The structures of the α-alums RbCr(SO4)2·12H2O and CsCr(SeO4)2·12H2O at 293 and 12 K.

The crystal structures of the α-alums rubidium chromium bis(­sulfate) dodecahydrate, RbCr(SO₄)₂·12H₂O, and caesium chromium bis[tetraoxoselenate(VI)] dodecahydrate, CsCr(SeO₄)₂·12H₂O, have been determined by X-ray diffraction at 293 and 12 K. The metal atoms lie on sites and the anions lie on threefold rotation axes. The accurate and extensive data sets lead to much more precise determinations than are available from earlier work, particularly at 12 K. The changes in the atomic displacement parameters between 293 and 12 K correspond to the respective predominances of intermolecular and intramolecular vibrational effects. [ABSTRACT FROM AUTHOR]