Pressure and temperature studies of Raman scattering of the molecular crystal sym-C6Cl3F3

Abstract

Earlier experimental results have been considerably strengthened by Raman scattering from crystalline sym-C6Cl3F3. Measurements have been made as a function of temperature at ambient pressure, and as a function of pressure at room temperature. Apart from a new first-order phase transition observed at 62 kbar but as yet uncharacterised, the crystal structure maintains the P63/m space group symmetry. Anomalies in phonon frequencies and line-widths at 296 K indicate a phase transition, which is 'isostructural' order-disorder. Similar features are seen at 15 kbar, suggesting that the phase transition is of the same nature as that observed at 296 K. The half-width of the phonon associated with dynamic disorder is found by fitting to be Gamma 12/(T)=0.032+0.0061 T+3400 exp(- Delta Ea/kT)cm-1, with Delta Ea=35 kJ mol-1. Although in-plane 120 degrees molecular reorientational jumps may play a role, this may not be a major role at room temperature due to a rather high activation energy. The full understanding of the results requires there to be another minor activation process that also obeys Ag symmetry. The variation in the crystal structure parameters and the external mode frequencies with pressure up to 60 kbar is calculated using a 6-exp atom-atom potential with the rigid-body approximation. The molecular orientation changes by only 0.5 degrees between 0 and 60 kbar. The calculated external mode frequencies agree with Raman experiment up to 60 kbar within 5-15%. A molecular dynamics simulation for 300 K indicates a residence time between reorientations greater than 500 ps, giving a quasi-elastic Lorentzian broadening of less than 0.01 cm-1.