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Ice XIX represents the latest discovery of ice polymorphs and exists in the medium pressure range near 1–2 GPa. Ice XIX is a partially hydrogen-ordered phase, by contrast to its disordered mother phase ice VI, which shares the same oxygen-atom network with ice XIX. Ice XIX differs in terms of the ordering of the hydrogen-atom sublattice, and hence the space group, from its hydrogen-ordered sibling ice XV, which also features the same type of oxygen network. Together, ice VI, XV, and XIX form the only known trio of ice polymorphs, where polymorphic transformations from order to order, order to disorder, and disorder to order are possible, which also compete with each other depending on the thermodynamic path taken and the cooling/heating rates employed. These transitions in the H-sublattice have barely been investigated, so we study here the unique triangular relation in the ice VI/XV/XIX trio based on calorimetry experiments. We reveal the following key features for H-sublattice transitions: (i) upon cooling ice VI, domains of ice XV and XIX develop simultaneously, where pure ice XV forms at ≤0.85 GPa and pure ice XIX forms at ≥1.60 GPa, (ii) ice XIX transforms into ice XV via a transient disordered state, (iii) ice XV recooled at ambient pressure features a complex domain structure, possibly containing an unknown H-ordered polymorph, (iv) recooled ice XV partly transforms back into ice XIX at 1.80 GPa, and (v) partial deuteration slows down domain reordering strongly. These findings not only are of interest in understanding possible hydrogen-ordering and -disordering processes in the interior of icy moons and planets but, more importantly, also provide a challenging benchmark for our understanding and parameterizing many-body interactions in H-bonded networks.

It is difficult to investigate the hydrogen-bonding dynamics of hydrogen-disordered ice VI. Here, we present a comparative method based on our previous study of its counterpart hydrogen-ordered phase, ice XV. The primitive cell of ice XV is a 10 molecule unit, and the vibrational normal modes were analyzed individually. We constructed an 80 molecule supercell of ice VI to mimic the periodic unit and performed first-principles density functional theory calculations. As the two vibrational spectra show almost identical features, we compared the molecular translation vibrations. Inspired by the phonon analysis of ice XV, we found that the vibrational modes in the translation band of ice VI are classifiable into three groups. The lowest-strength vibration modes represent vibrations between two sublattices that lack hydrogen bonding. The highest-strength vibration modes represent the vibration of four hydrogen bonds of one molecule. The middle-strength vibration modes mainly represent the molecular vibrations of only two hydrogen bonds. Although there are many overlapping stronger and middle modes, there are only two main peaks in the inelastic neutron scattering (INS) spectra. This work explains the origin of the two main peaks in the far-infrared region of ice VI and illustrates how to analyze a hydrogen-disordered ice structure.

Amorphous water ice comes in at least three distinct structural forms, all lacking long-range crystalline order. High-density amorphous ice (HDA) was first produced by compressing ice I to 11 kilobar at temperatures below 130 kelvin, and the process was described as thermodynamic melting1, implying that HDA is a glassy state of water. This concept, and the ability to transform HDA reversibly into low-density amorphous ice, inspired the two-liquid water model, which relates the amorphous phases to two liquid waters in the deeply supercooled regime (below 228 kelvin) to explain many of the anomalies of water2 (such as density and heat capacity anomalies). However, HDA formation has also been ascribed3 to a mechanical instability causing structural collapse and associated with kinetics too sluggish for recrystallization to occur. This interpretation is supported by simulations3, analogy with a structurally similar system4, and the observation of lattice-vibration softening as ice is compressed5,6. It also agrees with recent observations of ice compression at higher temperatures-in the 'no man's land' regime, between 145 and 200 kelvin, where kinetics are faster-resulting in crystalline phases7,8. Here we further probe the role of kinetics and show that, if carried out slowly, compression of ice I even at 100 kelvin (a region in which HDA typically forms) gives proton-ordered, but non-interpenetrating, ice IX', then proton-ordered and interpenetrating ice XV', and finally ice VIII'. By contrast, fast compression yields HDA but no ice IX, and direct transformation of ice I to ice XV' is structurally inhibited. These observations suggest that HDA formation is a consequence of a kinetically arrested transformation between low-density ice I and high-density ice XV' and challenge theories that connect amorphous ice to supercooled liquid water.

The recent discovery of a low-temperature endotherm upon heating hydrochloric-acid-doped ice VI has sparked a vivid controversy. The two competing explanations aiming to explain its origin range from a new distinct crystalline phase of ice to deep-glassy states of the well-known ice VI. Problems with the slow kinetics of deuterated phases have been raised, which we circumvent here entirely by simultaneously measuring the inelastic neutron spectra and neutron diffraction data of H2O samples. These measurements support the deep-glassy ice VI scenario and rule out alternative explanations. Additionally, we show that the crystallographic model of D2O ice XV, the ordered counterpart of ice VI, also applies to the corresponding H2O phase. The discovery of deep-glassy ice VI now provides a fascinating new example of ultrastable glasses that are encountered across a wide range of other materials., We thank the Royal Society for a University Research Fellowship (CGS, UF150665), the Austrian Science Fund for a Schrödinger fellowship (AA, J4325), and ISIS for granting access to the TOSCA instrument. Furthermore, this project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement no. 725271).

More than 20 crystalline and amorphous phases have been reported for ice so far. This extraordinary polymorphism of ice arises from the geometric flexibility of hydrogen bonds and hydrogen ordering, and makes ice a unique presence with its universality in the wide fields of material and earth and planetary science. A prominent unsolved question concerning the diversity is whether a hydrogen-disordered phase of ice transforms into only one hydrogen-ordered phase, as inferred from the current phase diagram of ice, although its possible hydrogen configurations have close energies. Recent experiments on a high-pressure hydrogen-disordered phase, ice VI, revealed an unknown hydrogen-ordered form ($\beta$-XV) besides the known ordered phase, ice XV, which would be a counterexample of the question. However, due to lack of experimental evidence, it has not been clarified whether $\beta$-XV is a distinct crystalline phase. Herein we report a second hydrogen-ordered phase for ice VI, ice XIX, unambiguously demonstrated by neutron diffraction measurements. The phase boundary between ice VI and ice XIX shows that ice VI contracts upon the hydrogen ordering, which thermodynamically stabilizes ice XIX in higher-pressure region than ice XV because of its smaller volume than ice XV. The pressure-driven phase competition between hydrogen-ordered phases, also theoretically suggested in other ice polymorphs, can induce hydrogen ordering of ice in different manners. Thus, this study demonstrates a hitherto undiscovered polymorphism of ice.

We report on the discovery and characterization of ice β-XV, which represents the second hydrogen ordered polymorph related to ice VI., In the last twelve years five new ice phases were experimentally prepared. Two of them are empty clathrate hydrates and three of them represent hydrogen ordered counterparts of previously known disordered ice phases. Here, we report on hydrogen ordering in ice VI samples produced by cooling at pressures up to 2.00 GPa. Based on results from calorimetry, dielectric relaxation spectroscopy, Raman spectroscopy, and powder X-ray diffraction the existence of a second hydrogen ordered polymorph related to ice VI is suggested. Powder X-ray data show the oxygen network to be the one of ice VI. For the 1.80 GPa sample the activation energy from dielectric spectroscopy is 45 kJ mol–1, which is much larger than for the known hydrogen ordered proxy of ice VI, ice XV. Raman spectroscopy indicates the 1.80 GPa sample to be more ordered than ice XV. It is further distinct from ice XV in that it experiences hydrogen disordering above ≈103 K which is 26 K below the ice XV to ice VI disordering transition. Consequently, below 103 K it is thermodynamically more stable than ice XV, adding a stability region to the phase diagram of water. For the time being we suggest to call this new phase ice β-XV and to relabel it ice XVIII once its crystal structure is known.

The vibrational spectrum of ice II was investigated using the CASTEP code based on first-principles density functional theory (DFT). Based on good agreement with inelastic neutron scattering (INS), infrared (IR), and Raman experimental data, we discuss the translation, libration, bending, and stretching band using normal modes analysis method. In the translation band, we found that the four-bond and two-bond molecular vibration modes constitute three main peaks in accordance with INS ranging from 117 to 318 cm&minus, 1. We also discovered that the lower frequencies are cluster vibrations that may overlap with acoustic phonons. Whale et al. found in ice XV that some intramolecular vibrational modes include many isolated-molecule stretches of only one O&ndash, H bond, whereas the other O&ndash, H bond does not vibrate. This phenomenon is very common in ice II, and we attribute it to local tetrahedral deformation. The pathway of combining normal mode analysis with experimental spectra leads to scientific assignments.

It is always difficult to assign the peaks of a vibrational spectrum in the far-infrared region. The two distinct peaks seen in many ice phases are still a mystery to date. The normal modes of ice XV were calculated using the CASTEP code based on first-principles density functional theory. On the basis of vibrational modes analysis, we divided the translational modes into three categories: four-bond vibrations, which have the highest energy levels, two-bond vibrations, which have medium levels of energy, and relative vibrations between two sublattices, which have the lowest energy. Whale et al. found that some intramolecular stretching modes include the isolated vibration of only one O&ndash, H bond, whereas the others do not vibrate in ice XV. We verified this phenomenon in this study and attributed it to local tetrahedral deformation. Analysis of normal modes, especially in the translation and stretching band of ice XV, clarified the physical insights of the vibrational spectrum and can be used with other ice phases.

Using a first-principles density functional theory method, we calculated the vibrational spectrum of ice XIII. In combination with spectroscopic experiments, we demonstrated that the two characteristic hydrogen bond (H-bond) peaks in the translational region of ice V (the hydrogen-disordered counterpart of ice XIII) recorded by inelastic neutron scattering originate from two basic H-bond vibrational modes. We first observed this phenomenon when studying ice Ic and then confirmed it when studying ice XIV, XVI, and XVII. This study of high-pressure ice XIII supports our theory that two basic H-bond modes exist as a general rule among the ice family. The “isolated O–H vibration modes” found by Whale et al. when studying ice XV are also discussed in this work.

Powder neutron diffraction and Raman spectroscopy experiments for ice XV, the recently discovered proton-ordered polymorph of ice VI, suggest that the protons arrange in an antiferroelectric structure with P1 symmetry, contrary to several density functional theory predictions of a ferroelectric Cc structure. Here, we find that higher-level fragment-based second-order perturbation theory (MP2) and coupled cluster theory (CCSD(T)) electronic structure calculations predict that the experimentally proposed proton ordering is indeed slightly more stable than the other possible structures. These calculations reveal a close competition between the structure with the strongest local hydrogen bonding (Cc) and the one with the most favorable “delocalized” hydrogen bond cooperativity effects (P1), with the latter being preferred by ∼0.4 kJ/mol per molecule. The results reiterate the importance of viewing ice networks as a whole instead of focusing on pairwise hydrogen-bonding interactions.

Most ice polymorphs have order–disorder “pairs” in terms of hydrogen positions, which contributes to the rich variety of ice polymorphs; in fact, three recently discovered polymorphs— ices XIII, XIV, and XV—are ordered counter forms to already identified disordered phases. Despite the considerable effort to understand order–disorder transition in ice crystals, there is an inconsistency among the various experiments and calculations for ice XV, the ordered counter form of ice VI, i.e., neutron diffraction observations suggest antiferroelectrically ordered structures, which disagree with dielectric measurement and theoretical studies, implying ferroelectrically ordered structures. Here we investigate in-situ neutron diffraction measurements and density functional theory calculations to revisit the structure and stability of ice XV. We find that none of the completely ordered configurations are particular favored; instead, partially ordered states are established as a mixture of ordered domains in disordered ice VI. This scenario in which several kinds of ordered configuration coexist dispels the contradictions in previous studies. It means that the order–disorder pairs in ice polymorphs are not one-to-one correspondent pairs but rather have one-to-n correspondence, where there are n possible configurations at finite temperature.

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 $\ensuremath{\sim}130\text{ }\text{ }\mathrm{K}$ 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 ($Cc$).

Our recent discovery of three new phases of ice has increased the total number of known distinct polymorphs of ice to fifteen. In this Perspective article, we give a brief account of previous work in the field, and discuss some of the particularly interesting open questions that have emerged from recent studies. These include (i) the effectiveness of acid and base dopants to enable hydrogen-ordering processes in the ices, (ii) the comparison of the calorimetric data of some of the crystalline phases of ice and low-density amorphous ice, (iii) the disagreement between the experimental ice XV structure and computational predictions, (iv) the incompleteness of some of the hydrogen order/disorder pairs and (v) the new frontiers at the high and negative pressure ends of the phase diagram.

Most ice polymorphs have order–disorder “pairs” in terms of hydrogen positions, which contributes to the rich variety of ice polymorphs; in fact, three recently discovered polymorphs— ices XIII, XIV, and XV—are ordered counter forms to already identified disordered phases. Despite the considerable effort to understand order–disorder transition in ice crystals, there is an inconsistency among the various experiments and calculations for ice XV, the ordered counter form of ice VI, i.e., neutron diffraction observations suggest antiferroelectrically ordered structures, which disagree with dielectric measurement and theoretical studies, implying ferroelectrically ordered structures. Here we investigate in-situ neutron diffraction measurements and density functional theory calculations to revisit the structure and stability of ice XV. We find that none of the completely ordered configurations are particular favored; instead, partially ordered states are established as a mixture of ordered domains in disordered ice VI. This scenario in which several kinds of ordered configuration coexist dispels the contradictions in previous studies. It means that the order–disorder pairs in ice polymorphs are not one-to-one correspondent pairs but rather have one-to-n correspondence, where there are n possible configurations at finite temperature.

The D2O ice VI to ice XV hydrogen ordering phase transition at ambient pressure is investigated in detail with neutron diffraction. The lattice constants are found to be sensitive indicators for hydrogen ordering. The a and b lattice constants contract whereas a pronounced expansion in c is found upon hydrogen ordering. Overall, the hydrogen ordering transition goes along with a small increase in volume which explains why the phase transition is more difficult to observe upon cooling under pressure. Slow-cooling ice VI at 1.4 GPa gives essentially fully hydrogen-disordered ice VI. Consistent with earlier studies, the ice XV obtained after slow-cooling at ambient pressure is best described with P-1 space group symmetry. Using a new computational approach, we achieve the atomistic reconstruction of a supercell structure that is consistent with the average partially ordered structure derived from Rietveld refinements. This shows that C-type networks are most prevalent in ice XV but other structural motifs outside of the classifications of the fully hydrogen-ordered networks are identified as well. The recently proposed Pmmn structural model for ice XV is found to be incompatible with our diffraction data and we argue that only structural models that are capable of describing full hydrogen order should be used., Comment: 19 pages, 8 figures

The vibrational spectra of the condensed phases of water often show broad and strongly overlapping spectral features which can make spectroscopic interpretations and peak assignments difficult. The Raman spectra of hydrogen-ordered H2 Oa nd D2 Oi ce XV are reported here, and it is shown that the spectra can be fully interpreted in terms of assigning normal modes to the various spectral features by using density functional theory (DFT) calculations. The calculated lattice-vibration spectrum of the experimental antiferroelectric P1 structure is in good agreement with the experimental data whereas the spectrum of a ferroelectric Cc structure, which computational studies have suggested as the crystal structure of ice XV, differs substantially. Moreover, the calculated P 1c oupled O–H stretch spectrum also seems in better agreement with the experiment than the calculated spectrum for the Cc structure. Both the hydrogen bonds as well as the covalent bonds appear to be stronger in hydrogen-ordered ice XV than in the hydrogen-disordered counterpart ice VI. A new type of stretching mode is identified, and itisspeculatedthatthiskindofmodemightberelevantforothercondensedwaterphasesaswell.Furthermore,theiceXVspectra are compared to the spectra of ice VIII which is the only other high-pressure phase of ice for which detailed spectroscopic assignments have been made so far. In summary, we have established a link between crystallographic data and spectroscopic information in the case of ice XV by using DFT-calculated spectra. Such correlations may eventually help interpreting the vibrational spectra of more structurally-disordered aqueous systems. Copyright © 2012 John Wiley & Sons, Ltd.

The reversible phase transition from hydrochloric-acid-doped ice VI to its hydrogen-ordered counterpart ice XV is followed using differential scanning calorimetry. Upon cooling at ambient pressure fast hydrogen ordering is observed at first followed by a slower process which manifests as a tail to the initial sharp exotherm. The residual hydrogen disorder in H2O and D2O ice XV is determined as a function of the cooling rate. We conclude that it will be difficult to obtain fully hydrogen-ordered ice XV by cooling at ambient pressure. Our new experimental findings are discussed in the context of recent computational work on ice XV., Comment: 15 pages, 4 figures

Ice XV is the hydrogen-ordered form of the ice VI phase whose structure was predicted to be Cc and ferroelectric using periodic DFT approaches. However, neutron diffraction and Raman spectroscopy data show the structure to have P1̅ symmetry and to be antiferroelectric. Recent work1 using fragment-based MP2 and CCSD(T) approaches predicts the experimental structure as the ground state. We have reconsidered this problem using fully periodic MP2 and RPA approaches and find that the ferroelectric Cc structure is the lowest energy configuration. However, ubiquitously employed tinfoil boundary conditions stabilize polar structures. We suggest that ferroelectric Cc crystals can grow within a paraelectric ice VI matrix but may become unstable once a fraction of the matrix has become hydrogen-ordered. The reduction in dielectric constant causes P1̅ and other structures with small polarization to become favored, providing a possible resolution between observation and theoretical predictions.

Ice VI is a hydrogen bond disordered crystal over its known region of stability. In this work, we predict that ice VI will transform into a hydrogen bond ordered phase near 108 K, and have identified the likely low-temperature phase as ferroelectric (space group Cc) with an antiferroelectric structure (space group P2 1 2 1 2 1 ) close by in energy. Electronic density functional theory calculations provide input to our calculations, which are extended to cells large enough for statistical simulations by using graph invariants. A significant decrease in the configurational entropy is predicted as hydrogen bonds exhibit partial order above the transition, provided that the hydrogen bonds can equilibrate on an experimental time scale. Conversely, partial disorder is predicted at temperatures below the transition. Although some evidence for ordering of ice VI has been observed in experiments, a low-temperature proton ordered phase has not been identified experimentally.

Ice VI, a high-pressure form of density 1.31 g cm-(3), has a tetragonal cell of dimensions a = 6.27 A, c = 5.79 A, space group P4(2)/nmc, each cell containing ten water molecules. The structure is built up of hydrogen-bonded chdins of water molecules that are analogs of the tectosilicate chains out of which the fibrous zeolites are constructed. The chains in ice VI are linked laterally to one another to form an open, zeolite-like framework. The cavities in this framework are filled with a second framework identical with the first. The two frameworks interpenetrate but do not interconnect, and the complete structure can thus be considered a "self-clathrate." This structural feature is a natural way to achieve high density in tetrahedrally linked framework structures.

Ice polymorphs usually appear as hydrogen disorder-order pairs. Ice VI has a wide range of thermodynamic stability and exists in the interior of Earth and icy moons. Our previous work suggested ice β-XV as a second polymorph deriving from disordered ice VI, in addition to ice XV. Here we report thermal and structural characterization of the previously inaccessible deuterated polymorph using ex situ calorimetry and high-resolution neutron powder diffraction. Ice β-XV, now called ice XIX, is shown to be partially antiferroelectrically ordered and crystallising in a √2×√2×1 supercell. Our powder data recorded at subambient pressure fit best to the structural model in space group $$P\bar 4$$ P 4 ¯ . Key to the synthesis of deuterated ice XIX is the use of a DCl-doped D2O/H2O mixture, where the small H2O fraction enhances ice XIX nucleation kinetics. In addition, we observe the transition from ice XIX to its sibling ice XV upon heating, which proceeds via a transition state (ice VI‡) containing a disordered H-sublattice. To the best of our knowledge this represents the first order-order transition known in ice physics.

The nature of the hydrogen sublattice of an HCl-doped ice VI sample after cooling at 1.8 GPa has been a topic of recent interest. The samples are interpreted either as the new H-ordered ice phase ice β-XV with a thermodynamic stability region in the phase diagram [T. M. Gasser et al., Chem. Sci., 2018, 9, 4224], or alternatively as H-disordered, deep glassy ice VI [A. Rosu-Finsen and C. G. Salzmann, Chem. Sci., 2019, 10, 515]. Here we provide a comprehensive Raman spectroscopic study on ice β-XV, ice XV and ice VI, with the following key findings: (i) the Raman spectra of ice β-XV differ fundamentally from those of ice VI and ice XV, where the degree of H-order is even higher than in ice XV. (ii) Upon cooling ice VI there is competition between formation of ice XV and ice β-XV domains, where ice XV forms at 0.0 GPa, but ice β-XV at 1.8 GPa. Domains of ice β-XV are present in literature "ice XV" at 1.0 GPa. This result clarifies the puzzling earlier observation that the degree of H-order in ice XV apparently improves upon heating and recooling at ambient pressure. In reality, this procedure leaves the H-order in ice XV unaffected, but removes ice β-XV domains by transforming them to ice XV. (iii) Upon heating, the samples experience the transition sequence ice β-XV → ice XV → ice VI, i.e., an order-order transition at 103 K followed by an order-disorder transition at 129 K. The former progresses via a disordered transient state. (iv) D2O ice β-XV forms upon cooling DCl-doped D2O-ice VI, albeit at a much lower pace than in the hydrogenated case so that untransformed D2O ice VI domains are present even after slow cooling. The librational band at 380 cm-1 is identified to be the characteristic spectroscopic feature of deuterated ice β-XV. Taken together these findings clarify open questions in previous work on H-ordering in the ice VI lattice, rule out a glassy nature of ice β-XV and pave the way for a future neutron diffraction study to refine the crystal structure of D2O ice β-XV.

The structures of deuterated ices VI, VII, and VIII have been studied under their conditions of stability by neutron powder diffraction. The mode of ordering of (tetragonal) ice VIII is clearly established, and no evidence is found of partial ordering as the temperature is raised. Ice VII is accurately cubic with D2O molecules disordered around their center of mass; there is no evidence of partial ordering at any temperature. The water molecule geometry is normal in both phases, and the hydrogen‐bonded first neighbor in ice VIII is confirmed to be more distant than the first nonbonded neighbor. The transition temperature between the two phases occurs at 263±2 K, some 11 K lower than expected. Hydrogen bond lengths in both phases are equal at the transition. Although the ice VI data are not as good, we can see no evidence of the antiferroelectric ordering proposed by Kamb from work on recovered samples. Our results are consistent with thermodynamic measurements indicating disorder in ice VI at 193 K. We co...