Liquid-liquid immiscibility in single-component network-forming fluids: model calculations and implications for polyamorphism in water

Analytical calculations and Monte Carlo simulations of a lattice model of a tetravalent network-forming fluid show liquid-liuqid immiscibility at low temperatures. Three types of phase behaviour are found to occur: vapor-liquid equilibrium, vapor-liquid equilibrium plus liquid-liquid immiscibility with an upper critical temperature, and vapor-liquid equilibrium plus closed-loop liquid-liquid immiscibility. The coexisting liquids differ appreciabily in density, structure, and molecular dynamics. The low-density liquid displays more structured, open networks, and slower translational and netowrk-restructuring dyanmcis than the high-density liquid. The model provides useful insights into the moelcular basis of low-temperature immiscibility in network-forming fluids, and its relation to the unusual transition between distinct amorphous phases in vitreous water.