The dynamics of simple molecular systems showing glassy properties has been explored by dielectric spectroscopy and nuclear quadrupole resonance (NQR) on the halogenomethanes CBr2Cl2 and CBrCl3 in their low-temperature monoclinic phases. The dielectric spectra display features which correspond to alpha- and beta-relaxation processes, commonly observed in canonical glass formers. NQR experiments, also performed in the ergodic monoclinic phase of CCl4, enable the determination of the microscopic mechanism underlying the beta dynamics in these simple model glasses: Molecules that are nonequivalent with respect to their molecular environment perform reorientational jumps at different time scales. Thus our findings reveal another mechanism that can give rise to typical beta-relaxation behavior, raising some doubt about the existence of a universal explanation of this phenomenon.

The spatial extent of orientational short-range order in liquid and rotationally disordered phases of two halogen-substituted methanes are examined by means of neutron diffraction. The pair distribution functions measured within the liquids are compared to those corresponding to the rotator-phase crytals where the separation between orientational and positional correlations is enabled by the presence of time-averaged fcc lattices. The two condensed systems under scrutiny comprise a material devoid ͑CCl 4 ͒ and another ͓C͑CD 3 ͒ 3 Cl͔ having a relatively strong molecular dipole moment. Our results show orientational correlations to be confined below the second coordination sphere. Furthermore, contrary to what could be expected, angular correlations within the nonpolar material are of comparable strength to those present within the polar C͑CD 3 ͒ 3 Cl.

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The polymorphism of dibromodichloromehtane (CBr 2 Cl 2 ) 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 highpressure orientationally disordered rhombohedral phase (a R E 14.6 AE 0.3 A ˚and a E 89.2 AE 0.21). 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.

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