Vibrational spectra of potassium chromate/potassium sulphate mixed crystals

Abstract

The internal modes of potassium chromate and potassium sulphate were studied in a series of potassium chromate/potassium sulphate mixed crystals over the entire relative concentration range. The comparison of infrared and Raman spectra of mixed crystals with those of mixtures and pure compounds were presented and discussed. Vibrational multiplet structure in the internal modes can be classed as originating from site group effects and correlation field or factor group effects. These two effects occur simultaneously, however, they can be investigated separately in mixed crystals. Potassium chromate and potassium sulphate formed a continuous concentration range of mixed crystals and the internal vibrational modes of potassium chromate and potassium sulphate falled in different frequency regions so each vibrational unit would couple only with its own kind in the mixed crystals. In the low concentration limit of one component in the mixed crystals, the vibrational unit of that component would be vibrationally uncoupled and sensitive only to static (site group) effects. It was expected that factor group splittings would vanish in the spectra of a low concentration of potassium chromate in potassium sulphate host and in the spectra of a low concentration of potassium sulphate in potassium chromate host. The expected pattern of behavior was evident in this study. The spectral data from a pure crystal to dilution limit of mixed crystals showed that the magnitude of factor group shplitting frequency was 10 cm.-1 In the analysis of the vibrational spectra of crystals it was common to find that the site group effects were more important than factor group effects. The result in this study suggested that such conclusions should be treated with caution because the site group effects may contain the factor group effects. It was also demonstrated that hidden symmetry of crystal structure C3 not evident in the space group Pnma (D2h16), played a key role in the vibrational spectra of mixed crystals and pure compounds.