Metastable extensions of phase equilibrium lines and singular points of simple substance
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SORDER, AND PHASE TRANSITIONS IN CONDENSED SYSTEMS
Metastable Extensions of Phase Equilibrium Lines and Singular Points of Simple Substance V. G. Baidakov* and S. P. Protsenko Institute of Thermal Physics, Ural Division, Russian Academy of Sciences, ul. Amundsena 106, Yekaterinburg, 620016 Russia * e-mail: [email protected] Received July 5, 2006
Abstract—The thermodynamic properties of crystal, liquid, and gas in the stable and metastable states have been determined by molecular dynamics simulation of a system of 2048 Lennard-Jones particles. The spinodals of a superheated crystal, a superheated liquid, and a supersaturated vapor have been approximated; the spinodal for a supercooled liquid turns out to be nonexistent. The liquid–vapor, liquid–crystal, and crystal–vapor equilibrium curves and their extensions beyond the triple point have been calculated. It has been shown that, as distinct from the metastable extension of the saturation curve, which terminates at the zero isotherm, the metastable melting and sublimation curves terminate at, respectively, the stretched liquid and superheated crystal spinodals. The properties of the critical end points of metastable equilibrium of extended phases are considered. PACS numbers: 64.60.My DOI: 10.1134/S1063776106120053
1. INTRODUCTION First-order phase transitions imply the existence of metastable states [1]. In a simple one-component system, the number of coexisting phases in equilibrium can be as large as three. Each of the phases—crystal, liquid, and gas—can be metastable. Metastable phase states are the states of partial equilibrium of a thermodynamic system. The system is unstable to large-scale perturbations that lead to the formation of viable nuclei of competing phases. A superheated (stretched) crystal is unstable with respect to the formation in it of the incipient liquid phase or discontinuities; the supersaturation of the vapor phase is removed by nucleation and growth of liquid or crystal nuclei; superheated (stretched) and supercooled (supercompressed) liquids are unstable with respect to the formation of supercritical vapor and crystalline phases, respectively. Spontaneous formation of a sufficiently large amount of a new phase is highly improbable; therefore, in the absence of ready nucleation centers and external factors that initiate nucleation, a metastable system can persist for a long time. Being unstable with respect to nucleation, a metastable system retains its restoring response to infinitesimal (continuous) changes in state parameters. The boundary of essential instability (spinodal) is determined in this case by the conditions [2] ∂p –1 – ⎛ -------⎞ = ( v β T ) = 0, ⎝ ∂v ⎠ T
T ⎛ ∂T ------⎞ = ----- = 0. ⎝ ∂s ⎠ p cp
Here, p, v = 1/ρ, T, and s are the pressure, specific volume, temperature, and entropy, respectively; βT is the isothermal compressibility; and cp is the heat capacity at constant pressure. The thermodynamic stability boundary (1) corresponds to the long-wave limit of the perturbation spectrum. In most cases, entry into the metastabl
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