We present a spin-1, three-state Ising model for the unusual thermodynamics of fluid water. Thus, besides vacant cells, we consider singly occupied cells with two accessible volumes in such a way that the local structures of low density, energy, and entropy associated with water’s low-temperature “icelike” order are characterized. The model has two order parameters that drive two phase transitions akin to the standard gas-liquid transition and water’s hypothesized liquid-liquid transition. Its mean-field equation of state enables a satisfactory description of results from experiments and simulations for the ST2 and TIP4P/2005 force fields, from the phase diagram, the density maximum, or the deeply “stretched” states to the behavior of thermodynamic response functions at low temperatures at which water exists as a supercooled liquid. It is concluded that the model may be regarded as a most basic prototype of the so-called “two-critical-point scenario.”
REFERENCES
An alternative (but entirely equivalent) derivation starts from Eq. (5) in the form p = −U/V + TS/V + μρ, with ρ ≡ N/V the number density.
Ciach et al.31 originally reported that a waterlike spin-1 lattice model exhibits two phase transitions. Our approach differs from theirs in which we characterize the problem by allowing the individual cell volumes of the lattice to fluctuate.
Note also that spin-1 Ising models can lead to very complex phase diagrams, as it is the case of three-component systems.29,30 But recall that we are dealing with a one-component fluid.
The existence of such an optimal density was first postulated by Poole et al.19