Mahin Hemmati

Is there a second critical point in liquid water

The supercooled and stretched regions of the phase diagram of simulated liquid water are investigated by calculating the equation of state of the ST2 and TIP4P pair-potentials. We find that simulated water does not display a re-entrant spinodal and that the projection of the density maximum line in the plane of pressure and temperature becomes positively sloped on stretching. The well-known anomalous behavior of supercooled water is tentatively associated with the existence of an inaccessible critical point. Evidence is presented that suggests the association of this new critical point with the transition between low density and high density amorphous solid water. We show how the observed transformation behavior of the two forms of amorphous solid water can be explained in terms of a first order phase transition, via a consideration of the limits of metastability associated with this kind of transition, and support this interpretation with simulations of the amorphous solid. We therefore propose a phase diagram which accounts for the behavior of both liquid and amorphous solid water.

Interpretation of the molten BeF2 viscosity anomaly in terms of a high temperature density maximum, and other waterlike features

In an effort to understand the anomalous behavior of the viscosity of liquid beryllium fluoride relative to other liquids in the strong/fragile classification we have carried out ion dynamics computer simulations of BeF2 over a temperature range which overlaps with the experimental viscosity data. Using the simple rigid ion potentials which seem to be suitable for the nonpolarizable ions of this substance, we obtain diffusivity data which are in good agreement with values obtained from the experimental viscosities when converted to diffusivities using the Eyring equation for jump transport processes. The diffusivity data show a highly anomalous fragile region of behavior at temperatures just above the limits of laboratory measurement, which reconciles the observed viscosity with that of other liquids. This strongly curved region is interpreted, using the Adams–Gibbs equation, in terms of a strongly negative liquid expansivity regime associated with a large heat capacity (hence strongly temperature-depende...

IR absorption of silicate glasses studied by ion dynamics computer simulation. I. IR spectra of SiO2 glass in the rigid ion model approximation

Abstract IR spectra, calculated by the ion dynamics computer simulation method (IDCS), are presented for a variety of pair potential models of silica. A number of factors entering the determination of the reliability of the calculated spectral frequencies and line shapes are investigated. Major differences are found between characteristic IR frequencies calculated from the different pair potentials, all of which give comparable radial distribution functions. All pair models have a common failing: they predict principal IR peak separations which are too small. We relate this finding to a common failure of pair models to account for the experimental intertetrahedral bond angle. The greater sensitivity to pair potential of the IR spectrum over radial distribution function is consistent with the proposed sensitivity series: structure

Diffusivity and short-time dynamics in two models of silica

We discuss the dynamic behavior of two silica models, the BKS model (by van Beest, Kramer, and van Santen) and the WAC model (by Woodcock, Angell, and Cheeseman). Although BKS is considered the more realistic model for liquid silica, the WAC model has the unique property that it is very close to having a liquid-liquid critical point (LLCP), and this makes it particularly useful in studying the dynamics of models that do have a LLCP. We find that the diffusivity is a good indicator of how close a liquid is to criticality--the Si diffusivity shows a jump of 3-4 orders of magnitude when the pressure is reduced, which may be interpreted as an abrupt (though not first-order) transition from a high-density liquid state to a low-density liquid state. We show that this transition is captured by the Adam-Gibbs relation, which also allows us to estimate the configurational entropy of the system.

Novel Features in the Equation of State of Metastable Water

The thermodynamic properties of two commonly used water pair-potentials, ST2 and TIP4P, are calculated from molecular dynamics simulations. In particular, the properties of supercooled and stretched states are found, yielding a determination of the equation of state (EOS) in this region. A unexpected feature in the calculated EOS appears at the lowest temperatures T, in the form of an inflection in the liquid phase isotherms of pressure P versus density ρ. This EOS does not exhibit a re-entrant liquid spinodal, and also predicts that the line of density maxima in the phase diagram has a maximum in T as a function of P. These same behaviors are observed in simulations of SiO2 (using a rigid ion potential), indicating that such behavior may be a generic feature of liquids forming tetrahedral networks. The form of the calculated EOS for water suggests that a critical point may occur in the liquid state phase diagram at lower T. Simulations of the amorphous solid states of water confirm the possibility that this critical point may be the end-point of a line of first order phase transitions, itself originally proposed from experimental observations, separating low and high density amorphous ice.