Pressure Induced Orientational Ordering in \rho -Terphenyl

Three different interaction potentials existing in the literature along with a model proposed here have been used to model \rho -terphenyl under standard conditions. Of these, the model that predicts the room-temperature crystal structure well has been used to understand the behavior of \rho -terphenyl under pressure. Lattice parameters show good agreement with the X-ray diffraction values reported by Puschnig et al. (Puschnig, P.; Heimel, G.; Weinmeier, K.; Resel, R.; Ambrosch-Draxl, C. High Pressure Res. 2002, 22, 105). The nonplanar structure of \rho -terphenyl transforms to a planar structure with gradual disappearance of disorder associated with ring flipping. We show that the transformation is accompanied by a change in the potential energy profile from W-shaped to a U-shaped form, which is associated with complete planarization between 1.0 and 1.5 GPa. Our results reported here are in excellent agreement with X-ray diffraction results which also suggest the existence of a similar transition as a function of pressure in polyphenyls such as biphenyl and \rho -hexaphenyl. Interestingly, the amplitude of the torsional motion is largest at an intermediate pressure of 1.0 GPa. This is attributed to the rather flat potential energy landscape which occurs during the transition from W- to U-shaped potential