Abstract
The orientational order/disorder phase transition of urea-paraffin inclusion compounds is discussed. Translation-rotation coupling between the orientational order of the included molecule and transverse acoustic phonons of the urea host lattice is shown to provide an indirect coupling between the orientations of adjacent paraffin molecules analogous to that found between cyanide ions in the disordered phase of alkali cyanide crystals. This facilitates the ordering phase transition. Analysis of the various contributions shows that the herringbone ordered structure is stabilized by next nearest neighbour interactions in the urea lattice rather than by direct interactions between paraffin molecules. It is suggested that the presence of translation-rotation coupling may be detected in reduced elastic constants and phonon spectra and explains the anomalous thermal expansion in the orthorhombic phase.
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