Crystal structure of the solid solution NH{sub 4}Al{sub 0.62}Cr{sub 0.38}(SO{sub 4}){sub 2} · 12H{sub 2}O

The solid solution NH{sub 4}Al{sub 0.62}Cr{sub 0.38}(SO{sub 4}){sub 2} · 12H{sub 2}O was studied by X-ray diffraction. The crystal structure was determined in a series of the maximal subgroups Pa3̄ > R3̄ > P1̄ > P1. Reflections forbidden in sp. gr. Pa3̄ are indicative of symmetry lower than cubic. In the centrosymmetric models under consideration, all sulfate groups are oppositely oriented with respect to each other. In non-centrosymmetric sp. gr. P1, four of the eight sulfate groups have the same orientation, whereas the other four groups are oriented in an opposite direction.