A Temperature Dependent X-ray Study of the Order-Disorder Enantiotropic Phase Transition of p-Terphenyl

A temperature dependent (100-296 K) X-ray diffraction study of the structural changes associated with the order-disorder enantiotropic phase transition of crystalline para-terphenyl (PTP) is presented. Reflections were collected every degree through the phase transition region 183-196 K. The temperature dependent X-ray results characterize a transition from a disordered monoclinic phase (P2(1)/a, non-standard monoclinic space group) at room temperature to an ordered triclinic phase (C-1, non-standard space group) with reticular, pseudo-merohedral twinning below 193 K. The present study confirms general conclusions of prior temperature dependent neutron scattering and low temperature X-ray studies, and it makes the following new contributions: (a) quantitative assessment of the temperature dependent residual order present in the high-temperature disordered-monoclinic phase (e.g. weak temperature dependent superlattice reflections in the long-range disordered-monoclinic phase up to 296 K), (b) quantitative measure of the temperature dependent disorder present in the low-temperature ordered-triclinic phase (e.g. down to 188 K), (c) identification of the twin symmetry law (i.e. twin components related to each other by a mirror-plane perpendicular to the b-axis or a twofold rotation parallel to the b-axis of the triclinic cell), (d) the minor/major twin components are found to be temperature dependent in the low temperature ordered-triclinic phase, (e) a revised low temperature triclinic structure (C-1 non-standard space group) with reticular, pseudo-merohedral twinning is reported, with an improved R1 value of 0.039 at 100 K. The low temperature structure retains four unique PTP enantiomers, with improved torsional angles. Temperature dependent X-ray study of the structural changes associated with the order-disorder phase transition of para-terphenyl (e.g. torsion angles, order, disorder, and twinning ratio).