Phase Diagram for Supercooled Water and Liquid-Liquid Transition

Molecular dynamics simulations have been carried out at constant pressure and temperature to examine phase behaviors of supercooled water. The anomalies of supercooled water in thermodynamic response functions at atmospheric pressure, the phase transition between low and high density amorphous ices (LDA and HDA) and a fragile-strong transition are accounted for by reconciling an idea introducing a second critical point separating LDA and HDA ices with a conjecture that LDA is a different phase from a normal water, called water II. It is found that there exist large gaps around temperature 213 K in thermodynamic, structural and dynamic properties, suggesting liquid-liquid phase transition. This transition is identified as an extension of the experimentally observed LDA-HDA transition in high pressure to atmospheric pressure. In a new phase diagram, a locus of the second critical point is moved into negative pressure and the divergences are accounted for in terms of the critical point and the spinodal-like instability.