Metastable phases and "metastable" phase diagrams

TL;DR

This paper explores the nature of metastable phases and phase diagrams, emphasizing their reversible transitions at moderate conditions and irreversible transformations at high pressures, with implications for polymer synthesis.

Contribution

It provides a detailed analysis of metastable phase behavior, highlighting non-classical transformations and the potential for synthesizing polymeric phases under normal conditions.

Findings

Metastable phases are common in organic compounds and can undergo reversible transitions.

High-pressure conditions induce irreversible transformations to polymeric phases.

Polymeric phases like polyethylene can be synthesized at normal pressure using classical chemistry techniques.

Abstract

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