RAMAN AND IR SPECTROSCOPIC STUDIES OF ALUNITE-SUPERGROUP COMPOUNDS CONTAINING Al, Cr3+, Fe3+ AND V3+ AT THE B SITE

Raman and IR spectroscopies, X-ray diffraction and chemical analysis have been used to characterize synthetic compounds of the alunite supergroup of formula AB(3)(SO4)(2)(OH)(6), with A = Na+, K+, H3O+ and B = Al, Cr3+, Fe3+, V3+. For each A-site cation, the proportion of B-site vacancies decreases in the same order as the effective ionic radius of the B-site cations (Al > Cr > Fe > V). Raman and IR spectra of the compounds with B = Al and Fe3+ (alunite- and jarosite-group phases) show very close agreement with previously published spectra. The spectra for Cr- and V-based analogues show similarities to the alunite- and jarosite-group phases, and particularly the latter. The Raman and IR spectra are similar in the high-wavenumber ranges (around 3400-3500 cm(-1)), but are different in the intermediate and, particularly, the lower ranges, < 700 cm(-1), probably owing to the different responses of Raman and IR to SO4 and metal-O (M-O) group symmetries. The a parameter correlates well with v(1)SO(4), v(2)SO(4) and v(4)SO(4) Raman and v(4)SO(4) IR wavenumbers. There are differences in the OH-stretching regions, and shifts in the main vSO(4) and M-O peaks of the alunite-group phases, and Cr-, Fe- and V-based analogues with substitution of K, Na or H3O at the A site in the Raman and IR spectra. Raman and IR spectroscopies are therefore useful in distinguishing these compounds.