Laser-Raman spectra of crystalline K2Cr04, Rb2CrO4 and Cs2CrO4

Abstract

The Raman spectra of oriented single crystals of K2CrO4 and Rb2CrO4 and of fine randomly oriented crystals of Cs2CrO4 are reported. The spectra are assigned in terms of the factor group symmetry of the crystals. The forms of the Ag internal modes are indicated by comparison of the magnitudes of the changes in the three αjj components of the polarizability tensor for K2CrO4. A trend towards lower frequency and increasing degeneracy occurs with increasing cation size for both internal and external modes. The ordering and intensities of the internal modes are similar for all compounds, and the frequencies of these modes decrease linearly with the square of the cation-chromium distances in the crystals. This relationship does not apply to the external modes. The ordering and intensities of the external modes are unique in each case, suggesting that rotatory and translatory modes have different dependencies on the mass of the cation.