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Ice_VI

Ice VI is a high-pressure polymorph of water ice, distinguished by its tetragonal crystal structure (space group P4₂/nmc) and a density of 1.31 g/cm³, forming a unique self-clathrate framework of two interpenetrating, hydrogen-bonded networks of water molecules with disordered protons.^[1]^[2] It is stable across a broad pressure range of approximately 0.6 to 2.1 GPa and temperatures from near 0 K up to 81°C, bordering the liquid–ice VI–ice VII triple point, allowing recovery at ambient conditions upon rapid quenching to low temperatures.^[3] The unit cell of Ice VI measures a = 6.27 Å and c = 5.79 Å, accommodating 10 water molecules, and its structure enables it to persist as a proton-disordered phase even at relatively low temperatures compared to other high-pressure ices.^[1] Upon cooling below approximately 129–100 K under pressure, Ice VI can transition to the hydrogen-ordered phase known as Ice XV, highlighting its role in the complex hydrogen-ordering behavior of water's polymorphic landscape.^[2] This phase has been spectroscopically identified in natural diamond inclusions, underscoring its geological significance in high-pressure environments such as Earth's mantle or extraterrestrial bodies.^[2] Recent experimental advances using synchrotron X-ray diffraction have uncovered multiple dynamic freezing–melting pathways leading to Ice VI formation at room temperature within its stability field, particularly under supercompression above 1.6 GPa, revealing hidden structural evolutions from high-density to very-high-density amorphous water intermediates.^[4] These findings enhance understanding of water's phase behavior under extreme conditions, with implications for planetary interiors where Ice VI-like structures may prevail.^[4]

Discovery and Overview

Historical Discovery

Ice VI was first identified by Percy Williams Bridgman in 1912 during piston-cylinder high-pressure experiments on water, where he observed a distinct solid phase forming at pressures around 1 GPa and temperatures up to 80°C. Bridgman's apparatus utilized self-sealing packing to achieve these conditions, allowing him to explore the phase behavior of water beyond ordinary ice Ih.^[5] This discovery formed part of Bridgman's broader investigations into high-pressure physics, which revealed multiple polymorphs of ice and contributed to the early classification of high-pressure phases, including Ice VI as one of the denser forms stable above room temperature.^[5] His systematic mapping of water's phase diagram under compression laid the groundwork for understanding anomalous behaviors in condensed matter. Bridgman's work on high pressures, culminating in the 1946 Nobel Prize in Physics, highlighted the role of these experiments in advancing geophysics and materials science, though early efforts were hampered by equipment limitations such as frequent vessel failures and the need for robust alloys to prevent leaks.^[5] In the early 20th century, synthesizing high-pressure ices like Ice VI involved overcoming imprecise pressure calibration and initial misinterpretations of phase boundaries, often due to metastable states and incomplete equilibration.^[5] Key milestones included Tammann's earlier 1900 identification of Ice II, which contextualized Bridgman's findings, and subsequent refinements by Bridgman in the 1930s using enhanced pressure cells that clarified stability ranges for phases including Ice VI.

General Characteristics

Ice VI is a proton-disordered, tetragonal high-pressure polymorph of water ice, characterized by its fully hydrogen-bonded structure with orientational disorder among water molecules.^[6] It exhibits body-centered tetragonal symmetry with space group P4₂/nmc and consists of two interpenetrating sublattices of water molecules. This phase has a density of approximately 1.31 g/cm³, making it denser than liquid water under standard conditions. Ice VI is stable over a pressure range of 0.6–2.2 GPa and temperatures from approximately 100 K (−173 °C) to 355 K (82 °C), positioning it as an accessible high-pressure form that can be synthesized by crystallizing liquid water under these conditions.^[6]^[2] Unlike many high-pressure ices, it is recoverable to ambient conditions in a metastable state, often preserved by rapid quenching in liquid nitrogen.^[6] As one of over 20 known crystalline ice phases, Ice VI highlights the structural diversity of water under compression.^[4] In the phase diagram of water, Ice VI acts as a crucial intermediate phase at moderate high pressures, bridging lower-pressure forms like Ice V and higher-pressure phases such as Ice VII.^[6]

Crystal Structure

Lattice and Framework

Ice VI exhibits a body-centered tetragonal unit cell with space group symmetry P4₂/nmc (No. 137). At approximately 1 GPa and room temperature, the lattice parameters are a = 6.1990(14) Å and c = 5.698(3) Å.^[7] The crystal structure comprises two independent, interpenetrating sublattices of H₂O molecules, each forming a self-interpenetrating diamond-like network. In these networks, oxygen atoms are tetrahedrally coordinated by four hydrogen bonds, mimicking the topology of diamond but with water molecules at the vertices.^[7] Notably, the sublattices do not share hydrogen bonds with one another, which contributes to the overall stability of the framework under compression by allowing independent flexibility while maintaining dense packing.^[7] This tetragonal arrangement, often depicted in diagrams as two offset, cage-like frameworks interlocked without direct bonding, contrasts with the cubic body-centered structure of Ice VII—where sublattices are more symmetrically equivalent—and the hexagonal wurtzite-like lattice of Ice Ih, highlighting Ice VI's unique adaptation to moderate pressures.^[7]^[8]

Hydrogen Bonding and Disorder

Ice VI exhibits a proton-disordered structure in which hydrogen atoms occupy positions randomly distributed among the possible sites along the oxygen-oxygen bonds, while strictly adhering to the Bernal-Fowler ice rules. These rules stipulate that each oxygen atom forms two short covalent O-H bonds and two longer hydrogen bonds with neighboring oxygen atoms, ensuring a tetrahedral coordination for every water molecule. This disorder arises within the fixed oxygen lattice framework, allowing for a large number of equivalent configurations that contribute to the phase's thermodynamic properties.^[9]^[10] The hydrogen-ordered counterpart to Ice VI is Ice XV, discovered in 2009, which forms upon cooling Ice VI under pressures around 1 GPa. In Ice XV, the protons align in an antiferroelectric arrangement, with ordering occurring below approximately 130 K, resulting in a more stable, low-entropy structure compared to the disordered parent phase. This ordering eliminates the random proton positions, leading to distinct crystallographic symmetry and potential applications in understanding pressure-induced phase behaviors in water.^[11]^[9] The proton disorder in Ice VI significantly influences its dielectric properties, primarily through the orientational freedom of water molecules, which enables dipole reorientation under an electric field. This results in a high static dielectric constant, typically on the order of 100 near room temperature, far exceeding that of ordered ice phases and reflecting the material's ability to exhibit dielectric relaxation. Such behavior underscores the role of hydrogen dynamics in the electrical response of high-pressure ices.^[12] Theoretical descriptions of the disorder in Ice VI often employ the Pauling entropy approximation, which estimates the configurational entropy due to the multiplicity of hydrogen arrangements satisfying the ice rules. For a fully disordered phase like Ice VI, this yields a residual entropy per water molecule of S = R \ln(3/2), where R is the gas constant, capturing the approximate six possible orientations per molecule reduced by local constraints. This model provides a foundational understanding of the entropy contribution from proton disorder, with more detailed calculations confirming its close applicability to Ice VI.^[10]^[13]

Physical Properties

Density and Mechanical Properties

Ice VI exhibits a density of 1.31 g/cm³ at approximately 1 GPa and 273 K, reflecting its compact tetragonal structure formed under high-pressure conditions.^[1] This density value surpasses that of ambient liquid water and increases further with applied pressure, attributable to the progressive reduction in intermolecular spacing and enhanced hydrogen bonding efficiency within the lattice.^[8] The compressibility of Ice VI is characterized by a bulk modulus of approximately 14 GPa, indicating moderate resistance to uniform compression compared to other ice phases.^[8] Its equation of state is commonly described by the third-order Birch-Murnaghan model, which relates pressure P to volume V as:

P(V) = \frac{3B}{2} \left[ \left( \frac{V}{V_0} \right)^{-7/3} - \left( \frac{V}{V_0} \right)^{-5/3} \right] \left\{ 1 + \frac{3}{4} (B' - 4) \left[ \left( \frac{V}{V_0} \right)^{-2/3} - 1 \right] \right\},

where B is the bulk modulus, V_0 is the reference volume, and B' \approx 4 represents the pressure derivative of the bulk modulus; fits to experimental data yield V_0 \approx 14.17 cm³/mol and B \approx 14.05 GPa at reference conditions.^[8]^[14] Due to its tetragonal symmetry, Ice VI displays elastic anisotropy, with elastic constants such as C_{11} = 32.8 GPa, C_{33} = 27.8 GPa, C_{12} = 11.8 GPa, C_{13} = 14.7 GPa, C_{44} = 6.3 GPa, and C_{66} = 5.9 GPa measured at 1.23 GPa,^[15] leading to directional variations in longitudinal and shear sound velocities (e.g., higher along the c-axis). This anisotropy contributes to its overall mechanical stability under shear and uniaxial stress. In comparison to liquid water under similar high-pressure conditions (where liquid density reaches about 1.05–1.10 g/cm³ at 1 GPa and 273 K), Ice VI's higher density ensures it sinks, promoting gravitational segregation in pressurized aqueous environments such as planetary interiors.^[1]^[8]

Thermal and Optical Properties

Ice VI possesses a thermal conductivity of approximately 1.6 W/m·K at 0 °C under pressures around 0.7 GPa, with a small positive dependence on pressure that results in relatively stable values across its stability range.^[16] This is lower than that of ordinary ice Ih (2.2 W/m·K at the same temperature), reflecting the denser packing and altered phonon scattering in the high-pressure phase.^[17] The specific heat capacity near 273 K is about 2.16 J/g·K, consistent with values for other dense ice polymorphs and enabling efficient heat storage in planetary interiors.^[18] Due to its tetragonal crystal symmetry, thermal expansion in Ice VI is anisotropic, with linear coefficients typically on the order of 10^{-5} K^{-1} along the a-axis and roughly twice that along the c-axis, contributing to volume changes under temperature variations within its stability field.^[19] Optically, Ice VI is transparent in the visible spectrum, allowing transmission of light with minimal absorption, which facilitates spectroscopic studies of high-pressure environments.^[20] Its birefringence arises from the tetragonal structure, yielding ordinary and extraordinary refractive indices of approximately n_o = 1.35 and n_e = 1.32 at visible wavelengths (around 532 nm) and pressures up to 2 GPa at room temperature, with both increasing modestly under compression.^[20] Raman spectroscopy reveals characteristic vibrational modes, including O-H stretching peaks centered near 3200 cm^{-1}, which broaden due to the disordered hydrogen network and shift slightly with isotopic substitution or pressure.^[21] The dielectric response of Ice VI involves a Debye-type relaxation process, with a characteristic relaxation time on the order of 10^{-11} s at ambient conditions within its phase space, reflecting rapid orientational dynamics of water molecules.^[22] This short timescale is modulated by the inherent hydrogen disorder, as detailed in studies of its bonding framework, enabling the material to exhibit significant permittivity changes under applied fields.^[23]

Thermodynamic Stability

Stability Conditions

Ice VI is thermodynamically stable in the pressure-temperature range of approximately 0.63 to 2.21 GPa and 0 to 81°C, where it coexists in equilibrium with liquid water along its melting curve.^[24] The lower pressure boundary is defined by the ice V–ice VI–liquid triple point at 0.6324 GPa and 0.16°C, while the upper boundary corresponds to the ice VI–ice VII–liquid triple point at 2.21 GPa and 81°C.^[25] Within this envelope, ice VI exhibits the global minimum in Gibbs free energy relative to other ice polymorphs and the liquid phase, ensuring its thermodynamic preference.^[25] The slopes of these phase boundaries are governed by the Clapeyron equation, \frac{dP}{dT} = \frac{\Delta H}{T \Delta V}, where \Delta H is the enthalpy change and \Delta V is the volume change across the transition.^[25] For the ice V–ice VI boundary, positive \Delta H and negative \Delta V (due to the denser structure of ice VI) yield a positive slope, consistent with experimental melting curves. Similarly, the ice VI–liquid boundary shows a shallow positive slope, reflecting small volume contraction upon freezing. Enthalpy minima for ice VI occur near the triple points, with fusion enthalpies around 100 J/g derived from integrated Clapeyron relations along the coexistence lines.^[25] At ambient pressure, ice VI is metastable when rapidly quenched to cryogenic temperatures, such as below 77 K, where high nucleation barriers to the stable ice Ih phase prevent immediate transformation.^[26] Upon warming, it undergoes an irreversible transition to cubic ice Ic followed by hexagonal ice Ih, with rates increasing markedly above -100°C and associated enthalpy release of approximately 67 J/g (16 cal/g). Lifetime estimates indicate stability for days to weeks at liquid nitrogen temperatures (77 K), limited by thermal activation over nucleation barriers estimated at 50–100 kJ/mol from kinetic studies, but shortening to minutes at -50°C due to lowered barriers.^[26] Impurities and confinement can perturb this stability field. For instance, dissolved NaCl at 1 molal shifts the ice VI–ice VII–liquid triple point to higher pressures (∼2.57 GPa) and lower temperatures (∼67°C), expanding the ice VI regime slightly while altering enthalpy changes via solute incorporation into the lattice.^[27] In nanoporous media, such as silica pores of 2–5 nm diameter, confinement stabilizes ice VI-like structures at pressures below 0.6 GPa by suppressing nucleation of lower-density phases, with effective volume changes reduced by up to 20% due to interfacial effects, extending metastability lifetimes by orders of magnitude.^[28]

Phase Transitions

Ice VI transforms to Ice V upon decompression to lower pressures, approximately 0.63 GPa, via a reconstructive mechanism that requires the breaking and reformation of hydrogen bonds to rearrange the oxygen lattice from body-centered cubic in Ice VI to monoclinic in Ice V.^[29] This process is sluggish due to the significant structural reorganization, often resulting in kinetic barriers that prevent rapid equilibration under dynamic compression or decompression conditions.^[30] In contrast, the transition from Ice VI to Ice VII occurs at higher pressures around 2.1 GPa and features a relatively small volume change of ΔV ≈ -1.05 cm³/mol, reflecting the shared body-centered cubic oxygen sublattice between the two phases.^[31] The similarity in coordination environments—both phases exhibit sixfold hydrogen bonding per oxygen—facilitates faster kinetics through a more displacive mechanism, with minimal bond breaking compared to the Ice V transition.^[32] The melting curve of Ice VI rises nearly linearly with increasing pressure, with temperatures increasing from about 273 K at 0.63 GPa to 354 K at 2.21 GPa.^[24] Laboratory observations of Ice VI phase transitions reveal notable hysteresis effects, including superheating of the solid phase by up to 10 K above the equilibrium melting temperature and supercooling of the melt, which arise from the nucleation challenges in these high-pressure environments. Defects such as dislocations and stacking faults play a critical role in lowering energy barriers for nucleation, enabling the eventual progression of the transformation despite the metastable persistence of phases.^[33]^[34]

Comparisons to Other Phases

Relation to Ice V and Ice VII

Ice VI shares proton disorder with its neighboring high-pressure phases, Ice V and Ice VII, where hydrogen atoms are distributed randomly along the bonds while satisfying the Bernal-Fowler rules, leading to paraelectric behavior in all three. Unlike Ice V, which features a monoclinic structure with a single tetrahedral framework of oxygen atoms, Ice VI adopts a tetragonal arrangement consisting of two interpenetrating, self-clathrate frameworks that are rotated relative to each other.^[35]^[36] In contrast, Ice VII exhibits a simpler cubic structure with body-centered cubic-like oxygen positions and also forms a self-clathrate, but with more compact hydrogen bonding geometry. The densities of these phases increase progressively with pressure, reflecting enhanced coordination efficiency: Ice V at approximately 1.27 g/cm³, Ice VI at 1.31 g/cm³, and Ice VII at 1.50 g/cm³ under their respective stability conditions near room temperature.^[35] This progression arises from the structural compression, where Ice V's looser packing gives way to the denser interpenetrating lattices in Ice VI and the even tighter bcc arrangement in Ice VII. Phase transitions involving Ice VI differ in their mechanisms and barriers. The transition from Ice V to Ice VI is characterized by a lower-energy diffusive process involving proton rearrangements and framework adjustments at pressures around 0.6–1.0 GPa.^[37] In comparison, the Ice VI to Ice VII transition at higher pressures (above ~2.1 GPa) features a higher barrier and proceeds via a martensitic-like mechanism, with rapid shear-dominated structural shifts that can be influenced by compression rates.^[37] Spectroscopically, Ice VI is distinguished by unique vibrational modes in the 500–800 cm⁻¹ range, corresponding to lattice vibrations influenced by its interpenetrating frameworks and hydrogen disorder, which differ from the broader or shifted bands observed in Ice V and Ice VII.^[38] These modes, probed via Raman and inelastic neutron scattering, highlight the distinct hydrogen-bonding dynamics in Ice VI compared to the single-framework vibrations in Ice V and the more symmetric ones in Ice VII.^[38]

Broader Context in Ice Polymorphs

Ice VI represents one of 21 experimentally confirmed crystalline polymorphs of water ice, a family that continues to expand with post-2019 discoveries such as the superionic Ice XVIII, observed under extreme pressures exceeding 100 GPa and temperatures above 2000 K, where oxygen ions form a lattice while hydrogen ions diffuse freely like a liquid. Similarly, Ice XIX, identified in 2021 as a partially hydrogen-ordered phase related to Ice VI itself, emerges at pressures around 1-2 GPa and low temperatures, featuring an orthorhombic structure that deviates from full ordering.^[39] The most recent addition, Ice XXI discovered in 2025, forms metastably at room temperature under rapid compression to over 20,000 times atmospheric pressure, exhibiting a unique atomic arrangement distinct from prior phases and persisting briefly outside equilibrium conditions.^[40] These advancements underscore the structural diversity of ice, with 21 phases now documented across varied pressure-temperature regimes.^[41] In the high-pressure domain above 0.6 GPa, Ice VI plays a pivotal role by bridging low-density, open-cage structures exemplified by ambient Ice Ih (density ~0.92 g/cm³) to the high-density, close-packed configurations like Ice X (density ~1.5 g/cm³ at >100 GPa), where its body-centered cubic framework of water molecules accommodates partial hydrogen disorder while maintaining tetrahedral coordination.^[13] This intermediate positioning highlights Ice VI's significance in the P-T phase diagram, where it dominates a broad stability field up to ~2.2 GPa and 81 °C, facilitating transitions among medium-pressure ices and influencing mantle dynamics in icy satellites.^[42] Unlike low-pressure hexagonal or cubic ices, Ice VI's self-interstitial topology allows for greater compressibility, evolving toward denser proton-ordered states under further compression. Hydrogen ordering in ice polymorphs often yields analogs with distinct electrical properties, and for Ice VI, this manifests as Ice XV, its fully ordered counterpart stable below ~130 K at 0.8-1.5 GPa, featuring a triclinic structure with antiferroelectric hydrogen ordering.^[11] This pairing exemplifies a broader trend across phases, where ordered variants like Ice XI (from Ice Ih) exhibit ferroelectricity due to net dipole moments from aligned hydrogen bonds, contrasting with typically antiferroelectric arrangements in others such as ordered Ice VII. In Ice XV, subtle deviations from perfect antiferroelectricity suggest potential ferroelectric tendencies under doping or strain, contributing to emerging interest in polar ice phases for geophysical modeling.^[43] Prior literature on ice polymorphs has shown incomplete integration of 2020s discoveries, such as Ice XIX and Ice XXI, which challenge established phase boundaries and necessitate revisions to the water phase diagram, particularly in the medium-pressure regime where multiple disordered-ordered pairs coexist.^[39] For instance, Ice XXI's formation via non-equilibrium pathways from supercompressed water implies additional metastable regions adjacent to Ice VI's field, potentially altering predictions for hydrogen ordering kinetics and phase coexistence in planetary interiors.^[44] These gaps highlight ongoing refinements, with computational screenings now predicting dozens of candidate structures beyond the confirmed 21, emphasizing the dynamic evolution of ice science.^[41]

Applications and Significance

Planetary Science Implications

Ice VI is anticipated to occur within the outer mantles of the ice giant planets Uranus and Neptune under pressures of 1–2 GPa and temperatures ranging from approximately 100 to 250 K, conditions where it may form as a stable high-pressure phase of water ice. Recent interior models from the 2020s indicate that high-pressure ices constitute a substantial portion of these planets' mantles when combined with other volatiles like ammonia and methane, though specific phases like Ice VI may be limited to shallower layers. As of 2025, models challenge the traditional 'ice giant' classification, suggesting rock-dominated compositions with potentially lower overall ice content.^[45]^[46] In the context of exoplanet habitability, Ice VI plays a key role in water-rich worlds by allowing convective transport of salts and nutrients through high-pressure ice layers, potentially sustaining subsurface oceans beneath thick icy shells. On ocean-dominated exoplanets, the solubility of salts in Ice VI—up to 2.5 wt% NaCl—prevents impermeable barriers at the ocean-ice interface, enabling geochemical cycles that could support life in deep, high-pressure environments.^[47] Post-2019 advancements, including integration of Juno mission data revealing Jupiter's dilute water distribution and low ice content in its interior, have refined models of volatile layering in giant planets and highlighted discrepancies in pre-2020 predictions for ice phases. Similarly, James Webb Space Telescope observations of icy exomoons and waterworld candidates have exposed compositional complexities that underscore gaps in earlier simulations of high-pressure ice stability and mantle dynamics.^[48]^[49]

Potential Technological Uses

Ice VI's interpenetrating body-centered cubic lattice structure enables the formation of hydrogen-filled variants, such as the C0 phase, where hydrogen molecules occupy interstitial cages, offering potential for hydrogen storage with capacities exceeding 5 wt% at moderate pressures around 25 bar and cryogenic temperatures below 80 K.^[50] This approach leverages the phase's ability to reversibly adsorb and desorb hydrogen, with adsorption enthalpies ranging from 2 to 5 kJ/mol, making it suitable for cryo-adsorption systems in fuel cell applications, though practical implementation requires temperatures below 130 K for stability.^[51] In high-pressure experimentation, Ice VI functions as a reliable pressure-transmitting medium within diamond anvil cells, providing hydrostatic conditions up to approximately 2 GPa due to its mechanical stability and high optical transparency, which allows for unobstructed in situ optical and spectroscopic observations of samples.^[52] Its formation from water under compression ensures uniform pressure distribution without significant shear stresses, enhancing the accuracy of measurements in studies of material properties at elevated pressures.^[53] The metastability of recovered porous forms of Ice VI, such as Ice XVII obtained by emptying hydrogen from the C0 phase, enables applications in cryogenic engineering for ultra-cold storage in biotechnology, remaining stable at ambient pressure and temperatures below 120 K.^[51] This property supports the preservation of sensitive biological materials by providing a low-density, channel-structured matrix that maintains structural integrity under liquid nitrogen conditions, potentially improving efficiency in sample cryopreservation.^[51] Emerging research in the 2020s has examined salt-doped variants of Ice VI as solid-state electrolytes for all-solid-state batteries, demonstrating lithium-ion conductivities of 10^{-3} S cm^{-1} at room temperature and wide electrochemical stability windows up to 3.8 V, formed via pressure-induced phase transitions from aqueous salt solutions.^[54] These developments highlight challenges in scalability and integration into practical devices, as noted in recent studies on high-pressure ice phases for energy storage.^[54]

Research Approaches

Experimental Methods

Ice VI is primarily synthesized in laboratory settings using piston-cylinder apparatuses, which can achieve moderate pressures up to approximately 3 GPa suitable for its stability range.^[55] These devices typically involve enclosing deionized water in a polytetrafluoroethylene (PTFE) sample capsule, pre-cooling the assembly to around 250 K to initiate freezing, and then applying pressure while controlling temperature to form the phase.^[55] Hydrostatic conditions are ensured by surrounding the sample with pressure-transmitting media such as 16:3:1 methanol-ethanol-water mixtures, which remain fluid up to about 10 GPa and minimize non-uniform stress on the ice lattice.^[56] For in-situ structural characterization, diamond anvil cells (DACs) are employed to generate pressures up to 2 GPa, allowing direct observation of Ice VI formation and properties under load. These cells facilitate the growth of single crystals from distilled water loaded with a metal gasket, enabling high-resolution studies without sample recovery.^[57] When combined with neutron diffraction, DACs have been used at facilities like the Institut Laue-Langevin (ILL) for powder and single-crystal analyses of deuterated Ice VI, revealing hydrogen disorder and lattice parameters under stability conditions.^[58] Similarly, experiments at the Spallation Neutron Source (SNS) have utilized DACs on the SNAP beamline to refine Ice VI structures quantitatively, leveraging high neutron flux for precise bond length determinations.^[59] Post-2015 advancements include synchrotron X-ray diffraction for monitoring real-time phase transitions involving Ice VI, such as its formation from liquid water during dynamic compression in DACs.^[60] These techniques, often at facilities like ESRF or APS, provide time-resolved snapshots of lattice evolution at rates exceeding 1 GPa per second, capturing intermediate states during Ice VI to Ice VII transitions.^[61] Recent 2025 studies have further advanced this using dynamic diamond anvil cells (dDACs) with X-ray free-electron lasers (XFEL) at the European XFEL (EuXFEL), achieving compression rates up to 120 GPa/s at room temperature and pressures of 0.6–2.0 GPa. These experiments, combined with synchrotron diffraction at PAL and PETRA III, micro-Raman spectroscopy, and high-speed imaging, have revealed at least five freezing-melting pathways for Ice VI, including transitions through the metastable body-centered tetragonal phase Ice XXI (density 1.413 g/cm³).^[4] Cryogenic cooling protocols, involving rates of 0.2–40 K/min down to 80 K under pressure, have enabled the study of ordered variants like Ice XV and Ice XIX by promoting hydrogen sublattice ordering in doped samples.^[42] To recover metastable Ice VI samples for ex-situ analysis, rapid quenching techniques are applied, such as immersion in liquid nitrogen at cooling rates of ~40 K/min while maintaining pressure, which kinetically traps the phase below its equilibrium boundary.^[62] Post-recovery purity is verified using differential scanning calorimetry (DSC), where scans detect endothermic peaks indicative of phase purity or residual disorder, with transition enthalpies around 1400 J/mol confirming Ice VI integrity.^[62] Additionally, 2025 molecular dynamics simulations have explored the growth rates of Ice VI under pressure, showing rates similar to Ice Ih and minor pressure effects, using adapted Wilson-Frenkel theory for phase-specific predictions.^[63]

Theoretical and Computational Studies

Theoretical and computational studies of Ice VI have primarily employed ab initio density functional theory (DFT) to optimize its structure and compute phonon spectra, providing insights into its vibrational properties and stability. Using the generalized gradient approximation with the RPBE functional in the CASTEP code, researchers constructed a 2×2×2 supercell of 80 water molecules to model the hydrogen-disordered structure, optimizing lattice parameters at 2 GPa to achieve a density of 1.30 g/cm³, closely matching experimental values of 1.33 g/cm³. Phonon density of states calculations revealed distinct vibrational modes, including low-energy translational bands (peaking at 179.5–294.6 cm⁻¹) attributed to non-hydrogen-bonded, two-bonded, and four-bonded oxygen environments, with the translational band blue-shifted and stretching band red-shifted relative to the ordered counterpart, Ice XV. These spectra elucidate the origins of far-infrared peaks in inelastic neutron scattering data, confirming the nondegenerate phonon states due to disorder.^[38] Molecular dynamics (MD) simulations, often using the TIP4P/2005 water model, have been instrumental in exploring Ice VI's structure, phase stability, and hydrogen disorder. Direct coexistence simulations between fluid and solid phases demonstrate Ice VI's stability in the phase diagram up to pressures around 20 kbar at low temperatures, with the model accurately reproducing melting lines consistent with the Clausius-Clapeyron equation. These simulations highlight the proton-disordered nature, where hydrogen positions satisfy the ice rules but exhibit dynamic fluctuations. Further MD studies with TIP4P/2005 reveal hydrogen diffusion coefficients on the order of D_H \approx 10^{-9} m²/s at 300 K, reflecting limited proton mobility within the disordered framework before transitioning to higher-pressure phases. Validation against experimental densities and Raman spectra confirms the model's predictive power for structural optimization.^[64]^[65] Path-integral Monte Carlo (PIMC) methods incorporate quantum effects to investigate proton delocalization and ordering in Ice VI and related polymorphs. By treating water molecules as rigid rotors in the TIP4P/2005 potential, PIMC simulations quantify nuclear quantum delocalization, which enhances proton positional uncertainty and influences phase boundaries. For the proton-ordered counterpart Ice XV, these quantum-inclusive approaches predict an ordering transition temperature around 102 K, aligning with experimental observations of dielectric anomalies and heat capacity changes, thereby bridging classical MD limitations in capturing zero-point motion. Such computations underscore quantum effects' role in stabilizing the disordered state of Ice VI at ambient conditions.^[66] Recent advancements from 2021 to 2025 have extended theoretical models to superionic conduction near the Ice VI–Ice VII boundary and refined phase diagrams using machine learning potentials (MLPs). MLPs trained on PBE-DFT data enable large-scale simulations of high-pressure ices, revealing gradual increases in proton diffusivity (exceeding $10^{-8} m²/s) as Ice VI approaches Ice VII conditions above 40 GPa and 1000 K, marking the onset of superionic behavior where protons diffuse through a body-centered cubic oxygen lattice. A deep potential model, benchmarked against ab initio calculations, maps the full water phase diagram up to 50 GPa, accurately locating Ice VI's stability field and nucleation barriers while filling gaps in pre-2020 empirical potentials. These developments enhance predictions for spectroscopic signatures and thermodynamic properties, with brief cross-validation against neutron diffraction data affirming structural fidelity.^[37]^[67] In 2025, NPH ensemble MD simulations using TIP4P/Ice have efficiently determined the Ice VI phase diagram at 1.19 GPa, tracking temperature and energy evolution for stability assessment.^[68] Deep potential-driven explorations have further probed Ice VI/XV structures, identifying spiral and interpenetrating hydrogen-bond networks in polymorphs.^[41] Tools like IceCoder, developed in 2024, now enable identification of Ice VI phases in MD trajectories by extending classification to high-pressure ices and hydrates.^[69]

References

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[PDF] IDENTIFICATION OF ICE VI ON THE HUGONIOT OF ICE In
Using this value of y and assuming a value of 2.16 kJ/(kg K) for the specific heat of ice VI at constant volume, shock and post-shock temperatures of 400 ...
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Thermal expansion anomaly of ice VI related to the order–disorder ...
Feb 15, 1979 · Thermal expansion anomaly of ice VI related to the order–disorder transition Available. O. Mishima;. O. Mishima. Department of Material Physics ...
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Interferometric measurements of refractive index and dispersion at ...
Mar 10, 2021 · New data on liquid water and ice VI up to 2.21 GPa at room temperature illustrate how higher precision measurements of the index and its optical ...
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Vibrational Spectra of the Ices. Raman Spectra of Ice VI and Ice VII
The Raman spectra at 77°K and atmospheric pressure of Ices VI and VII made from both H2O and D2O have been obtained in the O–H and O—D stretching regions ...Missing: peaks | Show results with:peaks
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Dielectric properties of ice VI at low temperatures - AIP Publishing
Jun 1, 1976 · The relaxation time follows an Arrhenius law with constant activation energy down to the lowest temperatures, in contrast to ice Ih, in which ...Missing: s | Show results with:s
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Experiments indicating a second hydrogen ordered phase of ice VI
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 ...
[24]
International Equations for the Pressure Along the Melting and ...
May 1, 1994 · ... ice VI, and ice VII, which only contain one to three fitted coefficients, cover the pressure range from the ''normal'' triple point to 20000 MPa ...
[25]
TRANSFORMATIONS OF ICE VI AND ICE VII AT ATMOSPHERIC ...
The rates of transformation depend on the temperature. The heat evolved is 16 ± ~2 cal/g for ice VI and 31 ± ~4 cal/g for ice VII.
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[PDF] The science of Solar System ices (ScSSI)
cates that VI-VII-liquid triple point lies at 67±2 °C and. 2.57±0.1 GPa at the studied bulk composition. The addition of 1 molal NaCl to H2O thus results in ...<|control11|><|separator|>
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Fluids and Electrolytes under Confinement in Single-Digit Nanopores
Mar 10, 2023 · Confined fluids and electrolyte solutions in nanopores exhibit rich and surprising physics and chemistry that impact the mass transport and energy efficiency.
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The Raman spectra of ice V and ice VI and evidence of partial ...
We present the Raman spectra of ice V at a pressure of 0.54 GPa and ice VI at 0.68 GPa in the temperature range 250 to 83 K. Features in the lattice ...Missing: spectroscopy peaks
[29]
Hexagonal ice transforms at high pressures and compression rates ...
Dec 10, 2009 · ... ice V is observed in the stability fields of ices II and VI. Only at much higher pressures of > 1 GPa the stable phase ice VI grows from ice V.
[30]
[PDF] Explanation of the Phase Anomalies of Water (P1-P12)
volume when ice Ih changes to ice II (ΔV = 3.92 cm3 ... cm3 mol-1), ice V changes to ice VI (ΔV = 0.7 cm3 mol-1), and ice VI changes to ice VII (ΔV = 1.05.
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[PDF] Modeling the Ice VI to VII Phase Transition - Notre Dame Physics
Jul 31, 2009 · This project focuses on two forms: Ice VI (space group P42/nmc) and Ice ... (Equivalently, we could say that the particle density ρ is constant.)”.Missing: 1.31 | Show results with:1.31
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New Equations for the Sublimation Pressure and Melting Pressure ...
Dec 5, 2011 · The sublimation-pressure equation covers the temperature range from 50 K to the vapor–liquid–solid triple point at 273.16 K. The ice Ih melting ...
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Origin of metastable ice VII and its crystal growth kinetics
Mar 10, 2025 · We report on the structural verification of metastable ice VII solidifying in the phase space of ice VI at 1.80 GPa at room temperature.Missing: 4.4 | Show results with:4.4
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Experimental evidence for the existence of a second partially ...
Feb 18, 2021 · The phase boundary between ice VI and ice XIX shows that ice VI contracts upon hydrogen ordering, which thermodynamically stabilizes ice XIX in ...
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Structure of ice. V - IUCr Journals
Bridgman's density for ice V and not with his density for ice IV. This reinforces the identification of the form of ice studied here as ice V. X-ray data.
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Structure of Ice VI - jstor
Phase diagram for water. The solid phases (ice polymorphs) are identi- fied by the roman numeral designations assigned by Bridgman (14, 9, 15), upon whose ...
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Thermodynamics of high-pressure ice phases explored with ... - Nature
Aug 10, 2022 · Our study thus clarifies the phase behaviour of the high-pressure ices and reveals peculiar solid–solid transition mechanisms not known in other systems.
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Comparative Analysis of Hydrogen-Bonding Vibrations of Ice VI
May 25, 2021 · Subject to Bernal–Fowler rules, (47) every water molecule in an ice phase forms a hydrogen bond (H bond) with each of its four neighbors ...
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Structure and nature of ice XIX | Nature Communications
May 26, 2021 · Upon cooling HCl-doped ice VI at pressures above ~1 GPa, the hydrogen-ordering phase transition from ice VI to ice XV is suppressed ...
[40]
Ice XXI: Scientists use X-ray laser to identify new room-temperature ...
Oct 10, 2025 · But even pure ice, which consists only of water molecules, has been discovered to exist in more than 20 different solid forms or phases that ...
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Article Deep potential-driven structure exploration of ice polymorphs
May 5, 2025 · In this work, we present an extensive exploration of ice polymorphs under pressure conditions ranging from 1 bar to 10 GPa.
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Experiments indicating a second hydrogen ordered phase of ice VI
Mar 26, 2018 · 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, ...
[43]
Ferroelectricity in high-density H 2 O ice - AIP Publishing
Apr 1, 2015 · We find that a ferroelectric variant of ice VIII is energetically competitive with the established antiferroelectric form under pressure. The ...
[44]
New Form of Ice Discovered: Ice XXI | Sci.News
Oct 15, 2025 · Scientists have demonstrated that supercompressed water transforms into ice VI at room temperature through multiple freezing-melting pathways, ...
[45]
High-pressure experiments provide insight into icy planets
Inside the planet Neptune, a mantle of water ice, methane ice, and ammonia ... At room temperature, for example, water first becomes ice VI at about ...
[46]
Icy or rocky? Convective or stable? New interior models of Uranus ...
Sep 30, 2025 · Overall, our findings challenge the conventional classification of Uranus and Neptune as 'ice giants' and underscore the need for improved ...
[47]
Proton Conduction in Water Ices under an Electric Field | Request PDF
Aug 7, 2025 · ... conductivity when a phase transition from ice VI to ice VII occurs. The electrical transport mechanisms of these two ice polymorphs can be ...
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How do ice giants maintain their magnetic fields? | Carnegie Science
Oct 14, 2021 · A layer of “hot,” electrically conductive ice could be responsible for generating the magnetic fields of ice giant planets like Uranus and Neptune.
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Salty ice and the dilemma of ocean exoplanet habitability - Nature
Jun 21, 2022 · Habitability of exoplanet's deepest oceans could be limited by the presence of high-pressure ices at their base. New work demonstrates that ...
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Jupiter's Interior as Revealed by Juno - Annual Reviews
Feb 11, 2020 · Jupiter is the most massive planet in our Solar System and may well have played a key role in deter- mining the architecture of our planetary ...
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Have Astronomers Finally Found an Exomoon? - Scientific American
Oct 14, 2025 · The latest not-quite-smoking-gun claim concerns a potential exomoon that may be erupting to spew debris onto and around its host planet. Using ...Missing: VI Juno icy
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Ice XVII as a Novel Material for Hydrogen Storage - MDPI
By storing hydrogen in the liquid state, one attains a high volumetric density (70 g/L), but the low liquid–vapour equilibrium temperature (20 K at standard ...Missing: VI | Show results with:VI<|control11|><|separator|>
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New porous water ice metastable at atmospheric pressure obtained ...
Nov 7, 2016 · Experimental evidence for the existence of a second partially-ordered phase of ice VI ... hydrogen storage applications. Methods. Procedure ...
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[PDF] The Diamond Anvil Pressure Cell
These initial studies demonstrated that pressure-dependent shifts in frequency and changes in intensity of infrared absorption bands could be measured, and also ...
[55]
Detailed crystallographic analysis of the ice VI to ice XV hydrogen ...
The ice VI high-pressure phase was discovered by Bridgman when he explored the phase diagram of H2O up to 2 GPa.1 The ice V/VI/liquid triple point is located at ...
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Salt Ice VI as Solid‐State Electrolytes - The Advanced Portfolio - Wiley
Oct 30, 2025 · After the liquid–solid phase transition, the hydrogen bonding is enhanced and becomes strong hydrogen bonding in the solid states of ice VI and ...
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Investigation of high-pressure planetary ices by cryo-recovery. II ...
To prepare the ice VI sample, deionized water was frozen inside a PTFE sample capsule, which was then put into the piston–cylinder cell, pre-cooled to ∼250 K in ...
[58]
(PDF) Effective Hydrostatic Limits of Pressure Media for High ...
Aug 7, 2025 · Plot of shear-wave travel times in 4:1 methanol-ethanol (squares) and 16:3:1 methanol-ethanol-water (circles) as a function of pressure above ...Missing: VI | Show results with:VI
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High-Pressure Single-Crystal Studies of Ice VI | Science
X-ray diffraction data can be obtained from single crystals of ice VI produced and maintained under high pressures.Missing: early | Show results with:early
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Structure and hydrogen ordering in ices VI, VII, and VIII by neutron ...
Oct 15, 1984 · The structures of deuterated ices VI, VII, and VIII have been studied under their conditions of stability by neutron powder diffraction.
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On single-crystal neutron-diffraction in DACs: quantitative structure ...
... ice VI. KEYWORDS: Single-crystal · neutron-diffraction · H-bonds · DAC · SNAP · TOPAZ ... This work was conducted at the Spallation Neutron Source (SNS), a ...
[62]
Accurate crystal structure of ice VI from X-ray diffraction with ...
Ice VI forms tetragonal crystals with P42/nmc (No. 137) space group symmetry. Water molecules connected by hydrogen bonds form two separate interpenetrating ...Missing: body- | Show results with:body-
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Time-resolved x-ray diffraction across water-ice-VI/VII ... - IOP Science
We present recent time-resolved x-ray diffraction data obtained across the solidification of water to ice-VI and -VII at different compression rates.
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Origin of the low-temperature endotherm of acid-doped ice VI
Jan 2, 2019 · Compared to pure ice VI, the glass transition temperature is lowered by more than 30 K by the acid dopant. Future work should focus on the deep ...
[65]
Room temperature electrofreezing of water yields a missing dense ...
Apr 26, 2019 · In the newly obtained phase diagram of ice with the TIP4P/2005 water model, ice χ is located in the high-pressure region between ice II and ice ...
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Phase diagram of the TIP4P/Ice water model by enhanced sampling ...
Aug 4, 2022 · We studied the phase diagram for the TIP4P/Ice water model using enhanced sampling molecular dynamics simulations.
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[PDF] arXiv:0906.3967v1 [cond-mat.stat-mech] 22 Jun 2009
Jun 22, 2009 · By using the path integral formulation for a rigid model we shall be studying atomic quantum delocalisation effects in the influential region.Missing: ordering XV
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Phase Diagram of a Deep Potential Water Model | Phys. Rev. Lett.
Satisfactory overall agreement with experimental results is obtained. The fluid phases, molecular and ionic, and all the stable ice polymorphs, ordered and ...