Knowledge Bank

The infrared and Raman spectra of single crystals of KBr, KCl and KI doped with $BeF42-$ and $CrO42-$ anions along with some divalent cations have been investigated at several temperatures. The analysis of the spectra have allowed for an identification of several site symmetries ($Cs,C2v,C3v$ and $Td$) possessed by the tetrahedral anions in the alkali halide crystals. The relative intensities of the bands assigned to various symmetry sites are explained in terms of the relative population of these sites governed by the energy-entropy competition among the same. The $CrO42-$ doped samples give intense Raman spectra, presumably due to resonance effects. This has enabled us to investigate the low-frequency bending modes of the $CrO42-$ ion in solid solution. These modes cannot be obtained in the infrared spectra due to intense absorption by the alkali halides below $400cm-1$.