The polymorphism of 2,2-dichloropropane ((CH 3 ) 2 CCl 2 ) has been further investigated by both thermal and X-ray powder diffraction experiments. From the former the phase transitions between the different phases have been characterized at normal pressure as well as at high pressures (up to 200 MPa). From the p-T slopes of the two-phase coexistence lines, the volume variations at the transition points have been calculated and compared with those obtained by means of X-ray powder diffraction characterization. The existence of two low-temperature stable ordered phases (III and II), one high-temperature orientationally disordered phase (rhombohedral, Ib) and one additional monotropic orientationally disordered phase Ia, has been confirmed. The structure of the low-temperature ordered phase II has been determined by X-ray powder diffraction and Rietveld profile refinement as monoclinic C2/c, with lattice parameters a )10.6402(3) Å, b ) 5.4074(2) Å, c ) 10.7295(3) Å, and β ) 116.274(3)°at 175.2 K. The strength of intermolecular interactions as well as the anisotropy have been analyzed by means of the thermal-expansion tensor.

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A DTA apparatus for the temperature range −200°C to +150°C and up to 3000 bar is described. The apparatus is applied to the calorimetric investigation of the phase behaviour of pentane, hexane, heptane, octane, nonane, decane, isopentane, 2,2‐dimethylbutane, 2,3‐dimethylbutane, cyclopentane, cyclohexane, and methylcyclohexane. A new solid phase has been found in cyclohexane at elevated pressures. A strange phase behaviour is observed in 2,3‐dimethylbutane that exhibits endothermic and exothermic transitions. The well‐known odd‐even effect of the melting temperatures of the n‐alkanes is correlated with the slopes of the melting curves: For sufficiently high pressures the melting curve of an even n‐alkane intersects the melting curve of the following odd n‐alkane. Volume changes accompanying the phase transitions have been calculated using the Clausius‐Clapeyron equation.

The low frequency relaxation times tau//, which characterize the flip-flop molecular motions in liquid crystalline phases, recently determined in high-pressure experiments for eight liquid crystalline substances, were reanalyzed considering a relation proposed for the glass-forming liquids [C. Dreyfus, Phys. Rev. E 68, 011204 (2003); R. Casalini and C. M. Roland, Phys. Rev. E 69, 062501 (2004)]. The data, measured at constant pressure, constant temperature, and constant molar volume, could be rescaled onto a master line in the ln tau// vs 1/(T V(gamma)m) plot, with gamma as an adjustable parameter (Vm=1/rho is the specific volume). The obtained gamma values are in good agreement with other estimations; here, the value of gamma parameter was determined for the crystal-like smectic- E phase.

Dielectric studies of a thermotropic-liquid-crystalline substance [pentylcyanobiphenyl (5CB)] under high pressure are reported. Measurements of a complex dielectric permittivity were carried out in the pressure range 0.1 -160 MPa, the frequency range 1 kHz-13 MHz, and the temperature range 287 -343 K. Static permittivities measured at constant pressures are governed by the [2S(T)+1]/T factor. An increase in permittivity with pressure at a constant temperature is interpreted as a result of an increasing molecular ordering and of a breaking of the dipole-dipole correlations between neighboring molecules.The relaxation time~~~characterizing molecular rotations around the short axes in the nematic phase is analyzed as a function of temperature and pressure, yielding an activation enthalpy of approximately 60 kJ mol ', which decreases with increasing pressure, and an activation volume of about 60 cm' mol ', decreasing with increasing temperature. An activation energy in isochoric conditions was evaluated to be 37 kJmol ' at V =247 cm'mol ', 25 kJmol ' at 243 cm'mol '. A relation between the different activation parameters is discussed.PACS number(s): 61.30.Eb, 64.70.Md, 77.40.+i