Vibrational spectra of hexaaquacomplexes. XII. On the possible anion disorder in selenate alums: prediction of the crystal structure and vibrational spectra of KAl(SeO4)2·12H2O and related alums

摘要

A survey is given for the crystallographic and vibrational spectroscopic results of a number of alums. From both types of results (structural and spectroscopic) it is positively known that the sulfate α-alums exhibit orientational disorder of the sulfate anions along the threefold symmetry axis. Both IR and Raman spectra confirm the finding for sulfate disorder in KAl(SO 4 ) 2 ·12H 2 O. Only the Raman spectra show clearly that the sulfate anions in many K, Tl and Rb are indeed disordered, in excellent agreement with the crystallographic results [1]. The disorder depends on the nature and size of M I cations in the structure, the smaller the radius, the larger the disorder. No anion disorder has thus far been detected in selenate alums. The structure prediction of KAl(SeO 4 ) 2 ·12H 2 O allows the existence of disorder of the selenate groups. The latter seems to be corroborated by the study of the Raman spectra of selenate α alums. It is thus worthwhile to have the crystal structure of KAl(SeO 4 ) 2 ·12H 2 O refined, in order to check this prediction and as an additional check of the general explanation for the sulfate anion disorder in alums, offered earlier [1].