COMPETITION BETWEEN VITRIFICATION AND CRYSTALLIZATION OF METHANOL AT HIGH-PRESSURE

We have studied methanol at high pressure up to 33 GPa at room temperature with x-ray diffraction, optical (polarization) microscopy, Raman spectroscopy, and detection of hydrostaticity. A competition between crystallization and vitrification is observed when methanol is superpressed beyond the freezing pressure of 3.5 GPa: between 5.0 and 10.5 GPa crystals can nucleate, but if this region is surpassed quickly enough (within a few seconds), methanol remains amorphous. For the first time the nucleation rate and the crystal growth velocity have been studied as a function of pressure. These kinetic properties can be described by classical nucleation theory in agreement with, respectively, Turnbull-Fisher and Wilson-Frenkel type behavior using one and the same activated hard-sphere diffusion coefficient. The experimental nucleation rate and the crystal growth velocity are both effectively reduced to zero above 10.5 GPa, because the diffusion is suppressed. At these pressures methanol is compressed into a glass. (C) 1995 American Institute of Physics.