Abstract

The crystal structure of a weakly anisotropic aluminum, iron(III) alum solid solution, (NH4)Al0,45Fe0,55(SO4)2·12H2O,has been studied from single-crystal X–ray diffraction. X-ray reflection pattern contains forbidden reflections of space group Pa of hk0 with h odd, 0kl with k odd, and h0l with l odd. Forbidden reflections for space group P213 of h00 with h odd, 0k0 with k odd, and 00l with l odd also present. Full set of X-ray reflections displays a reducing of the cubic space group until triclinic. Because of that, crystal structure was solved and refined in the higher symmetry space group Pa and its maximal subgroups P and P1. It is built from octahedral cation [M(H2O)6] 3+ (M = Fe3+ and Al3+), hydrated ammonium group [(NH4) + (H2O)6] and two (SO4) 2- anions which are linked to three-dimensional network through intermolecular H-bonds. In the cubic space group Pa , the M 3+ and(NH4) 2+cations are placed on the 3-fold rotation–inversion axis and distributed on 4a and 4b positions respectively. The sulfate group is statistically oriented in two orientations: the S atom and O atom from main as well as additional orientations lie on the threefold axis, i.e. on 8c position. Small fluctuations in values of the site occupancy factors for M 3+cations in both triclinic space groups are observed. The M 3+ and (NH4) + cations in the space group P (Z=4) are placed onan inversion centre of symmetry. All sulfate groups are disordered in two pseudo-reversed orientations. In non-centro symmetrically space group P1,four of the eight sulfate groups oriented only in one orientation