High-pressure phase transitions in molecular and plastic crystals

Abstract

The phase transitions of some molecular and plastic crystals have been studied by pVT and dielectric constant measurements up to 3000 bar. The pVT data show that the density of solid dodecane immediately below the freezing point is comparable with the density of solid undecane immediately below the rotational transition temperature. The phase diagram of acetonitrile exhibits a solid–solid transition (denoted as an α–β solid–solid transition at atmospheric pressure), the pressure dependence of which has not yet been reported. Also cyclohexanone has been studied; it exhibits three solid phases (I, II and III), one of which (II) is only observed at elevated pressures. The dielectric results for cyclohexanone are discussed in terms of the Kirkwood–Fröhlich–Onsager theory. The Kirkwood correlation g-factor is about unity both for liquid and solid (I) cyclohexanone. However, a small increase in the g-factor is observed at the freezing point and a considerable decrease at the I–II solid–solid transition.