Structure of calcium aluminate decahydrate (CaAl2O4.10D2O) from neutron and X-ray powder diffraction data.

Calcium aluminate decahydrate is hexagonal with the space group P6(3)/m and Z = 6. The compound has been named CaAl(2)O(4).10H(2)O (CAH(10)) for decades and is known as the product obtained by hydration of CaAl(2)O(4) (CA) in the temperature region 273-288 K - one of the main components in high-alumina cements. The lattice constants depend on the water content. Several sample preparations were used in this investigation: one CAH(10), three CAD(10) and one CA(D/H)(10), where the latter is a zero-matrix sample showing no coherent scattering contribution from the D/H atoms in a neutron diffraction powder pattern. The crystal structure including the positions of the H/D atoms was determined from analyses of four neutron diffraction powder patterns by means of the ab initio crystal structure determination program FOX and the FULLPROF crystal structure refinement program. Additionally, eight X-ray powder diffraction patterns (Cu Kalpha(1) and synchrotron X-rays) were used to establish phase purity. The analyses of these combined neutron and X-ray diffraction data clearly show that the previously published positions of the O atoms in the water molecules are in error. Thermogravimetric analysis of the CAD(10) sample preparation used for the neutron diffraction studies gave the composition CaAl(2)(OD)(8)(D(2)O)(2).2.42D(2)O. Neutron and X-ray powder diffraction data gave the structural formula CaAl(2)(OX)(8)(X(2)O)(2).gammaX(2)O (X = D, H and D/H), where the gamma values are sample dependent and lie between 2.3 and 3.3.