Crystal Structure Investigations on Cation-Substituted Alums by X-Ray and Neutron Diffraction

摘要

The crystal structures of the three alums: NH sub 4 Al(SO sub 4 ) sub 2 .12H sub 2 O, (NH sub 3 CH sub 3 )Al(SO sub 4 ) sub 2 .12H sub 2 O and (NH sub 3 OH)Al(SO sub 4 ) sub 2 .12H sub 2 O 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 NH sub 4 , (NH sub 3 CH sub 3 ) and (NH sub 3 OH) 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 (SO sub 4 ) exp 2- groups in the NH sub 4 -alum are disordered with about 17% of the sulfate tetrahedra being in a reversed orientation around the sulfur atom. The disorder in (NH sub 3 CH sub 3 ) and (NH sub 3 OH)-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. (Atomindex citation 11:564914)