Crystal structure investigations on cation-substituted alums by X-ray and neutron diffraction

The crystal structures of the three alums: NH4Al(SO4)2.12H2O, (NH3CH3)Al(SO4)2.12H2O and (NH3OH)Al(SO4)2.12H2O have been determined from three-dimensional neutron diffraction data enhanced by X-ray diffraction when necessary. These compounds crystallize cubic in space group Pa3. The structures of the three alums exhibit partial occupancies of crystallographic sites for the NH4, (NH3CH3) and (NH3OH) group atoms. This can be explained by a quantized rotation of the three groups around an axis perpendicular to the [111] direction. Some of the (SO4)2- groups in the NH4-alum are disordered with about 17% of the sulfate tetrahedra being in a reversed orientation around the sulfur atom. The disorder in (NH3CH3) and (NH3OH)-alums is only 4,3% and 3.0% respectively. The atoms in the alum structures are held together by a system of hydrogen bonds between the water molecules and between the water molecules and the sulfate oxygen atoms. In these three structures there is a strong indication that shorter hydrogen bonds tend to be nearly linear. (orig.)