Abstract
Phase behavior of crystals of rather simple and flexible molecules is described putting a special emphasis on a role played by intramolecular motional degrees of freedom. Freezing of intramolecular crankshaft motion of trans-azobenzene and trans-stilbene molecules results in a glass transition around 110 and 170K, respectively. Structural phase transitions of p-polyphenyl crystals are closely related to the molecular twisting degrees of freedom. Their properties are changed, by the modification of intramolecular potential curve responsible to the twisting motion, from order-disorder to displacive type rather continuously and is described consistently in terms of a unified theory of structural phase transitions. Effects of impurity that is different in the relevant intramolecular degree of freedom are investigated using p-polyphenyls as host crystals. The effect depends on the type of structural phase transition. The behavior of the order-disorder phase transition in p-terphenyl mixed crystals is consistent with the percolation theory while that of the displacive phase transition in biphenyl mixed crystals is much stronger than that predicted by the theory. The remarkable impurity effect is explained in terms of spatial localization of the soft mode.
