Infrared study of the vibrational behavior of SO42− guest ions matrix-isolated in metal (II) chromates (Me = Ca, Sr, Ba)

Infrared spectra of matrix-isolated SO42− guest ions in host chromate matrices - CaCrO4·2H2O, SrCrO4 and BaCrO4 are reported and discussed with respect to the SO stretching and OSO bending modes. When the larger CrO42− ions are replaced by the smaller SO42− ions the mean frequencies of the asymmetric stretching and bending modes (ν¯3 and ν¯4) as well as the frequencies of ν1 of the SO42− guest ions are shifted to lower wavenumbers as compared to those in the respective neat sulfates due to the smaller compression that the guest ions undergo at the host lattice sites (larger values of the unit-cell volumes of the neat chromates than those of the neat sulfates). The SO42− guest ions exhibit three bands corresponding to the ν3 modes as deduced from the site group analysis (C1 site symmetry in CaCrO4·2H2O and SrCrO4 and Cs site symmetry in BaCrO4). However, in the bending mode region the SO42− guest ions matrix-isolated in BaCrO4 and SrCrO4 display a higher local symmetry than the crystallographic one (probably D2d). An adequate measure for the SO42− guest ion distortion is the site group splitting Δνas and Δνmax (the difference between the highest and the lowest wavenumbered components of the SO stretching modes). The distortion of the SO42− guest ions in CaCrO4·2H2O is stronger than that in SrCrO4 and BaCrO4 due probably to the smaller ionic radius of the Ca2+ ions than those of the Sr2+ and Ba2+ ions, i.e. to the shorter CaO bond distances than the respective SrO and BaO bond distances.