The Crystal Structures of [Al2(OH)2(H2O)8](SO4)2.2H2O and [Al2(OH)2(H2O)8](SeO4)2.2H2O.

Abstract

The crystal structures of a basic aluminium sulfate, Al2O3�2SO3�11H2O, and the isomorphous selenate have been investigated by means of X-ray crystallographic methods. The crystals are monoclinic and the space group is P21 /n. The unit cell dimensions are: a = 8.15 �, b = 12.48 �, c = 8.40 �, b = 101.9� for the sulfate and a = 8.26 �, b = 12.76 �, c = 8.48 �, b = 102.1� for the selenate. The structure contains discrete aluminium-oxygen complexes of the composition Al2O10, which are built up from two AlO4 octahedra sharing an edge. The hydrogen atoms have been located by indirect methods. Their positions indicate that the formula for the Al2O10 group should be written [Al2(OH)3(H2O)3]4+. These ions are joined by means of hydrogen bonds to the oxygen atoms of the sulfate of the selenate groups and the water molecules.