Metastable phases and ‘metastable’ phase diagrams

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Abstract
The work discusses the qualitative nature of phase transitions for metastable states of substances. The objects of the physics of condensed media are primarily the equilibrium states of substances with metastable phases viewed as an exception, while in chemistry the overwhelming majority of organic substances under investigation are metastable. It turns out that at normal pressure many simple molecular compounds based on light elements (these include: most hydrocarbons; nitrogen oxides, hydrates, and carbides; carbon oxide (CO); alcohols, glycerin) are metastable substances too, i.e. they do not match the Gibbs free energy minimum for a given atomic chemical composition. At moderate temperatures and pressures, the phase transitions for particular metastable phases are reversible throughout the entire experimentally accessible time period with the equilibrium thermodynamics laws obeyed. At sufficiently high pressures (1-10 GPa), most molecular phases irreversibly transform to more energy efficient polymerized phases. These transformations are not consistent with the equality of the Gibbs free energies between the phases before and after transition, i.e. they are not phase transitions in the `classical' meaning. The resulting polymeric phases at normal pressure can exist at temperatures above the melting one for an initial metastable molecular phase. Striking examples of such polymers are polyethylene and a polymerized modification of CO. Many energy-intermediate polymeric phases can apparently be synthesized by the `classical' chemistry techniques at normal pressure. At higher pressures (10-100 GPa) polymerized modifications transform to a mixture of simple stable phases.
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