Pressure Effect on the Solid I−Liquid and Solid III−Solid I Equilibrium Forms of Cyclohexane

This paper describes the changes in equilibrium forms of cyclohexane as functions of temperature and pressure. The crystals were grown by increasing pressure using a diamond anvil cell (DAC). After single crystals were prepared in the DAC under high pressure, the crystals were maintained for a long period under constant pressure and temperature. During the changes in the shape of the single crystal, both cubic and orthorhombic shapes appeared. The cubic crystals (the solid phase I of cyclohexane) were grown from the liquid and the orthorhombic crystals (the solid phase III) were grown from the solid phase I. These crystal shape variations, depending on pressure and temperature, were consistent for the anisotropies of the cubic and orthorhombic unit cells predicted from the molecular arrangements in the unit cells. We may be able to control the morphology of the organic ring compounds strongly bound by the interaction between their ring components using pressure variation.