Can high pressure experiments shed light on the puzzles of glass transition? The problem of extrapolation

The issues of the extrapolation to multi-GPa pressures of the experimental data obtained at moderate pressures are considered for different classes of glass-forming substances. For covalent glass-forming substances, the phase transitions and structural changes are major factors that hamper extrapolation. Organic glass-forming liquids are not ground thermodynamic states of matter; under high pressures they transform to polymeric substances and then to mixtures of simple inorganic compounds. Therefore, extrapolation of the data obtained at moderate pressures is hardly possible for this class either. Metallic melts and rare gas liquids are the only substances whose properties can be extrapolated into the megabar region. However, such an extrapolation is highly uncertain due to the low viscosity and weak pressure dependences of the properties of these liquids. New experimental studies of rare gas and metallic liquids in the pressure region of tens of GPa are urgently needed for the extrapolation to be reliable.