Structure of the alums. I. On the sulfate group disorder in the alpha-alums

The crystal structures at room (296 K) and low (173 K) temperature of sever al cr-alums have been refined by single-crystal X-ray structure analysis. M any cr-alums of known structure are disordered, the sulfate anions occupyin g one of two possible sites. All those studied here exhibited such disorder and the relative occupancies of the two sites are in excellent agreement w ith those obtained by Raman spectroscopy, where the nu(1)(SO4) mode is seen as a doublet owing to the presence of two different types of sulfate ion. No phase transitions were noted on cooling but there is less disorder.