Large-Scale Structure and Hyperuniformity of Amorphous Ices

We investigate the large-scale structure of amorphous ices and transitions between their different forms by quantifying their large-scale density fluctuations. Specifically, we simulate the isothermal compression of low-density amorphous ice (LDA) and hexagonal ice to produce high-density amorphous ice (HDA). Both HDA and LDA are nearly hyperuniform; i.e., they are characterized by an anomalous suppression of large-scale density fluctuations. By contrast, in correspondence with the nonequilibrium phase transitions to HDA, the presence of structural heterogeneities strongly suppresses the hyperuniformity and the system becomes hyposurficial (devoid of “surface-area fluctuations”). Our investigation challenges the largely accepted “frozen-liquid” picture, which views glasses as structurally arrested liquids. Beyond implications for water, our findings enrich our understanding of pressure-induced structural transformations in glasses.

Figure 1

(a) Coefficient $A$ during compression of $Ih$ (black, continuous) and LDA (red, dashed). Inset: zoom for $1.4<p<3.0\mathrm{GPa}$. (b) Coefficient $B$ during compression of $Ih$ (black, continuous) and LDA (red, dashed). The peaks of $A$ signal the $Ih$-to-HDA and LDA-to-HDA transformations, in correspondence with which the system becomes hyposurficial, as shown by the sharp drop of $B$.

Figure 2

Translational order metric $\tau$ during compression of $Ih$ (black, continuous) and LDA (red, dashed). Sudden drops of $\tau$ occur in correspondence with the PTs. Inset: $\tau$ of $Ih$ at low pressures.

Figure 3

Snapshots of representative configurations before (left), near (center), and after (right) the $Ih$-to-HDA transformation. Oxygen sites are represented by blue or red spheres, respectively, for $Ih$ or HDA character based on the local structure index. The corresponding $S(k)$ is reported below each snapshot.

Figure 4

(a) $H=S(0)/S(k_{\mathrm{peak}})$ during compression of $Ih$ (black, continuous) and LDA (red, dashed). The peaks in correspondence with the PTs ($Ih$ to HDA and LDA to HDA) reach values that are 2 orders of magnitude larger than the qualitative threshold for hyperuniformity ($H\sim {10}^{-3}$). (b) Zoom of $H$ before the LDA-to-HDA PT. (c) Zoom of $H$ after the $Ih$-to-HDA PT. (d) $H$ during cooling of liquid water down to $T=80\mathrm{K}$ at $p=0.1\mathrm{GPa}$. Averages are taken over 10 independent runs.