Infrared study of the vibrational behavior of CrO42− guest ions matrix-isolated in metal (II) sulfates (Me=Ca, Sr, Ba, Pb)

Infrared spectra of matrix-isolated CrO42− guest ions in host sulfate matrices - CaSO4·2H2O, SrSO4, BaSO4 and PbSO4 are reported and discussed with respect to the Cr-O stretching and O-Cr-O bending modes. An adequate measure for the CrO42− guest ion distortion is the site group splitting Δνas and Δνmax (the difference between the highest and the lowest wavenumbered components of the stretching and bending modes). When the smaller SO42− ions are replaced by the larger CrO42− 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 CrO42− guest ions are shifted to higher wavenumbers as compared to those in the respective neat chromates due to the larger repulsion potential at the host lattice sites (smaller values of the unit-cell volumes of the neat sulfates than those of the neat chromates). The CrO42− guest ions exhibit three bands corresponding to the ν3 modes as deduced from the site group analysis (C2 site symmetry in CaSO4·2H2O and Cs site symmetry in SrSO4, BaSO4 and PbSO4). However, the bending modes ν4 and ν2 of the CrO42− guest ions in SrSO4, BaSO4 and PbSO4 show an effectively higher local symmetry than the 'rigorous' crystallographic one (two bands for ν4 and one band for ν2 instead of a triplet and a doublet expected, respectively). Such different apparent site symmetries observed in various spectral regions may be attributed to the different influence of energetic and geometrical distortions of the polyatomic entities at particular site on various modes.