angstrom. The structure has two calcium-oxygen coordination polyhedra: Ca1 is eight coordinated and has Ca-O bond lengths in the range 2.31(3) to 2.89(2) angstrom and Ca2 is nine coordinated and has one Ca-O-water bond length of 2.43(3) angstrom, and eight Ca-O bonds in the range 2.30(4) to 2.86(4) angstrom. Two sulfate ions have S-O bonds in the range 1.47(3) to 1.49(4) angstrom, and 1.47(3) to 1.50(3) angstrom, respectively. The water molecule forms a hydrogen bond of 2.55(4) angstrom to an oxygen atom in one of the sulfate ions. The structure of the hemi-hydrate beta-CaSO4 center dot 0.5H(2)O has one-dimensional channels running parallel to the c-axis where the water molecules are located. This relates the structures of alpha- and beta-CaSO4 center dot 0.5H(2)O and soluble anhydrite AIII-CaSO4, which all have similar channel structures. The water molecules in the structure of beta-CaSO4 center dot 0.5H(2)O are packed in the channels with a three fold (3(1)) symmetry in a different way as compared to the pseudo hexagonal found in the structure of alpha-CaSO4 center dot 0.5H(2)O.