Ice XIX

Ice XIX is the second phase of ice, which is related to ice VI. Ice XIX is prepared by cooling HCl-doped ice VI at a pressure above 1.6 GPa down to about 100 K, but its crystal structure has not been elucidated yet (see below).

History of the discovery of ice XIX

The first report regarding ice XIX was published in 2018 by Thomas Loering's group from Austria . They quenched HCl-doped ice VI to 77 K at different pressures between 1.0 and 1.8 GPa to collect DSC thermograms, dielectric spectrum, Raman spectrum, and X-ray diffraction patterns. In the Raman spectra, there was an endothermic feature at about 110 K in addition to the endotherm corresponding to ice XV-VI transition. Additionally, the Raman spectra, dielectric properties, and the ratio of the lattice parameters differed from what ice XV had. Therefore, they proposed the existence of the second hydrogen-ordered phase of ice VI and named it ice beta-XV.

In 2019, Rosu-Finsen and Salzman argued that there was no need to consider the new phase of ice (ice beta-XV) and proposed "deep-glassy" state scenario . According to their DSC data, the degree of the endothermic feature depends not only on quench-recovery pressure but also on the heating rate and annealing duration at 93 K. They also collected neutron diffraction profiles of quench-recovered DCl-doped D₂O ice VI/XV prepared at different pressures of 1.0, 1.4 and 1.8 GPa to show that there were no significant differences among them. They conclude that the low-temperature endotherm originated from kinetic features related to glass transitions of deep glassy states of disordered ice VI.

Distinguishing between the two scenarios (new hydrogen-ordered phase vs. deep-glassy disordered ice VI) became an open question and the debate between the two groups has continued. Thoeny et al. (Loerting's group) collected another series of Raman spectra of ice beta-XV to additionally report that (i) ice XV prepared by the protocol reported previously contains both ice XV and ice beta-XV domains; (ii) upon heating, Raman spectra of ice beta-XV showed loss of H-order. In contrast, Salzmann's group again argued for the plausibility of a 'deep-glassy state' scenario based on neutron diffraction and neutron inelastic scattering experiments . Based on their experimental results, ice VI and deep-glassy ice VI share very similar features from both elastic (diffraction) scattering and inelastic scattering experiments, being clearly different from the properties of ice XV.

In 2021, the crystallographic evidence for a new phase (ice XIX) was individually reported by three groups: Yamane et al. (Hiroyuki Kagi and Kazuki Komatsu's group from Japan), Gasser et al. (Loerting's group), and Salzmann's group. Yamane et al. collected neutron diffraction profiles in situ (i.e. under high pressure) and found new Bragg features which are completely different from both ice VI and ice XV. They performed the Rietveld refinement of the profiles based on ${\sqrt {2}}\times {\sqrt {2}}\times 1$ supercell of ice XV and proposed some leading candidates for the space group of ice XIX: P-4, Pca21, Pcc2, P21/a, and P21/c. They also measured dielectric spectra in situ and determined phase boundaries of ices VI/XV/XIX; the sign of the slope of the boundary turns negative from positive at 1.6 GPa indicating the existence of two different phases (Clausis-Clapeyron relation). Gasser et al. also collected powder neutron diffractograms of quench-recovered ices VI, XV, and XIX and found similar crystallographic features as Yamane et al. reported, concluding that P-4 and Pcc2 are the plausible space group candidates. Both Yamane et al.'s and Gasser et al.'s refinements resulted in partially hydrogen-ordered structure. Months later, Salzmann et al. published a paper based on in-situ powder neutron diffraction experiments of ice XIX . Differently from their previous reports, they accepted the idea of the new phase (ice XIX) as they observed similar features of the previous two reports. However, they refined the diffraction profiles based on a disordered structural model (Pbcn) and argued that new Bragg reflections can be explained by distortions of ice VI, still regarding ice XIX as a deep-glassy state of ice VI. The crystal structure of ice XIX including hydrogen order/disorder is still under debate up to date.