A neutron and X-ray diffraction investigation of aluminum chloride hexahydrate

Abstract

The structure of aluminum chloride hexahydrate, A1C13.6H20, has been studied with X-ray and neutron diffraction techniques. Precise values for the dimensions of the hexagonal unit cell have been obtained: a= 11-827 + 0.006, c= 11.895 + 0.003/~. Single-crystal X-ray data have been used to confirm the space group, R3c, and, by least-squares refinement with the intensity data, to determine more precise values for the coordinates of the oxygen atoms and the chlorine ions than had been obtained previously. The coefficients of the anisotropic temperature factors for these atoms and for the aluminum ion have been obtained and reduced to directed (r.m.s.) amplitudes of vibration. The coordinates of the atomic positions of the hydrogen atoms have been deduced from single-crystal neutron diffraction data; isotropic temperature factor coefficients for all of the atoms have also been obtained. The structure is shown to consist of chains of the type -AI(HzO)3+-3C1--AI(H20)~+-3C1 --, proceeding parallel to the hexagonal c axis. The six water molecules surrounding a central aluminum ion form an essentiaUy regular close-packed octahedron. The two hydrogen atoms associated with a given oxygen atom lie very nearly on the lines connecting the oxygen atom with the two nearest-neighbor chlorine ions, one of which is a member of the same chain as the oxygen atom, while the other is a member of a neighboring chain.