Searching for crystal-ice domains in amorphous ices

Abstract

We employ classical molecular dynamics simulations to investigate the molecular-level structure of water during the isothermal compression of hexagonal ice (Ih) and low-density amorphous (LDA) ice at low temperatures. In both cases, the system transforms to high-density amorphous ice (HDA) via a first-order-like phase transition. We employ a sensitive local order metric (LOM) [Martelli et. al., Phys. Rev. B, 97, 064105 (2018)], that can discriminate among different crystalline and non crystalline ice structures and is based on the positions of the oxygen atoms in the first and/or second hydration shell. Our results confirm that LDA and HDA are indeed amorphous, i.e., they lack of polydispersed ice domains. Interestingly, HDA contains a small number of domains that are reminiscent of the unit cell of ice IV, although the hydrogen-bond network (HBN) of these domains differ from the HBN of ice IV. The presence of ice IV-like domains provides some support to the hypothesis that HDA could be the result of a detour on the HBN rearrangement along the Ih-to-ice IV pressure induced transformation. Both nonequilibrium LDA-to-HDA and Ih-to-HDA transformations are two-steps processes where a small distortion of the HBN first occurs at low pressures and then, a sudden, extensive re-arrangement of hydrogen bonds at the corresponding transformation pressure follows. Interestingly, the Ih-to-HDA and LDA-to-HDA transformations occur when LDA and Ih have similar local order, as quantified by the site-averaged LOMs. Since Ih has a perfect tetrahedral HBN, while LDA does not, it follows that higher pressures are needed to transform Ih into HDA than that for the conversion of LDA to HDA. In correspondence with both first-order-like phase transitions, the samples are composed of a large HDA cluster that percolates within the Ih/LDA samples. Our results shed light on the debated structural properties of amorphous ices and indicate that the kinetics of the Ih-to-HDA and LDA-to-HDA transformations require an in depth inspection of the underlying HBN. Such investigation is currently ongoing.

Evidence for high-density liquid water between 0.1 and 0.3 GPa near 150 K

“…They described uHDA obtained from PIA of ice I at low temperatures as a derailed state along the polymorphic transition path from crystalline ice I to crystalline ice IV. Similarly, Martelli et al (27) demonstrate in MD simulations that, even though HDA is indeed amorphous (lacking “polydispersed icelike structures”), there exist some small domains structurally reminiscent of ice IV. The bump in the uHDA line is reproducible and implies that near ∼0.5 GPa T x (uHDA) appears to approach T x (eHDA).…”

X-ray studies of the transformation from high- to low-density amorphous water

“…That is, LDA-I derived from uHDA crystalizes into hexagonal ice, whereas LDA-II derived from eHDA crystallizes to stacking disordered ice (or also commonly denoted cubic ice) [49][50][51][52], and they concluded that uHDA contains crystalline hexagonal remnants [55]. Recent MD simulations [56] of both uHDA and eHDA at high pressures showed a 'lack of polydisperse ice-like structures', but a weak indication that uHDA contains a 'nonnegligible number of Ih-like molecules' at elevated pressures of 1.4 GPa, clusters composed of 5-10 molecules [56]. We do not find direct evidence for uHDA containing crystalline hexagonal ice, because e.g.…”

Nature of Water’s Second Glass Transition Elucidated by Doping and Isotope Substitution Experiments

“…Also, here the (missing) doping-induced enhancement of the H-atom dynamics clearly differs from the findings for crystalline ices, such as ice I, ice V, and ice XII. Let us also draw attention to recent classical molecular dynamics simulations, which demonstrate that polydispersed ice domains are lacking in both LDA and eHDA [58].…”