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

Abstract

Abstract The crystal structure of the hydroxylammonium aluminium sulfate dodecahydrate alum, (NH3OH)Al(SO4)2 · 12H2O, has been determined from three dimensional neutron diffraction data. The compound crystallizes cubic with a - 12.328(4) Å in the space group Pa3. The (NH3OH) + complex around ½½½, the centre of symmetry, is arranged with equal probability over two orientations related to each other by a centre of symmetry. The elastic diffraction experiment gives a space and time-average picture of two orientations obtained by a rotation of the (NH3OH)+ complex around an axis perpendicular to the N − O bond. The positional and thermal parameters have been refined by full-matrix least-squares analysis to a weighted R-value of 1.9%. Disorder in the (SO4)2− complex has been considered and only 3% disorder has been found.