Polymorphism of CBr2Cl2

The polymorphism of dibromodichloromehtane (CBr2Cl2) has been studied by means of a wide set of experimental techniques as a function of temperature and pressure. From the p - V - T diagram and the derived p - T diagram the volume variations at the transition points have been calculated and compared with those obtained by means of X-ray and neutron powder diffraction. By combining the experimental techniques, it has been demonstrated the existence of a high-pressure orientationally disordered rhombohedral phase (a(R)approximate to 14.6 +/- 0.3 angstrom and a alpha approximate to 89.2 +/- 0.2 degrees). The existence of a glass transition within the monoclinic (C2/c, Z=32) low-temperature ordered phase associated with the freezing of exchange positions between Cl and Br atoms is analyzed considering the asymmetry of the intermolecular interactions by means of the study of the thermal expansion tensor. The change of the derivative of the aspherism index, previously reported as a possible "fingerprint'' for this kind of glass transitions, is found