Pressure-induced transformations in computer simulations of glassy water

Abstract

Glassy water occurs in at least two broad categories: low-density amorphous (LDA) and highdensity amorphous (HDA) solid water. We perform out-of-equilibrium molecular dynamics simulations to study the transformations of glassy water using the ST2 model. Specifically, we study the known (i) compression-induced LDA-to-HDA, (ii) decompression-induced HDA-to-LDA, and (iii) compression-induced hexagonal ice-to-HDA transformations. We study each transformation for a broad range of compression/decompression temperatures, enabling us to construct a "P-T phase diagram" for glassy water. The resulting phase diagram shows the same qualitative features reported from experiments. While many simulations have probed the liquid-state phase behavior, comparatively little work has examined the transitions of glassy water. We examine how the glass transformations relate to the (first-order) liquid-liquid phase transition previously reported for this model. Specifically, our results support the hypothesis that the liquid-liquid spinodal lines, between a lowdensity and high-density liquid, are extensions of the LDA-HDA transformation lines in the limit of slow compression. Extending decompression runs to negative pressures, we locate the sublimation lines for both LDA and hyperquenched glassy water (HGW), and find that HGW is relatively more stable to the vapor. Additionally, we observe spontaneous crystallization of HDA at high pressure to ice VII. Experiments have also seen crystallization of HDA, but to ice XII. Finally, we contrast the structure of LDA and HDA for the ST2 model with experiments. We find that while the radial distribution functions (RDFs) of LDA are similar to those observed in experiments, considerable differences exist between the HDA RDFs of ST2 water and experiment. The differences in HDA structure, as well as the formation of ice VII (a tetrahedral crystal), are a consequence of ST2 overemphasizing the tetrahedral character of water. © 2013 AIP Publishing LLC.

Heating-induced glass-glass and glass-liquid transformations in computer simulations of water

“…In addition, we compare the present results, based on isobaric heating runs, to those reported in Ref. 38, based on compression-decompression runs. This enables us to construct a phase diagram that includes pressure, as well as heating-induced LDA-HDA transformations.…”

“…This work complements previous computational studies where we explored the interplay between the LLPT and glass transition in glassy water 37 as well as the LDA-HDA transformations induced by pressure. 38 Here, we extend the heating studies from Ref. 37 to very high (P ≈ 2000 MPa) and very negative (P ≈ −1000 MPa) pressures.…”

“…However, the density values and structure of LDA and HDA are better predicted by the SPC/E model than by the ST2 model. 38 The main qualitative difference between these models is that equilibrium MD simulations indicate the existence of a LLPT in ST2 water 24,[39][40][41][42][43][44][45] (see, however, Refs. [46][47][48][49].…”

“…To our knowledge, only a few fullatom water models have been employed in computational studies of glassy water: the TIP4P, [29][30][31] SPC/E, [32][33][34][35][36] and ST2 model. 24,29,37,38 As is the case in the liquid state, these models have limitations in reproducing experimental data. For example, compared to experiments, the ST2 model reproduces better the qualitative behavior of the pressure dependence of density during the pressure-induced LDA-HDA transformations.…”

“…We note that standard (out-of-equilibrium) MD computer simulations can only be performed with time scales that are much shorter than the corresponding experimental time scales. 38 Moreover, one expects that the (T, P) loci for the pressure-induced and heating-induced transformations between LDA and HDA will quantitatively depend on heating/cooling and compression/decompression rates. However, in most cases, the qualitative features of these loci will be only weakly dependent on rates.…”