Metastable phases and ‘metastable’ phase diagrams

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. 1. The phase transition effect falls equally within the area of both physical and chemical studies. The notion of the "phase" of was introduced by J.W. Gibbs [1] as a condition of substance with a certain "phase", meaning a set of pulses and coordinates of the particles (atoms or molecules) that make up the substance. At a later time, the meanings of the term "phase" evolved so that currently the definitions of the phase of a substance used in physics and chemistry literature are somewhat different [1,2,3,4,5,6]. In chemistry the thermodynamic equilibrium condition is not mandatory for the phase. In physics the thermodynamic non-equilibrium is allowed in principle but with reservations of the kind as in [2] "sometimes the non-equilibrium metastable state of a substance is also called the "phase (metastable phase)". This seemingly minor distinction in the definitions accepted in physics and chemistry has a profound meaning. An incomplete understanding of the specifics of metastable molecular organic and inorganic compounds often gives rise to an erroneous interpretation of experimental data on phase transitions in these systems. In the present paper, the focus of our attention is the analysis of quasi-equilibrium and non-equilibrium phase transformations in metastable phases.The name "metastable phase" is given to a nonequilibrium state of a substance whose properties change reversibly over a period of the experiment or observation. Strictly speaking,...

Modeling of a random network of extended co solids

“…It turns out that there is a set of irreversible transitions to poly-CO structures corresponding to different applied pressures, which is consistent with the qualitative model suggested by Brazhkin in [4]. Starting from 7-8 GPa, some of the CO molecules are intact as in the δ crystal phase, but with different densities, and certain parts of the molecules form chains, four and five fold rings with O in the cage and carbonyl groups decorating the rings.…”

“…We started our simulation from construction of the δ-phase of solid CO, based on experimental information from [4]. We then compressed the unit cell at the various pressures starting from 2 GPa to 18 GPa in 1 GPa increments.…”

Multiple Amorphous–Amorphous Transitions

“…In this sense, structural transitions in amorphs have been called ''phase'' transitions, even though amorph (nonequilibrium nature) and phase (equilibrium nature) are in direct contradiction. Glasses and amorphous solids are nonergodic, nonequilibrium states, and a strict physical meaning of the ''phase'' can be applied to them only conditionally (''metastable phase'') [21]. The use of terminology reserved for true equilibrium transitions (phase, latent heat, first-order nature, nucleation, etc.)…”

The Emergence of Bioenergetics: The Formation of a Gluconeogenesis System and Reductive Pentose Phosphate Pathway of CO2 Fixation in Ancient Hydrothermal Systems

“…Thermodynamic calculations of the Gibbs standard free energy and analysis of the geochemical constraints showed that the abiotic synthesis of organic compounds in hydrothermal systems is limited by the metastable equilibrium that results from kinetic barriers, which prevent the achievement of stable equilibrium [32,[35][36][37]. Most of the organic substances in condensed and dissolved phases are in a metastable state, i.e., these substances do not reach the minimum Gibbs free energy for the given composition of elements, and thus are "kinetic" or "metastable" phases [38,39].…”