Raman and IR spectroscopic studies of alunite-supergroup compounds containing AI, 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 AB3(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,