Theory of distortion-induced IR and Raman intensities of forbidden modes of molecular units in crystals. I. Polar site symmetry case

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Abstract
A quantitative theory of distortion-induced infrared and Raman intensities due to forbidden modes of molecular units at polar site symmetries in crystals has been discussed. This relates the intensity of such modes with the amount of distortion in the unit which can be evaluated easily from the crystal structural data. It is shown that even for the distortion commensurate with C1 symmetry, not all forbidden modes of the unit attain first-order or even second-order activity in all cases. An example of the IR absorbance due the v1 mode of SO42- ion in (NH4)2SO4 crystal has been discussed with a view to demonstrating the usefulness of the results of this theory in the study of phase transitions in crystals.