Two Phase Transitions of Octa(ethylsilsesquioxane) (C2H5SIO1.5)8 (PREPRINT)

摘要

Crystals of the ethyl substituted octa-silsesquioxane (C2H5SiO1.5)8 (ethyl POSS) undergo two phase transitions within the temperature range from 300K to 100K. At room temperature, crystalline ethyl POSS exhibits a plastic phase (phase I). When the temperature drops below about 255K, the molecular motion in the crystal slows to a rigid limit. This transition from phase I to phase II lowers the symmetry from rhombohedral to triclinic, corresponding to the change from one sixth of a molecule per asymmetric unit in the rhombohedral phase to half an independent molecule in phase II. In addition, this transition is fully reversible and is accompanied by the appearance (or disappearance when reversed) of threefold non-merohedral twinning. The second phase transition -- from phase II to phase III -- occurs around 240K and lowers the symmetry again: there are two half molecules per asymmetric unit in phase III, which is also triclinic. Even though the transition to phase III destroys the crystal, warming it to temperatures above 255K restores the plastic phase and the crystal 'heals'.