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-C,CI,F3. 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 P6& 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 r,,2(T) = 0.032 + 0.0061 T + 3400exp(-AE,/ k T ) cm-', with A E , = 35 kJ mol-'. Although in-plane 120" 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 A, 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" between 0 and 60 kbar. The calculated external mode frequencies agree with Raman experiment up to 60 kbar within 5 1 5 % . A molecular dynamics simulation for 300 K indicates a residence time between reorientations greater than 500 ps, giving a quasielastic Lorentzian broadening of less than 0.01 cm-'.

Lattice dynamics of the fullerenesMxC60

“…The values of A C-C for the carbon atom was taken as in previous studies on organic molecules and C 60 . 7 The value of A M -M was assumed to be zero, i.e., no short range attractive interaction was assumed between the alkali-metal atoms.…”

Simulations of structures of liquid copper under pressure

“…In recent decades the molecular dynamics simulation technique, which has been proved to be a unique way of interpreting, uncovering and forecasting atomic motions, structures, and nature, has been widely applied to the liquid state field to obtain the short-range order, thermodynamic properties and a detailed, atomistic interpretation of the correlation functions and the response spectra that are evaluated. Therein studies of liquid and the effects of high pressure on the substance structure are receiving a lot of attention from many researchers [1][2][3][4][5], but results in this research field have been insufficient up to now; moreover, a study using many-body potential is scarcely reported.…”