An undistorted SO4 group with tetrahedral symmetry has four modes of vibration (Table 4). Of these, only two modes fall in the spectral range of this study. The frequencies of these modes are 1102 and 981 cm−1 . The former is an asymmetric stretching, triply degenerate, IR-active band designated ν3SO4; the latter is a symmetric stretching, non-degenerate, Raman-active band designated ν1SO4. If the symmetry of the SO4 group is lowered, the degeneracy of the ν3SO4 mode is lifted and the mode inactive in tetrahedral symmetry (ν1SO4) becomes IR active. Therefore, each crystallographically distinct sulfate contributes 1 (single ν3) to 4 (three ν3 and one ν1) bands to these spectra depending on the site symmetry at the sulfur positions (Table 1). The symmetry apparent from the spectra depends
on the degree of distortion of the SO4 group. Even if the symmetry of SO4 group is lower than Td, the splitting may be too small and become obscured by overlap of the individual sulfate bands. The problem is further compounded by possible overlap of the S―O stretching and O―H bending vibrations.