The hydrates and deuterates of ferrous sulfate (FeSO₄): a Raman spectroscopic study

A comparative analysis has been carried out on the Raman spectra of FeSO 4 · n H 2 O ( n = 1, 4, 7) including the 2 D‐analogs. The effects of changing the degrees of hydration have been found from the lattice, SO 4 2− internal, and H 2 O internal modes. Increasing degrees of hydration shift the intense ν 1 (SO 4 ) peak to lower wavenumbers and reduce the amount of splitting on the ν 3 (SO 4 ) peaks. Some of the water librational bands cause the broadening of the ν 4 (SO 4 ) peaks in FeSO 4 ·7H 2 O and the ν 2 (SO 4 ) peaks in FeSO 4 ·7D 2 O. The ν 2 (H 2 O) band in FeSO 4 ·H 2 O is red‐shifted in excess of 100 cm −1 relative to the unperturbed H 2 O band. Between 240 and 190 K and between 140 and 90 K in the spectra of FeSO 4 .4H 2 O, two potential phase transitions have been identified from the changes in the lattice and water‐stretching regions. The resolution of the ν 1 (H 2 O) and ν 3 (H 2 O) bands in FeSO 4 ·4H 2 O and FeSO 4 ·H 2 O also improved sharply at low temperatures. The capability of distinguishing various forms of FeSO 4 hydrates unambiguously makes the Raman technique a potential analytical tool for the identification of sulfate minerals on planetary surfaces.