Neutron diffraction study of alums L The crystal structure of hydroxylammonium aluminium alum L The crystal structure of hydroxylammonium aluminium alum

X-Ray and neutron diffraction study of alums

The crystal structure of the alums, (NH3CH3)Al(SO4)2 · 12H2O and (NH4)Al(SO4)2 · 12H2O, has been determined and refined from X-ray and neutron diffraction data. The compounds crystallize cubic in space group Pa3 (Z = 4) with a 0 = 12.322(3) Å and a 0 = 12.242(1) Å respectively. The positional and thermal parameters of all atoms including hydrogens have been refined by full matrix least-squares analysis resulting in Rw values from the X-ray and neutron data of 0.030 and 0.029 respectively for the methylammonium alum and of 0.024 and 0.014 respectively for the ammonium alum.

The atoms of the cation groups (NH3CH3)+ and (NH4)+ are distributed on 8(c) and 24(d) positions in two orientations of equal probability on and around the [111] direction related to each other by an inversion centre. The disorder in the cation groups is explained by a quantized rotation. Disorder in the sulfate groups has been determined to 4.2% for the methylammonium alum and to about 17% for the ammonium alum. The disordered sulfate tetrahedra are distorted and in a reversed orientation along the threefold axis. The system of the hydrogen bonds is discussed.