High-density amorphous ice: Molecular dynamics simulations of the glass transition at 0.3 GPa

The glass transition in high-density amorphous ice

There has been a long controversy regarding the glass transition in low-density amorphous ice (LDA). The central question is whether or not it transforms to an ultraviscous liquid state above 136 K at ambient pressure prior to crystallization. Currently, the most widespread interpretation of the experimental findings is in terms of a transformation to a superstrong liquid above 136 K. In the last decade some work has also been devoted to the study of the glass transition in high-density amorphous ice (HDA) which is in the focus of the present review. At ambient pressure HDA is metastable against both ice I and LDA, whereas at > 0.2 GPa HDA is no longer metastable against LDA, but merely against high-pressure forms of crystalline ice. The first experimental observation interpreted as the glass transition of HDA was made using in situ methods by Mishima, who reported a glass transition temperature Tg of 160 K at 0.40 GPa. Soon thereafter Andersson and Inaba reported a much lower glass transition temperature of 122 K at 1.0 GPa. Based on the pressure dependence of HDA's Tg measured in Innsbruck, we suggest that they were in fact probing the distinct glass transition of very high-density amorphous ice (VHDA). Very recently the glass transition in HDA was also observed at ambient pressure at 116 K. That is, LDA and HDA show two distinct glass transitions, clearly separated by about 20 K at ambient pressure. In summary, this suggests that three glass transition lines can be defined in the p-T plane for LDA, HDA, and VHDA.

Simulation of high-density water: its glass transition for various water models

High-density amorphous water is simulated by use of isothermal-isobaric molecular dynamics at a pressure of 0.3 GPa making use of several water models (SPC/E, TIP3P, TIP4P variants, and TIP5P). Heating/cooling cycles are performed in the temperature range 80-280 K and quantities like density, total energy, and mobility are analysed. Raw data as well as the glass transition temperatures Tg observed in our studies depend on the water model used as well as on the treatment of intramolecular bonds and angles. However, a clear-cut evidence for the occurrence of a glass-to-liquid transition is found in all cases. Thus, all models indicate that high-density amorphous ice found experimentally may be a low-temperature proxy of an ultraviscous high-density liquid.

Interplay of the glass transition and the liquid-liquid phase transition in water

Water has multiple glassy states, often called amorphous ices. Low-density (LDA) and high-density (HDA) amorphous ice are separated by a dramatic, first-order like phase transition. It has been argued that the LDA-HDA transformation connects to a first-order liquid-liquid phase transition (LLPT) above the glass transition temperature T(g). Direct experimental evidence of the LLPT is challenging to obtain, since the LLPT occurs at conditions where water rapidly crystallizes. In this work, we explore the implications of a LLPT on the pressure dependence of T(g)(P) for LDA and HDA by performing computer simulations of two water models - one with a LLPT, and one without. In the absence of a LLPT, T(g)(P) for all glasses nearly coincide. When there is a LLPT, different glasses exhibit dramatically different T(g)(P) which are directly linked with the LLPT. Available experimental data for T(g)(P) are only consistent with the scenario including a LLPT.