I'm working on molecular crystals, where there is clear linewidth broadening with increase in temperature. This kind of materials are called plastic crystals, where molecules are expected to show disorder in their plastic crystalline phase (after order-disorder) transitions. Any plausible explanation for that? Any reference with proper explanation will really be helpful. What can be the explanation for linewidth broadening for higher frequency Raman spectra?
1) It is absolutely normal in Raman spectroscopy that a lineshape changes with the temperature since the width is correlated to the relaxation time of the vibration. I don't have a reference for your kind of system, but here's one for diamond:
Article Temperature dependence of the Raman line width in diamond: Revisited
2) Since you have a molecular solid: if your molecules are interacting strongly, the bonds within them may be weakened or strengthened. That is of course associated with increasing or decreasing force constants. If you have different amounts of interaction in your system, you will have several components scattering with mild differences which, in the envelope of your spectrum, will look like broadening. With higher temperatures, different geometries may occur and therefore a larger spread of interactions.
In order for a vibrational transition to be Raman active, the molecule must undergo a change in polarizability during the vibration. Polarizability refers to the ease of distorting electrons from their original position.
Thank you Dr. Whippert for your valuable explanation, really appreciate.
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