Modelling of the structure and vibrational properties of LDA, HDA, and VHDA amorphous ices

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Abstract

The models of three known amorphous ice structures—low-density (LDA), high-density (HDA) and very-high-density (VHDA) amorphous ices have been obtained using sequential steps of molecular dynamics simulation and the additional optimization scheme within the lattice dynamics approach at the different cell parameters. It has been shown that this combined approach leads to a more accurate and detailed structural description of HDA and LDA ices and yields results that are in excellent agreement with experiment. The dynamic properties of obtained structures have been analyzed within the lattice dynamics approach. It has been shown that in all considered structures there exist some extended vibrational modes, which involve up to 70% of molecules. These modes apparently are a consequence of existence of hydrogen bonds network in amorphous ices.

Introduction

The study of different forms of amorphous ices is necessary for understanding the phase transitions in water at low temperatures and high pressures. The first observation of phase transition hexagonal (Ih) ice—high-density amorphous ice at 77 K, 12 GPa and phase transition high-density amorphous (HDA)—low-density amorphous (LDA) ices was made by Mishima et al. in 1984 [1], [2]. Since then many investigations have been performed to study their structures both with X-ray and neutron scattering [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13] as well as their dynamical and thermodynamic properties. In 2001 Loerting et al. had found a new form of amorphous ice, called as very-high-density amorphous ice (VHDA), using isobaric heating of HDA from 77 K up to 165 and 177 K (at 1.1 and 1.9 Gpa, respectively) [14]. Such unexpected result focused still more attention on phase transitions between different phases of water.

To understand the phase transformations of different water phases it is very important to investigate their thermodynamic properties. Thermodynamic properties of different phases of water are determined mainly by their vibrational spectrum. Vibrational part of free energy can reach 25% of the potential part of energy in some cases. Recently, we investigated the dynamical properties of LDA and HDA ices and the existence of vibrational modes with high participation ratio was shown in these ices. Here we extend our calculations for VHDA ice. We have determined the spectra of all known amorphous ice phases and analyzed them in the region of lattice vibrations.

Section snippets

Computational details

In our research we used standard SPC potential, i.e. molecules of water were considered to be rigid and interaction between water molecules consisted of van der Waals and Coulomb parts. Van der Waals interaction was considered only between oxygen atoms in the form of Lennard–Jones potential with size and energy parameters equal to 3.16 Å and 0.65 kJ/mol, respectively. Coulomb interaction was taken into account by placing point charges on oxygen and hydrogen atoms equal −0.82 e and +0.41 e,

Results and discussion

To investigate the properties of amorphous ices, series of intermediate structures starting from ice Ic toward structures with density 1.4 g/cm3 were obtained. Each structure was first uniformly compressed, then a new structure was constructed using the MD simulations and after that the structure optimization was used to find equilibrium positions of each molecule. First succession of compressions was made at temperature 77 K and so we have obtained HDA structure. Then HDA structure was

Conclusions

The molecules in VHDA, HDA and LDA are organized in highly developed networks of hydrogen bonds. In all amorphous ices molecules participate in crystal-like vibrational modes similar to that in crystal phases of ice. These modes in amorphous ices involve the majority of molecules. The appearance of localized vibrations with different degrees of localization during the amorphization is expected. The calculations of VDOS show that the dynamical stability increase in raw HDA > LDA > VHDA. These

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    It was shown that quantum corrections have a profound effect on the thermodynamic properties as well as the location of important thermodynamic points in the water phase diagram. The combination of molecular dynamics and conjugate-gradient and lattice dynamics methods was applied in conjunction with experimental data and a detailed reconstruction of the networks of hydrogen bonds of VHDA, HDA and LDA solid ices was obtained [34]. In this approach, the amorphous structure was constructed using the MD method and the final configurations were compared with experiment using paired correlation functions.

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