Soft Modes in Molecular Crystals

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Abstract
The self-consistent phonon approximation is applied to study the temperature dependence of the libron and phonon modes for simple molecular crystals. Several linear chain models are studied to illustrate the soft mode behavior of these excitations near the orientational order—disorder transitions that occur in many molecular crystals. It is shown that both libron and phonon frequencies are strongly temperature dependent in the neighborhood of the transition in qualitative agreement with experimental observations. Both the cases of strong and weak orientational coupling are considered. The effects of the cubic anharmonic terms are also discussed.