Liquid-Liquid Immiscibility in Pure Fluids: Polyamorphism in Simulations of a Network-Forming Fluid

We report Monte Carlo simulations that show both vapor-liquid and closed-loop liquid-liquid equilibrium in a pure substance, for a model fluid that forms orientation-dependent intermolecular bonds. This is the first time that direct evidence of liquid-liquid immiscibility in a pure fluid has been found using exact methods. Because the model is formulated in terms of a tetrahedrally coordinated molecule, the results are relevant to a broad class of substances, particularly to the transitions between amorphous phases which are known to occur in ${\mathrm{H}}_2\mathrm{O}$, ${\mathrm{SiO}}_2$, and ${\mathrm{GeO}}_2$.