The structures of the alpha-alums RbCr(SO4)(2)center dot 12H(2)O and CsCr(SeO4)(2)center dot 12H(2)O at 293 and 12

The crystal structures of the alpha-alums rubidium chromium bis(sulfate) do decahydrate, RbCr(SO4)(2). 12H(2)O, and caesium chromium bis[tetraoxoselena te(VI)] dodecahydrate, CsCr(SeO4)(2). 12H(2)O, have been determined by X-ra y diffraction at 293 and 12 K. The metal atoms lie on (3) over bar sites an d the anions lie on threefold rotation axes. The accurate and extensive dat a sets lead to much more precise determinations than are available from ear lier work, particularly at 12 K. The changes in the atomic displacement par ameters between 293 and 12 K correspond to the respective predominances of intermolecular and intramolecular vibrational effects.