Mechanism of order–disorder phase transition in phenanthrene crystals: an interpretation based on the rotational potential energy surface for a phenanthrene molecule in a phenanthrene crystal

Abstract

Crystals of phenanthrene undergo an order–disorder phase change at 72°C. High temperature phase was found to be disordered with two possible molecular positions. In order to understand the fundamental mechanism of the phase transition, we have modeled the molecular movement within the phenanthrene crystal and estimated the rotational potential energy surface for a phenanthrene molecule, as it undergoes this flip-flop motion. The situation in phenanthrene crystal is explained by a model where the molecules exhibit large amplitude librations having been trapped in the potential wells. Fluctuations in the orientation of the neighboring molecules result in changes in the shape of the potential well and consequently giving rise to angular motions of molecules with large amplitude. The calculations show that the two minima of the potential energy surface are not equivalent, as a result, the occupancy of the two possible molecular positions will not be the same, hence the space group of the disorder phase will remain P21 and not change to P21/a.