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 H2O, SiO2, and GeO2.