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 AB₃(SO₄)₂(OH)₆, with A = Na⁺, K⁺, H₃O⁺ and B = Al, Cr ³⁺, 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-0 (M-O) group symmetries. The a parameter correlates well with v₁SO₄, v₂SO₄ and v₄SO₄ Raman and v₄SO₄ IR wavenumbers. There are differences in the OH-stretching regions, and shifts in the main vSO₄ and -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.