Yet another criticality of water

Abstract

A phase behavior around the transition between ice VII and a plastic phase of water is investigated by molecular dynamics simulation and the subsequent analysis on the basis of Landau theory. The prior works have predicted that the phase transition between ice VII and plastic ice is a first-order transition on the ground of a weak hysteresis and so on. A rigorous survey in the present report, however, augments their prediction with new evidence that a first-order phase transition line gives way to a second-order one at higher pressures, where a tricritical point joins these phase boundaries together. Critical phenomena are also observed whereby, other than that associated with the hypothetical critical point in the deeply supercooled state, which could influence the physical properties in a wide range of temperatures and pressures. A new critical behavior is affirmed by the result that the scaling law holds at any pressure on the second-order phase transition line for which the critical exponents are estimated. We introduce an appropriate order parameter to obtain the Landau free energy functional and the change in the functional against temperature accounts for the phase behaviors. This also enables an estimate of the coexistence and the spinodal lines at pressures below the tricritical point, all of which compensate those obtained directly by molecular dynamics simulations. These results allow us to anticipate that the critical fluctuations may give us a clue for determining the phase boundary experimentally.

The effect of polymorphism on the structural, dynamic and dielectric properties of plastic crystal water: A molecular dynamics simulation perspective

“…All these findings clearly indicate the existence of the plastic crystal phase in simulated water, a robust conclusion further supported by coherent results produced by a wide variety of detailed force-fields representations [27][28][29][30] , as well as toy models 27 . A unified and complete insight into the local structural order and related properties, together with an assessment of the effect of polymorphism upon these properties, however, has still not been provided.…”

Novel Enhanced Sampling Strategies for Transitions Between Ordered and Disordered Structures

“…9, nucleation of ice proceeds through the formation of a crystal nucleus of cubic symmetry (Ic), ending up in a final state with either a perfect cubic symmetry or made up of layers of cubic Ic and hexagonal Ih ice, consistently with experiments finding stacking-disordered ice I (Malkin et al 2012). At high pressure, simulations of the VHDA-ice VII transformation lead to observe two additional metastable configurations: a plastic phase previously proposed in Himoto et al (2014) and a new phase characterized by a tetragonal oxygen lattice and stacked layers of hydrogen-bond networks. This result illustrates how the approach does not constrain the system to sample configurations along a simple path connecting the two reference structures but rather allows it to follow complex mechanisms and discover new free-energy basins.…”

“…A classic example is the connection between the crystalline ice stable at ambient pressure (ice I) and the low-density amorphous (LDA) and high-density amorphous (HDA) ices: by compressing ice I up to 10 kbar at ≈80 K, one obtains HDA instead of ice VI (Mishima et al 1984), which may be transformed into LDA by decompression of HDA at 130 K (Klotz et al 2005) or by heating recovered HDA at ambient pressure to beyond 130 K; (Mishima and Stanley 1998) finally ice I is recovered by heating up LDA. Similar connections between crystalline and amorphous ices are found in the high-pressure region of the water phase diagram, where a very-highdensity amorphous (VHDA) ice, plastic ices, and crystalline structures with complex hydrogen-bond network (e.g., ice VII) have been observed or predicted (AmannWinkel et al 2016;Himoto et al 2014). Clearly, the experimental evolution of water structures is a complex matter, and the observed outcomes are often determined by kinetics rather than pure thermodynamics.…”

“…Despite the simplicity of its molecular unit, water is a challenging system, because of its uniquely rich polymorphism (Bartels-Rausch et al 2012), the existence of different amorphous forms (polyamorphism), as well as predicted but yet unconfirmed features, including an elusive liquid-liquid transition (Palmer et al 2014) and the possible formation of plastic phases (Himoto et al 2014). It turns out that it is possible to define a simple metric, capturing changes in the topology of the interatomic network as represented by the adjacency matrix a ij , being able to differentiate water (meta)stable structures.…”

Comment on “Spontaneous liquid-liquid phase separation of water”

“…2. This is not surprising because large translational displacements of molecules are required for the density fluctuation near the vapor-liquid critical point, whereas the conversion between low-and high-density domains can be caused by small changes in molecular orientation in supercooled water [1], As noted by Binder [12], some special treatments are required to simulate the critical phenomena by molecular dynamics simulations because correlation length and time diverge at the critical point [13]. We do not intend to observe the criticality itself in the present Reply and Ref.…”