Structure of a New Dense Amorphous Ice

The detailed structure of a new dense amorphous ice, VHDA, is determined by isotope substitution neutron diffraction. Its structure is characterized by a doubled occupancy of the stabilizing interstitial location that was found in high density amorphous ice, HDA. As would be expected for a thermally activated unlocking of the stabilizing “interstitial,” the transition from VHDA to LDA (low-density amorphous ice) is very sharp. Although its higher density makes VHDA a better candidate than HDA for a physical manifestation of the second putative liquid phase of water, as for the HDA case, the VHDA to LDA transition also appears to be kinetically controlled.

Figure 1

Intermolecular partial radial distribution functions of VHDA and HDA at 80 K. For clarity, the ordinates for $g_{\mathrm{O}\mathrm{H}}(r)$ and $g_{\mathrm{O}\mathrm{O}}(r)$ are shifted by 2 and 4, respectively.

Figure 2

Spatial density functions showing the distribution of water molecules in shells of increasing distance around a central water molecule for VHDA (left panels) and HDA (right panels) at 80 K. The HDA data are from the work in Ref. 9. The contour levels are chosen such that 30% of the molecules are included in the lobes, which therefore indicate the main orientational arrangements of the waters in the shells. The shell boundaries are chosen to relate to relevant features in the $g_{\mathrm{O}\mathrm{O}}(r)$s of both structures (Fig. 1): (a) $2.3\AA –3.1\AA$, (b) $3.1\AA –3.3\AA$, (c) $3.3\AA –3.7\AA$, and (d) $3.7\AA –4.1\AA$. The coordination numbers that are associated with each of these panels are (a) 4.1(0.1) for both VHDA and HDA, (b) 1.7(0.1) and 0.9(0.1) for VHDA and HDA, respectively, (c) 3.5(0.1) and 2.2(0.1) for VHDA and HDA, and last (d) 2.7(0.1) and 3.5(0.1) for VHDA and HDA. This gives a total occupancy for the full second neighbor shell $(\mathrm{b})+(\mathrm{c})+(\mathrm{d})$ of 7.9(0.1) molecules for VHDA and 6.6(0.1) for HDA. The viewport for each panel is set to $15\AA \times 15\AA$, with the water oxygen at the center.