Pressure-induced structural change from low-density to high-density amorphous ice by molecular-dynamics simulations

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Abstract

We have investigated the pressure dependence of the structure of amorphous ices by molecular-dynamics simulations to clarify the characteristic features of the structural change from the low-density amorphous (LDA) ice to the high-density amorphous (HDA) ice. We have found that, with increasing pressure at the temperature of 77 K, (1) the mean-square-displacement and its fluctuation as a function of time decrease, (2) the second peak of the oxygen–oxygen (Osingle bondO) correlation function g(r) shifts to shorter distances and changes its shape drastically, e.g. it splits into two peaks, though the first peak remains almost same, (3) the average coordination number estimated from the first peak of g(r) increases from 4 to 5, (4) the broad peak of the Osingle bondOsingle bondO bond-angle distribution around 110° decreases and shifts towards 60°. From these results we conclude that the four-fold coordinated tetrahedral local structure in LDA ice changes to the five-fold coordinated structure in HDA ice, where the ‘fifth’ neighboring water molecule corresponds to the ‘interstitial’ water molecule.

Introduction

It is well known that water is a complex liquid with more than sixty anomalies [1], which have not yet been consistently explained, though some of these anomalies can be explained by the effects of hydrogen bonds and electric dipole moments of water molecules. One of the controversial problem is the existence and the features of the liquid–liquid phase transition from low-density liquid (LDL) water to high-density liquid (HDL) water in the supercooled liquid region [2]. As for frozen water, i.e. ice, anomalous features are also found. Though the pressure-induced transition from low-density amorphous (LDA) to high-density amorphous (HDA) ices was discovered by Mishima et al. [3] in 1985 and many experimental and computer simulation studies have been carried out since then, the structural properties of LDA and HDA ice and the microscopic mechanism of the transition between them are still not completely understood (see, e.g. recent review articles [4], [5]).

In this paper we investigate the pressure dependence of the structure of amorphous ice by molecular-dynamics simulations to clarify the microscopic mechanism of the pressure-induced structural change from LDA to HDA ice.

Section snippets

Method of calculation

We perform molecular-dynamics (MD) simulations of a system of 400 water (H2O) molecules confined in a cubic box under a periodic boundary condition. As intermolecular interactions between water molecules, we employ the TIP4P model potential [6], in which each water molecule is treated as a rigid rotator consisting of three point charges and an oxygen atom; two positive point charges q1=0.52e (where e is a fundamental unit of charge) are located on each hydrogen atom at a distance of 0.9572 Å

Confirmation of the amorphous state

First of all, we have confirmed that we can reproduce the LDA ice by our MD simulation. For this purpose, we carried out MD simulations at various temperatures under the ambient pressure of 0.1 MPa and calculated the g(r) and MSD. In our simulation we first reproduce the stable liquid water at 0.1 MPa and 300 K and then rapidly quench the water at a rate of 2×1014 K/s to the temperatures of 260–77 K at 0.1 MPa. In Fig. 1 we show the MSD at seven temperatures from 300 to 77 K. Since the MSD=6Dt,

Conclusion

We conclude from the pressure dependence of the mean-square-displacements, the oxygen–oxygen correlation functions and the Osingle bondOsingle bondO bond-angle distributions of amorphous ices at 77 K obtained by our molecular-dynamics simulations, that, with increasing pressure at the temperature of 77 K, the four-fold coordinated tetrahedral local structure in LDA ice changes to the five-fold coordinated structure in HDA ice. The ‘fifth’ neighboring water molecule corresponds to the so-called ‘interstitial’ water

Acknowledgements

The present work was supported in part by the Grant-in Aid for Scientific Research (C) by the Ministry of Education, Culture, Sports, Science and Technology of Japan. The results shown in this paper were calculated using the Fujitsu Materials Explorer at Kyushu University.

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