(218a) Liquid Polyamorphism: From Metallic Hydrogen to Supercooled Water

â??Liquid polyamorphismâ? is the existence of two alternative amorphous structures in a single-component fluid. Liquid polyamorphism is found in a broad group of very different materials, such as triphenyl phosphite, silicon, silicon dioxide, cerium, and hydrogen, usually at extreme conditions. For example, at extremely high pressures, between 100 and 400 GPa and much above the liquid-gas critical temperature, there exists a fluid-fluid transition between metallic monoatomic hydrogen and non-metallic molecular hydrogen. This transition is projected to be terminated by a fluid-fluid critical point at about 100 GPa and 1500 K. Under another extreme condition, in deeply supercooled metastable water a liquid-liquid transition is hypothesized. Cold and supercooled water is assumed to exist in two forms, a low-density or high-density liquid. The existence of these two alternative structures could, at certain conditions, result in a metastable liquid-liquid separation in pure water. The hypothesized liquid-liquid metastable coexistence is not directly accessible in bulk-water experiments because it is presumably located a few degrees below the empirical limit of homogeneous ice formation. We consider thermodynamic constrains imposed on the existence of liquid polyamorphism in a single-component fluid. The first fundamental question to be addressed is the separation of time scales: a system with two inter-convertible fluid structures can be thermodynamically treated as a single-component fluid if the time of observation is longer than the time of conversion (fast conversion). In the opposite limit (slow conversion) the system thermodynamically behaves as a two-component mixture. The second fundamental question is the difference between the order parameters describing the liquid-gas transition and the liquid-liquid transition and the corresponding critical points. In particular, the order parameter for the liquid-gas transition is associated with density, while for the liquid-liquid transition, this is the â??reaction coordinateâ?, the fraction of conversion. The reaction coordinate is a function of temperature and pressure. The field, conjugate to the order parameter, is the logarithm of the reaction equilibrium constant. Correspondingly, while the relevant thermodynamic potential for the liquid-gas transition is the density of the Helmholtz energy, for the liquid-liquid transition in a single-component fluid, this is the Gibbs energy. The developed equation of state with a â??chemical reactionâ? between two alternative strictures generates a global fluid phase behavior with liquid-gas and liquid-liquid transitions in the same single-component fluid.