Phase transition associated with a soft mode of molecular libration in crystal

A simple theory is presented to account for phase transitions of molecular crystals that accompany a small entropy change and occur over a wide temperature range; for example, P2Cl10 undergoes a transition that extends from 75 to 110 °K with the entropy change of 1.67 J °K−1 · mol−1. The theory assumes that molecules gradually change their orientation with temperature at the expense of an increase in the potential energy, which is counterbalanced by an increase in entropy resulting from a change in frequency of librational modes coupled anharmonically to the uniform tilt of the molecules. The theory predicts first or second order transitions of small entropy change depending on the shape of the potential energy curve.