Vibrational spectra of hexaaqua complexes. X. Raman and IR studies of the sulfate group disorder in alpha-alums

Earlier X-ray diffraction and Raman spectroscopic studies indicated that th e sulfate groups in some cu-alums are orientationally disordered. The curre nt studies of the Raman and IR spectra of Various alums give evidence that the disorder is indeed present in a number of sulfate a-alums. This is best manifested through the appearance of surplus bands in the spectral regions of the symmetric (Raman) and antisymmetric (IR) stretching vibrations of t he sulfate anion, It was reconfirmed that the extent of disorder is cation- sensitive, the smaller the univalent cation, the larger the disorder. The s pectroscopic results are in an excellent agreement with the crystallographi c ones. A sulfate group disorder was detected in sulfate-doped selenate alu ms as well but no positive spectroscopic evidence was found for a selenate group disorder, again in agreement with the crystallographic results for pu re selenate alums. (C) 2001 Elsevier Science B.V. All rights reserved.