The crystal structure of the alums

Abstract

The literature on the structure of the alums is fairly extensive, for though the formula is complex, the problem is greatly simplified by the high symmetry of the crystal. Nevertheless, none of the proposed structures has been fully supported by X-ray measurements. The first published work was that of Vegard and Schjelderup, but, though they arrived at the correct unit cell, their structure involved an improbable arrangement of atoms, in which the identity of even the SO 4 group was lost. On this ground it was strongly criticized by Schaefer and Schubert, and Niggli showed that it was also incompatible with space-group theory. From Vegard and Schjelderup's measurements he assigned the alums to the space-group T h 2 (Pn3), but Wyckoff, by means of Laue and rotation photographs, showed that this was incorrect, and that the space-group was T h 2 (Pa3).