Ice VII Structure Research Articles

Induction of ice VII structure with secondary shock compression by backward Raman scattering in plasmas

Forward stimulated Raman scattering and backward stimulated Raman scattering (BSRS) are measured when an intense 532 nm nanosecond pulsed laser is focused into water and heavy water. The investigation reveals a significant observation: the formation of the ice VII structure exclusively occurs in the backward direction when optical breakdown takes place, provided that the input energy falls below 90 mJ for liquid water or 75 mJ for heavy water. This phase transition is attributed to secondary shock compression, which comes from energy transfer and compression between the BSRS in plasmas with the pump laser. The optical breakdown experiment under pre-pressure reveals that the shock compression in the back direction is approximately 2.3 times that of the forward direction. This research is useful for shock compression and dynamics in plasmas.

Atomic distribution and local structure in ice VII from in situ neutron diffraction

Ice polymorphs show extraordinary structural diversity depending on pressure and temperature. The behavior of hydrogen-bond disorder not only is a key ingredient for their structural diversity but also controls their physical properties. However, it has been a challenge to determine the details of the disordered structure in ice polymorphs under pressure, because of the limited observable reciprocal space and inaccuracies related to high-pressure techniques. Here, we present an elucidation of the disordered structure of ice VII, the dominant high-pressure form of water, at 2.2 GPa and 298 K, from both single-crystal and powder neutron-diffraction techniques. We reveal the three-dimensional atomic distributions from the maximum entropy method and unexpectedly find a ring-like distribution of hydrogen in contrast to the commonly accepted discrete sites. In addition, total scattering analysis at 274 K clarified the difference in the intermolecular structure from ice VIII, the ordered counterpart of ice VII, despite an identical molecular geometry. Our complementary structure analyses robustly demonstrate the unique disordered structure of ice VII. Furthermore, these findings are related to proton dynamics, which drastically vary with pressure, and will contribute to an understanding of the structural origin of anomalous physical properties of ice VII under pressures.

Crystal Structure of Symmetric Ice X in H2O-H2 and H2O-He under Pressure.

Ice VII and ice X are the two most dominant phases, stable over a large pressure range between 2 and 150 GPa and made of fundamentally different chemical bonding. Yet, the two ice phases share a similar bcc-based crystal structure and lattice constants, resulting in a challenge to discern the crystal structure of ice VII and ice X. Here, we present well-resolved X-ray diffraction data of H2O in quasi-hydrostatic H2 and He pressure media, clearly resolving the two ice phases to 130 GPa and the dissociative nature of ice VII to X transition occurring at 20-50 GPa in H2O-H2 and 60-70 GPa in H2O-He. The present diffraction data permits, for the first time, the accurate determination of the bulk moduli B0 of 225 (or 228) GPa for ice X and 6.2 (or 4.5) GPa for ice VII, in H2O-H2 (or H2O-He), which can provide new constraints for Giant planetary models.

Pathways for the formation of ice polymorphs from water predicted by a metadynamics method

The mechanism of how ice crystal form has been extensively studied by many researchers but remains an open question. Molecular dynamics (MD) simulations are a useful tool for investigating the molecular-scale mechanism of crystal formation. However, the timescale of phenomena that can be analyzed by MD simulations is typically restricted to microseconds or less, which is far too short to explore ice crystal formation that occurs in real systems. In this study, a metadynamics (MTD) method was adopted to overcome this timescale limitation of MD simulations. An MD simulation combined with the MTD method, in which two discrete oxygen–oxygen radial distribution functions represented by Gaussian window functions were used as collective variables, successfully reproduced the formation of several different ice crystals when the Gaussian window functions were set at appropriate oxygen–oxygen distances: cubic ice, stacking disordered ice consisting of cubic ice and hexagonal ice, high-pressure ice VII, layered ice with an ice VII structure, and layered ice with an unknown structure. The free-energy landscape generated by the MTD method suggests that the formation of each ice crystal occurred via high-density water with a similar structure to the formed ice crystal. The present method can be used not only to study the mechanism of crystal formation but also to search for new crystals in real systems.

Search for a ferroelectrically ordered form of ice VII by neutron diffraction under high pressure and high electric field

Neutron diffraction experiments of ice VII at pressures up to 6.2 GPa and 10.2 kV/mm were conducted to investigate the potential ferroelectrically ordered structure of ice VII induced by a high electric field. To accomplish this, we developed a high-pressure cell assembly to allow for powder neutron diffraction to be conducted under both high-pressure and high-electric field conditions. However, the subsequent observed diffraction patterns taken in situ at these conditions do not show sufficient evidence of the proposed ferroelectrically ordered structure of ice VII. We estimate the degree of the hydrogen ordering in ice VII under the applied conditions from high-pressure $P\ensuremath{-}E$ loop measurements, and discuss the future possibilities of detecting the ferroelectrically ordered structure.

Influence of the hydrogen bond quantum nature in liquid water and heavy water on stimulated Raman scattering

Stimulated Raman scattering (SRS) of liquid water and heavy water have been investigated using Nd:YAG laser. The SRS spectra of liquid heavy water indicate that ice–VII and ice–VIII structures are formed by shock-induced compression (SIC) in forward and backward directions, respectively. Simultaneously, the SRS spectra reveal of liquid water that only ice-VII structure is formed in the backward direction. The difference in ice structures formed by SIC in liquid water and heavy water could be attributed to the effect of the hydrogen bond quantum nature with H+. SRS spectra of 2M NaOH water solution with ice–VII and ice-VIII structures have been successfully obtained in forward and backward, respectively, as OH− greatly reduce the quantum nature of hydrogen bonds by neutralizing H+ in water. The hydrogen bond quantum nature is important for understanding isotope calibration test structure and isotopic effect.

Potassium chloride-bearing ice VII and ice planet dynamics

Accurate modeling of planetary interiors requires that the pressure–volume–temperature (PVT) properties of phases present within the body be well understood. The high-pressure polymorphs of H2O have been studied extensively due to the abundance of ice phases in icy moons and, likely, vast number of extra-solar planetary bodies, with only select studies evaluating impurity-laden ices. In this study, ice formed from a 1.6mol percent KCl-bearing aqueous solution was studied up to 32.89±0.19GPa and 625K, and the incorporation of K+ and Cl− ionic impurities into the ice VII structure was documented. The compression data at 295K were fit with a third order Birch–Murnaghan equation of state and yielded a bulk modulus (KT0), its pressure derivative (KT0′), and zero pressure volume (V0) of 24.7±0.9GPa, 4.44±0.09, and 39.2±0.2Å3, respectively. The impurity-laden ice was found to be 6–8% denser than ice VII formed from pure H2O. Thermal expansion coefficients were also determined for several isothermal compression curves at elevated temperatures, and a PVT equation of state was obtained. The melting curve of ice VII with incorporated K+ and Cl− was estimated by fitting experimental data up to 10.2±0.4GPa, where melting occurred at 625K, to the Simon–Glatzel equation. The melting curve of this impurity-laden ice is systematically depressed relative to that of pure H2O by approximately 45K and 80K at 4 and 11GPa, respectively. A portion of the K+ and Cl− contained within the ice VII structure was observed to exsolve with increasing temperature. This suggests that an internal differentiating process could concentrate a K-rich phase deep within H2O-rich planets, and we speculate that this could supply an additional source of heat through the radioactive decay of 40K. Our data illustrate ice VII can incorporate significant concentrations of K+ and Cl− and increasing the possibility of deep-sourced and solute-rich plumes in moderate to large sized H2O-rich planetary bodies.

Neutron diffraction observations of interstitial protons in dense ice

The motif of distinct H2O molecules in H-bonded networks is believed to persist up to the densest molecular phase of ice. At even higher pressures, where the molecule dissociates, it is generally assumed that the proton remains localized within these same networks. We report neutron-diffraction measurements on D2O that reveal the location of the D atoms directly up to 52 GPa, a pressure regime not previously accessible to this technique. The data show the onset of a structural change at ∼13 GPa and cannot be described by the conventional network structure of ice VII above ∼26 GPa. Our measurements are consistent with substantial deuteron density in the octahedral, interstitial voids of the oxygen lattice. The observation of this "interstitial" ice VII form provides a framework for understanding the evolution of hydrogen bonding in ice that contrasts with the conventional picture. It may also be a precursor for the superionic phase reported at even higher pressure with important consequences for our understanding of dense matter and planetary interiors.

A comparison of ice VII formed in the H2O, NaCl–H2O, and CH3OH–H2O systems: Implications for H2O-rich planets

High-pressure H2O polymorphs, namely ice VI, ice VII, and ice X, are hypothesized to make up a considerable portion of the interiors of large icy satellites and select extra-solar planets. The incorporation of foreign ions or molecules into these high-pressure phases is possible through ocean–ice interaction, rock–ice interaction at depth, or processes that occurred during accretion. Recent research concerning the effects charged ions have on ice VII has shown that these ions notably affect the structure of ice VII (Frank et al., 2006; Klotz et al., 2009). This study was designed to determine the effects of a molecular impurity on ice VII and compare those effects to both pure H2O ice and ice with an ionic impurity. Ice samples were formed in this study via compression in a diamond anvil cell from either H2O, a 1.60mol% NaCl aqueous solution, a 1.60mol% CH3OH aqueous solution, or a 5.00mol% CH3OH aqueous solution and were compressed up to 71GPa at room temperature. Ice formed from pure H2O had no impurities whereas ices formed in the NaCl–H2O and CH3OH–H2O systems contained the impurities Na+ and Cl− and CH3OH, respectively. Pressure–volume relations were observed in situ by using synchrotron based X-ray diffraction and were used to determine the equations of state for ices formed in the H2O, NaCl–H2O and CH3OH–H2O systems. The data illustrate that ice VII formed from a NaCl-bearing aqueous solution exhibited a depressed volume when compared to that of H2O-only ice VII at any given pressure, whereas ice VII formed from CH3OH-bearing aqueous solutions showed an opposite trend, with an increase in volume relative to that of pure ice VII. The ices within planetary bodies will most likely have both ionic and molecular impurities and the trends outlined in this study can be used to improve density profiles of H2O-rich planetary bodies.

Stimulated Raman scattering of lattice translational modes in liquid heavy water

A study was conducted on stimulated Raman scattering (SRS) when laser-induced plasma is formed in heavy water by focusing an intense picosecond pulsed Nd:YAG laser beam with wavelength 532 nm at room temperature. An unexpected 280 cm(-1) low frequency SRS line attributed to the lattice translational modes is observed. This SRS line and the internal-mode SRS lines indicate that the ice VII structure is formed in heavy water under the condition of laser-induced shockwave production.

Site Disorder in Ice VII Arising from Hydrogen Bond Fluctuations

Recently, multisite models used for the refinement of neutron diffraction data have suggested that the structure of ice VII is quite unlike that of its ordered counterpart, ice VIII. We investigate the oxygen site disorder by modeling the site displacement, obtained from periodic DFT calculations, as a function of the local hydrogen bond network. Then, using graph invariants to describe hydrogen bond fluctuations in the thermodynamic limit, we perform statistical mechanical calculations using the oxygen site displacement model developed here. We find that the probability distribution of the oxygen atom about its perfect lattice site more closely resembles the 100 model rather than the recently suggested 111 model, although both models represent a simplification of the actual site distribution. We also find a unimodel distribution for the hydrogen bonded oxygen-oxygen distance and a trimodel distribution for the nearest nonbonded oxygen-oxygen distance with a peak separation of approximately 0.1 A.

The preparation and structure of salty ice VII under pressure

It is widely accepted that ice, no matter what phase, is unable to incorporate large amounts of salt into its structure. This conclusion is based on the observation that on freezing of salt water, ice expels the salt almost entirely as brine. Here, we show that this behaviour is not an intrinsic physico-chemical property of ice phases. We demonstrate by neutron diffraction that substantial amounts of dissolved LiCl can be built homogeneously into the ice VII structure if it is produced by recrystallization of its glassy (amorphous) state under pressure. Such 'alloyed' ice VII has significantly different structural properties compared with pure ice VII, such as an 8% larger unit cell volume, 5 times larger displacement factors, an absence of a transition to an ordered ice VIII structure and plasticity. Our study suggests that there could be a whole new class of 'salty' high-pressure ice forms.

Temperature induced immiscibility in the NaCl–H 2O system at high pressure

High-pressure polymorphs of H 2O are a major component in many outer planets, extra solar bodies, and icy satellites. This study sought to examine the influence of ionic impurities on the phase stability, thermal expansion, and melting curve of ice VII. Powder diffraction patterns of ice VII formed from pure H 2O and 5 wt.% NaCl aqueous solutions were taken at room temperature up to 11.1 ± 0.3 and 26.6 ± 0.4 GPa, respectively. Thermal expansions, α, of all ice VII samples were recorded and modeled up to the melting point of the samples. Ice VII formed from a NaCl-bearing aqueous solution at pressures greater than 2.2 GPa and less than 500 K can be indexed by ice VII only, whereas at temperatures greater than 500 K, diffraction lines indicative of halite (NaCl) are observed and become more intense with increasing temperature and only disappear at the melting point of the high-pressure ice. This phenomenon was observed in all NaCl-bearing ice samples that were heated to greater than 500 K. The melting curves of ice VII formed from pure H 2O and a 5 wt.% NaCl aqueous solution suggest that the presence of Na + and Cl − in the ice VII structure results in a depression of the melting curve by approximately 40 K. The exsolution of halite from the NaCl-doped ice VII and the depression of the ice VII melting curve suggest that the presence of ionic impurities in ice VII may promote the formation of a self-segregating zone deep within ice-rich bodies. This zone could initiate the formation of solute-rich melt pockets that may ascend toward the surface and result in surface manifestations such as solute-bearing aqueous vents, unexplained domes/diapirism, and/or salt-rich regions.

Molecular dynamics study of crystalline water ices

The behavior of structures of H2O crystalline ices Ih, Ic, XI, VII, VIII, VI is studied in molecular dynamics experiment using the potential offered by Poltev and Malenkov. The behavior of the system consisting of one of the two identical interpenetrating, but without any common hydrogen bonds, water frameworks comprising the ice VI structure is also simulated. As a result of simulations, the ice VII structure has collapsed, whereas other systems proved to be stable. The reasons of instability of the ice VII and previously studied ice IV structures in molecular dynamics experiments are discussed. Based on the simulation results of the above-mentioned ices and previous simulation of ices II, III, IX, IV, and XII, the general regularities of dynamic properties of water molecules in crystalline water ices are formulated. Unreliability of results obtained by molecular dynamics in the investigation of self-organizing processes in aqueous systems is shown.

Experimental study of the NaCl–H 2O system up to 28 GPa: Implications for ice-rich planetary bodies

Recent studies have hypothesized that high-pressure H 2O polymorphs, specifically Ice VI and Ice VII, make up a significant portion of the interiors of select outer planets and their moons; most notably the Galilean satellites, Saturn's Titan and possibly Neptune's moon Triton as well as potential H 2O-rich extra-solar bodies. Several of these bodies have been conjectured to contain subsurface salty H 2O waters; therefore, any potential ice phases in the interior of these satellites could have interacted extensively with the salty oceans. Raman spectroscopy and synchrotron radiation have been used previously to study the bonding structure and unit cell parameters of pure Ice VII. However, no data exist on the effect of salts on the unit cell parameters and volume of solid H 2O at high pressure. To obtain pertinent data for use in planetary physics, it is important to understand the effect of impurities on H 2O at high pressure. The NaCl–H 2O system was chosen as a first order approximation of H 2O-rich planetary bodies. The unit cell parameters and OH stretching frequencies of Ice VII formed from 5 and 10 wt.% NaCl–H 2O solutions were studied in detail up to 27 GPa at 298 K by using a diamond anvil cell, synchrotron X-ray radiation and Raman spectroscopy. The data indicate that, over the range in pressure and temperature of this study, the maximum solubility of solutes in Ice VII was not pressure dependent. Our data suggest that the maximum concentration of NaCl that can be incorporated into Ice VII at 298 K is 7.5 ± 2.5 wt.% (or 2.4 ± 0.8 mol% NaCl). Ice VII formed from a 5 wt.% NaCl–H 2O solution has a density that is up to 5% greater at any given pressure relative to the density of Ice VII formed from pure H 2O. Additionally, the bulk modulus, 26.2 ± 1.4 GPa, was found to be approximately 10–20% greater relative to Ice VII formed from pure H 2O. Relative OH stretching frequency shifts from Ice VII formed from the NaCl–H 2O solutions were compared to Ice VII formed from pure H 2O. Ice VII formed from a 5 wt.% NaCl–H 2O solution shows a systematic increase of approximately 15 rel cm −1 in the OH stretching frequency relative to pure Ice VII for any given pressure. We hypothesize that the incorporation of Na + and Cl − into the body centered cubic structure of Ice VII results in systematic variations in the intensive thermodynamic properties of Ice VII formed from low salinity solutions. These new data will provide an improved first order approximation of high-pressure H 2O phases found within H 2O-rich bodies and should be used to model the density profiles of H 2O-rich bodies.

Structure of Ice-VII and Ice-VIII: A Quantum Mechanical Study

Ab initio quantum mechanical methods have been used to investigate the structures of Ice-VII and Ice-VIII. In the case of proton-ordered Ice-VIII, accurate structural data enable us to benchmark the methodology. The disorder in H-bond orientations of Ice-VII causes difficulties in understanding its structure from either an experimental or a theoretical viewpoint. In this work, we explore the consequence of randomness of H-bond arrangements in Ice-VII by examining all ice-rule-allowed configurations in a unit cell of 16 water molecules. Noticeable relative displacement (ε) between the two sublattices, previously thought to exist only in Ice-VIII, was found in many of these configurations. We find a correlation between the energy of a particular configuration of Ice-VII and ε, an energy gap of 1.5 mhartree that separates a unique ground state from all the others. Our results have proven that ab initio calculations can be used to resolve ambiguities in interpreting neutron diffraction data of Ice-VII.

Structural change of heavy water by laser-induced plasma generation

Stimulated Raman scattering (SRS) of –OD stretching vibration has been studied when laser-induced plasma is formed in heavy water by focusing an intense picosecond pulsed Nd:YAG laser beam (wavelength 532 nm, pulse duration 80 ps) at room temperature. The main peak of the SRS shows a remarkable blue-shift and an asymmetric intensity distribution. The Raman shift and active modes of the SRS indicate that the ice VII structure is formed and lasts 20–30 ps in the focal volume of the pumping beam in heavy water.

Spectroscopic Analysis of Stimulated Raman Scattering in the Early Stage of Laser-Induced Breakdown in Water

We have investigated laser-induced breakdown (LIB) in water by measuring stimulated Raman scattering (SRS) induced in the early stage of the LIB phenomenon. The SRS is emitted before the breakdown plasma continuum emission becomes dominant and lasts about 80 picoseconds. The time-resolved spectra of the SRS indicate that ice VII structure is formed partially and inhomogeneously in the focal volume of the exciting beam on a picosecond time scale. The mechanism of the SRS inducement is also discussed.

Compression and bonding of ice VII and an empirical linear expression for the isothermal compression of solids

The volume of ice VII was measured between 3.0 and 8.0 GPa at 298 K using a high−pressure x−ray diffraction technique. The specific volume (0.708, +0.023, −0.012 cm3/g), the isothermal bulk modulus (12.54±0.27 GPa), and the modulus’ pressure derivative (5.56±0.14) for ice VII at 298 K and zero pressure were determined using an empirical equation for isothermal compression. That equation is [P V02/(V0 − V)]1/2 = CT + ST [P (V0 − V)]1/2, where CT and ST are constants, V0 is the ambient volume, and V is the volume at pressure P. This linear relation, which describes the state of nonporous materials along their Hugoniots, is shown to characterize the isothermal compression of solids as well as does the Murnaghan equation. The zero pressure, 298 K oxygen−oxygen distance in ice VII extrapolated from the present data and a simple bonding model for the hydrogen−bonded oxygen atoms strongly support Kamb’s description of the ice VII structure as two interpenetrating ice Ic frameworks.

Self-diffusion coefficients and rotational correlation times in polar liquids. VI. Water

A new model for liquid water is presented. As a first approximation (A) the liquid is considered as a quasi-hard-sphere fluid and the scaled particle theory is employed to calculate the work (w) required to create a vacancy of molecular dimension using the observed density of water. In the second approximation (B) a quasilattice model is invoked which is based on the Ice VII structure and the density and isothermal compressibility of water are computed using the values of w calculated in approximation A. A final approximation (C) is based on the Ice Ic structure. In this approximation the temperature and pressure dependence of self-diffusion, viscosity, and nuclear relaxation times are evaluated. The agreement of the model with experiment is good and only a few parameters enter into the calculation, and these have magnitudes that are physically reasonable. Results of molecular dynamics calculations on water obtained by Rahman are presented which are interpreted to show that molecular rotation in water occurs in rather large excursions of relatively short time duration. For times between the large angular reorientations, librational motions occur which do not result in a large amount of net molecular reorientation. It is proposed that the large amplitude reorientations result from hard molecular collisions. These results confirm the conclusions of earlier work on other liquids. The anisotropy of the rotational correlation time of rank 2 (τ2) in an asymmetric top molecule is derived based on the finite step rotation hypothesis. Available experimental values of τ2 for liquid water are summarized and compared with the results of the model.