Lattice vibrational spectra of five crystal modifications of thiourea

Abstract

Lattice vibrational spectra of the five polymorphic phases of thiourea were measured. Polarized Raman measurements were made on an oriented single crystal which allowed an unambiguous assignment of the observed Raman active lattice modes to the irreducible representations of the factor group. I.r. measurements were made on polycrystalline material; hence only a tentative assignment of the modes could be made. The observed intensities and frequencies varied almost linearly with temperature. No discontinuities in the frequencies or intensities were observed at the phase transition temperatures. These results indicate; therefore, that no large changes in intermolecular forces or crystal structure occur during these transitions. Mixing of translational and librational rigid motions is indicated by the relative Raman intensities of the A 1 g vibrations. Because of this mixing, a description of these modes in terms of either translational or librational motions could not be made.