Infrared study of the vibrational behavior of SO 42− guest ions matrix-isolated in metal (II) chromates (Me = Ca, Sr, Ba)

Abstract

Infrared spectra of matrix-isolated SO 4 2− guest ions in host chromate matrices – CaCrO 4·2H 2O, SrCrO 4 and BaCrO 4 are reported and discussed with respect to the S O stretching and OSO bending modes. When the larger CrO 4 2− ions are replaced by the smaller SO 4 2− ions the mean frequencies of the asymmetric stretching and bending modes ( ν ¯ 3 and ν ¯ 4 ) as well as the frequencies of ν 1 of the SO 4 2− 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 SO 4 2− guest ions exhibit three bands corresponding to the ν 3 modes as deduced from the site group analysis ( C 1 site symmetry in CaCrO 4·2H 2O and SrCrO 4 and C s site symmetry in BaCrO 4). However, in the bending mode region the SO 4 2− guest ions matrix-isolated in BaCrO 4 and SrCrO 4 display a higher local symmetry than the crystallographic one (probably D 2d). An adequate measure for the SO 4 2− guest ion distortion is the site group splitting Δ ν as and Δ ν max (the difference between the highest and the lowest wavenumbered components of the S O stretching modes). The distortion of the SO 4 2− guest ions in CaCrO 4·2H 2O is stronger than that in SrCrO 4 and BaCrO 4 due probably to the smaller ionic radius of the Ca 2+ ions than those of the Sr 2+ and Ba 2+ ions, i.e. to the shorter Ca O bond distances than the respective Sr O and Ba O bond distances.