Ice XV: A New Thermodynamically Stable Phase of Ice

A new phase of ice, named ice XV, has been identified and its structure determined by neutron diffraction. Ice XV is the hydrogen-ordered counterpart of ice VI and is thermodynamically stable at temperatures below $\sim 130\mathrm{K}$ in the 0.8 to 1.5 GPa pressure range. The regions of stability in the medium pressure range of the phase diagram have thus been finally mapped, with only hydrogen-ordered phases stable at 0 K. The ordered ice XV structure is antiferroelectric ($P\overline{1}$), in clear disagreement with recent theoretical calculations predicting ferroelectric ordering ($Cc$).

Figure 1

(a) The phase diagram of water and ice, including the liquidus lines of metastable ices IV and XII (long dashed lines) and extrapolated equilibria lines at low temperatures (short dashed lines). Hydrogen-disordered and ordered phases are indicated by italic and regular Roman numerals, respectively. DCl doped ice VI was produced by heating doped ice $\mathrm{I}h$ to 250 K at $\sim 0.9\mathrm{GPa}$. The consecutive $p/T$ pathways are shown. (b) Schematic illustration of the hydrogen ordering process as followed by diffraction. Larger spheres indicate oxygen atoms and smaller spheres hydrogen atoms, respectively. Fully occupied atomic sites are shown in black and half occupied sites in gray.

Figure 2

Analysis of the powder neutron diffraction data (GEM instrument). (a) Changes of lattice constant $c$ upon heating and cooling at ambient pressure. The numbers in (a) and (b) correspond to the $p/T$ way points in Fig. 1a. (b) Observed, calculated, and difference neutron powder diffraction profiles for ice XV after slow cooling to 80 K at ambient pressure ($R_{wp}=0.0155$, $R_p=0.0186$). The crystal structure was refined using the GSAS program [24] with a space group $P\overline{1}$ structural model and experimental diffraction data ranging from 0.55 to 7 Å. Hydrogen occupancy constraints were used during the refinement to ensure that the ice rules are maintained. Tick marks indicate the positions of the Bragg peaks for ice XV, Ni, and V (top to bottom). (inset) Magnification of a region where new Bragg peaks appeared. The data shown were recorded at 80 K after slow cooling under pressure (bottom), at 138 K after heating at ambient pressure (middle) and at 80 K after slow cooling at ambient pressure (top). Arrows indicate the positions of new Bragg peaks. (c) ${\chi }^2$ values obtained after converged Rietveld refinements using the fully ordered models shown in Fig. 3b and space group symmetries $Cc$, $Pn$, $P{2}_1$, and $P\overline{1}$. The ${\chi }^2$ value obtained for the structure predicted in Refs 14, 15 is depicted as an open square.

Figure 3

Unit cell projections of (a) hydrogen-disordered ice VI ($P{4}_2/nmc$), (b) the six possible hydrogen-ordered structures of a single hydrogen bonded network ($P1$) and (c) hydrogen-ordered ice XV ($P\overline{1}$). The atomic site occupancies are indicated by a gray scale gradient from black (fully occupied) to white (not occupied), and the polyhedra show the front faces of the hexameric units. The oxygen atoms belonging to different networks are marked with “1” and “2” in (a).