(512d) From Metallic Hydrogen to Polymeric Sulfur: Peculiar Thermodynamics of Polyamorphic Fluids

“Fluid polyamorphism” is the existence of two alternative amorphous phases in a single-component fluid. Fluid polyamorphism is found in a broad group of physically different materials, such as, silicon, silicon dioxide, sulfur, phosphorous, cerium, and hydrogen, usually at extreme conditions. In particular, this phenomenon is hypothesized in metastable, deeply supercooled water, inaccessible for direct bulk-water experiments because it is presumably located a few degrees below the empirical limit of homogeneous ice formation. We present a generic phenomenological approach, based on the Landau theory of phase transitions and the concept of the order parameter, to describe fluid polyamorphism, applicable regardless of the difference in the microscopic origin of the phenomena. To specify this approach, we consider two alternative scenarios: (a) “discrete” - a fluid with “chemical reaction” equilibrium between two competing interconvertible molecular structures and (b) “continuous” – one possible example is liquid-liquid separation caused by a “two-scale” (soft repulsion ramp) intermolecular potential. We discuss the applicability of these scenarios to dissociation/polymerization reactions (hydrogen, sulfur, phosphorous) and to the degree of tetrahedrality in fluid structures (silicon, silicon dioxide, water).