Pressure dependence study of the chloranil phase transition by Raman scattering

The authors report detailed experimental studies of chloranil (C 6 Cl 4 O 2 ) soft-mode behaviour both at constant pressure and variable temperature, and at constant temperature and variable pressure. The phase diagram is inferred. When pressure is increased, the transition is linearly shifted to higher temperature with a slope of about 7.5K kbar -1 . This behaviour may be related to the very short distance between the carbon and oxygen atoms of adjacent molecules. Over the whole experimental pressure range the soft-mode temperature dependence is found to be in agreement with Landau theory down to a frequency of 2 cm -1 . Below this frequency the soft mode has been qualitatively observed with a Fabry-Perot interferometer up to the transition temperature.