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Kinetic ordering of cations in solid solutions of alums showing anomalous birefringence was studied by means of single crystal X-ray diffraction. The crystal structures of two samples of K(Al 0.6 Cr 0.4 )(SO 4 ) 2  · 12 H 2 O and (K 0.45 (NH 4 ) 0.55 )Al(SO 4 ) 2  · 12 H 2 O, respectively, taken from {111} growth sectors were refined in the ideal space group  Pa -3 and in two of its subgroups – trigonal  R -3 and triclinic  P -1 with  R 1 = 0.028 – 0.032. The final choice of trigonal symmetry is based on the analysis of the diffraction patterns and of the cation distributions over the crystallographic sites. The correctness of the crystal structure refinements is supported by the disappearance of both ordering and birefringence upon crystal annealing. Also, an optically isotropic sample of (Rb 0.3 (NH 4 ) 0.7 )Al(SO 4 ) 2  · 12 H 2 O (growth sector {111}) exhibiting random cation distribution is found to have cubic symmetry (space group  Pa -3,  R 1 = 0.027).

Symmetry reduction and cation ordering in alum solid solutions