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

The crystal structures at room (296 K) and low (173 K) temperature of several α‐alums have been refined by single‐crystal X‐ray structure analysis. Many α‐alums of known structure are disordered, the sulfate anions occupying one of two possible sites. All those studied here exhibited such disorder and the relative occupancies of the two sites are in excellent agreement with those obtained by Raman spectroscopy, where the ν1(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.