Highly Compressible Plastic and Superionic Ice

Highly Compressible Plastic and Superionic Ice | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Highly Compressible Plastic and Superionic Ice Zhu Mao, Luo Li, Zihan Zhang, Yingxin Yu, Xinyang Li, Xinyue Zhang, and 6 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8118931/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Water is a key constituent of icy planets and exhibits a remarkable diversity of crystalline ice phases that govern their internal structure and evolution. Recent discoveries of plastic and superionic ice before melting have greatly extended the stability range of solid ice, yet experimental observations remain limited. In particular, the relationships, physical properties, and transition mechanisms of these phases remain largely unexplored. Here, we combine in situ synchrotron X-ray diffraction and ab initio molecular dynamics at 8–80 GPa and 500–900 K to link hydrogen-bond dynamics to lattice response. Ice at 500 K follows the same ice-VII sequence as at 300 K, but at 700 K it transforms into a plastic phase at 13.7–37.4 GPa via rapid molecular reorientation. At 900 K, fast proton diffusion stabilizes a water-like superionic state at 14.3–25 GPa, which converts to an ice-like superionic state at 31.2 GPa. Disruption of hydrogen bonds and enhanced molecular freedom markedly increase compressibility from ice-VII to plastic and superionic ice, with plastic ice-VII at 13.7–37.4 GPa and 700 K even denser than ice-VII at 500 K. These findings redefine the high-pressure ice phase diagram and reveal how microscopic mechanisms govern both phase transitions and the exceptional compressibility of plastic and superionic ice, shedding light on the interiors and thermal evolution of icy planets. Earth and environmental sciences/Solid Earth sciences/Mineralogy Earth and environmental sciences/Planetary science/Giant planets Physical sciences/Physics/Condensed-matter physics/Structure of solids and liquids Earth and environmental sciences/Planetary science/Mineralogy Figures Figure 1 Figure 2 Figure 3 Main Understanding water at extreme pressures and temperatures (P-T) conditions is central to condensed-matter physics and to the evolution of icy planets and moons 1 – 6 . More than 20 crystalline ice phases have been identified, underscoring the structural complexity of water at high pressures 7 – 15 . This complexity is evident even at 300 K, where ice-VII transitions to ice VII’ (~ 40–50 GPa) and then ice X (~ 60–100 GPa), driven by progressive hydrogen-bond disorder (proton hopping) and eventual bond symmetrization 16 – 24 . These transformations remain consistent with the Bernal-Fowler rules, in which each oxygen is covalently bonded to two hydrogens and hydrogen-bonded to two others, forming a four-fold network 25 – 27 . By contrast, at elevated pressures and temperatures of planetary interiors, hydrogen positions are difficult to resolve experimentally, dynamic hydrogen-bond rearrangements strongly influence lattice compressibility, and the stability of intermediate ice phases remains largely unconstrained 28 – 36 . Defining the stability, structure, and properties of ice in these regimes is therefore essential for modeling the thermodynamics and dynamics of icy worlds. Under the P-T conditions relevant to the shallow interiors of icy planets, recent neutron scattering experiments revealed the formation of plastic phase at ~ 3–8 GPa and 450–600 K 32 . In this state, water molecules are orientationally disordered while the oxygen sublattice remains crystalline, and hydrogen bonds satisfy the Bernal-Fowler rules dynamically rather than statically 33 – 36 . Plastic ice is predicted to be highly compressible and to exhibit low thermal conductivity—properties that would strongly influence the structure and heat transport of icy worlds—yet these predictions remain untested 31 , 37 – 39 . Its stability beyond ~ 8 GPa and 600 K, and whether it connects to higher-temperature phases, are poorly constrained. At higher pressures and temperatures, water adopts superionic forms in which hydrogen ions diffuse through the oxygen lattice, first in a body-center-cubic (bcc) arrangement (~ 18 GPa, 890 K) and then possibly in a face-center-cubic structure (> 29 GPa, 1300 K) 37,40 . Whether the plastic and superionic states are connected through a continuous sequence of transformations or represent distinct regimes of dense ice remains a central open question for completing the phase diagram of water under planetary conditions 41 – 44 . Resolving this question requires moving beyond phase boundaries to the microscopic mechanisms that govern stability and physical properties 42 , 45 , 46 . In particular, how hydrogen-bond dynamics govern the transition from orientational disorder to full ionic diffusion, and how these processes affect key properties such as compressibility and thermal transport, are unresolved issues 24 , 47 – 54 . This lack of understanding limits our ability to assess the behavior of water under planetary conditions and to model the interiors of ice-rich worlds. To address these gaps, we combined high P-T experiments with ab initio molecular dynamics (AIMD) simulations to systematically probe hydrogen-bond dynamics and lattice compressibility across 8–80 GPa and 500–900 K. By directly linking hydrogen-bond rearrangements to compressibility anomalies, we show how the onset of molecular reorientation drives the transitions from ice-VII to plastic ice, and ultimately to the superionic state. This integrated experimental-computational framework delineates the stability fields and physical properties of these phases, providing essential constraints for planetary interior models 55 . Consistent Ice-VII Transition Behavior at 300 and 500 K We first tracked the structural evolution of ice at 500 K using synchrotron X-ray diffraction (XRD) up to ~ 73.5 GPa. To better mimic planetary conditions, where water ice rarely occurs in a pure form, we synthesized samples containing 0.5 mol.% NaCl. The pressure-volume relation of ice-VII at 500 K was established and compared with previous 300 K data (Fig. 1 and Extended Data Figs. 1 and 2 ) 56 . To quantify the thermal effect, we defined the volume offset as Δ V = V T - V 300 K−ice−VII , V T is the measured volume at a given temperature, T, and V 300 K−ice−VII is the volume calculated from equation of state of ice-VII at the same pressure. Between 8 and 30 GPa, Δ V remains nearly constant (~ 0.9 ų), consistent with the stability of ice-VII in this regime. From ~ 30 GPa, however, we observe an obvious volume drop at 500 K. This anomaly resembles the ice-VII to ice-X transition previously reported at ~ 50 GPa at 300 K, but the reduced transition pressure and the broad intermediate regime between 30 and 60 GPa leave its microscopic origin uncertain. Above ~ 60 GPa, the volume difference becomes nearly pressure-independent, suggestive of ice-X (Extended Data Fig. 3 ). Because XRD cannot directly resolve hydrogen positions at these conditions, the underlying mechanism of the compressibility anomalies cannot be identified from experiments alone. We therefore turned to AIMD simulations, which are particularly suited to capturing hydrogen dynamics while the oxygen sublattice remains in a bcc framework (Table 1 and Supplementary Information section 1). The simulations reveal that at 500 K, hydrogen atoms develop increasing mean square displacements (MSD), rising from ~ 0.1 Å 2 at 13.9–22 GPa to ~ 0.2 Å 2 at 27.2 GPa (Fig. 2 and Extended Data Fig. 4). This behavior marks the onset of orientational disorder and hydrogen-bond rearrangement, consistent with the transitional VII’ state inferred from the experiments. In this pressure range, protons frequently hop between neighboring oxygens, resulting a jump in dissociation fraction from 0 to 2.2%, shortening hydrogen bonds from 1.6 Å at 22 GPa to 1.54 Å at 27.2 GPa and enhancing compressibility, which provide microscopic explanation for the observed volume anomaly (Fig. 2 ). These findings demonstrate that hydrogen-bond symmetrization and dynamic disorder govern the VII-VII’-X sequence. The microscopic picture provided by AIMD also clarifies how our observations relate to previously reported high-temperature phases 23 , 24 , 30 , 41 , 54 . Neutron diffraction indicates that a plastic phase is stabilized between 4–7 GPa at 500 K 32 . Since our experiments began at 8 GPa, just above this field, ice may undergo a plastic to ice-VII transition near 7–8 GPa, a possibility that awaits direct confirmation. Beyond this boundary, the structural sequence we observed at 500 K mirrors that at 300 K, but shifted systematically to lower pressures. This systematic offset highlight how elevated temperature promotes hydrogen-bond disorder and symmetrization, thereby narrowing the stability of ordered ice phases. 700 K Ice: Emergence of Plastic Phase with Distinct Hydrogen Dynamics Synchrotron XRD measurements at 700 K reveal a markedly different compressional response compared with that at 300 and 500 K. Using the unit-cell volume of ice-VII at 300 K as a reference, we also calculated the Δ V at each pressure. Within 13.7–37.4 GPa, Δ V decreases sharply with compression, from 1.6 ų at 13.7 GPa to only 0.6 ų at 37.4 GPa (Fig. 1 ). From 25 GPa, the volume at 700 K not only converges with that at 500 K but, surprisingly, falls below it at higher pressures. Above ~ 50 GPa, the volumes at 700 and 500 K become nearly indistinguishable. These results demonstrate that ice at 700 K is markedly more compressible than ice-VII at lower temperatures, exhibiting a behavior that cannot be explained by the conventional stability field of ice-VII and pointing towards the emergence of distinct structural states under compression. Further AIMD simulations indicate that the anomalous compressibility observed experimentally in ice at 700 K and 13.7–37.4 GPa originates from the formation of plastic ice. In simulations, the plastic phase emerges at slightly higher temperature (~ 900 K) and lower pressure, reflecting the systematic temperature and pressure offset inherent to AIMD. In our AIMD calculations, hydrogen exhibits substantial local mobility at 15.5 and 22.8 GPa, with MSD rapidly increasing from 0 to ~ 0.2–0.3 Å 2 between 1 and 5 ps, roughly twice the value at 500 K (Fig. 2 and Extended Data Fig. 4). Covalent and hydrogen-bond lengths and angles remain largely similar to those at 500 K, indicating that the ice rules are locally preserved (Table 1 and Supplementary Information section 1). Bond-breaking analysis shows covalent-bond dissociation fraction increases from 0 at 500 K to ~ 10% at 900 K, accompanied by a marked extension of hydrogen trajectories. And MSD undergoes a three- to four-fold increase from 500 to 900 K. These indicate that hydrogen transitions from broken covalent bonds to molecular rotation, rather than undergoing the covalent-hydrogen-bond transition observed for ice VII’. We identify this phase as plastic ice VII. The disrupted hydrogen-bond network and enhanced rotational freedom of water molecules make this phase highly compressible, while the average bcc oxygen framework is retained (Fig. 2 and Extended Data Fig. 4). Although plastic ice VII appears at slightly higher temperature in simulations than in experiments, its high compressibility shown in AIMD simulations provides a direct mechanistic explanation for the enhanced volume reduction observed experimentally at 13.7–37.4 GPa and 700 K. At 27.9 GPa in AIMD calculations, covalent-hydrogen-bond transitions occur as the dissociation fractions of hydrogen and covalent bonds become comparable (Fig. 2 ). Correspondingly, the hydrogen MSD at 5 ps decreases from ~ 0.4 Ų at 15.5 GPa to ~ 0.3 Ų at 27.9 GPa, reflecting a gradual reduction in rotational motion. This reduced hydrogen mobility leads to lower molecular rotation and decreased compressibility, indicating the formation of a more constrained hydrogen-bond network and marking the transition to a less compressible plastic ice VII’. Importantly, this transition from highly compressible plastic ice VII to less compressible VII’ in AIMD simulations corresponds closely to the experimentally observed change in ice compressibility at 700 K and 37.4 GPa, confirming that the structural evolution of the plastic phase governs the compressibility trend (Figs. 1 and 2 ). Our combined experimental and AIMD results uncover that plastic ice not only exists at 700 K but also undergoes a distinct transition from plastic ice VII to plastic ice VII’. This sequence exhibits remarkably high compressibility, with plastic ice VII at 37.4 GPa reaching densities exceeding those of ice-VII under the same pressure at 500 K. The plastic phase persists to higher pressures than previously reported 31 , 37 , revealing an unexpectedly extended stability field and a pronounced structural response to compression. 900 K Ice: From Water-like Superionic to Plastic Phases Synchrotron XRD measurements at 900 K, using the unit-cell volume of ice-VII at 300 K as a reference, reveal that ice becomes even more compressible than the plastic phase observed at 700 K (Fig. 1 ). Between 14.3 and 22.3 GPa, the volume reduction is markedly enhanced, exceeding that of 700 K ice over the same pressure range. At ~ 25 GPa, a distinct volume discontinuity is observed along the isotherm, indicating a first-order phase transition. Beyond this transition, the compressibility decreases, and the pressure-volume relation between 33.8 and 50 GPa closely resembles that of 700-K plastic ice at the same pressure range. At higher pressures above ~ 50 GPa, compressibility is further reduced, consistent with the behavior of the less compressible plastic ice VII’. These experimental observations point to the emergence of new structural states at 900 K below ~ 30 GPa, distinct from both ice VII and the plastic phase observed at lower temperatures. AIMD simulations at ~ 1300 K, accounting for the systematic temperature offset relative to experiments, provide a mechanistic explanation for these experimental observations at 900 K. As shown in Fig. 2 , calculations of hydrogen-bond yield a full width at half maximum (FWHM) of ~ 0.6 Å (~ 40°) at 20.9 and 24.3 GPa. These values indicate that hydrogen-bond lengths (angles) are strongly perturbed relative to ice VII with FWHM of ~ 0.3 Å (~ 20°) (Extended Data Figs. 5 and 6). The regularities of hydrogen-bond distribution vanish, and the phase violates the ice rules (Table 1 and Supplementary Information section 1). Hydrogen atoms from the MSD and atomic trajectories exhibit liquid-like diffusivity while oxygen atoms remain vibrating at their lattice sites, revealing the emergence of a water-like superionic phase (Extended Data Fig. 7). The onset of water-like superionic phase is marked by extremely high MSD up to ~ 30 Å 2 at 5 ps and dissociation fraction up to ~ 60%, unequivocally signaling a breakdown of the Bernal-Fowler ice rules. This phase is distinguished by a disrupted hydrogen-bond network and markedly increased proton mobility. These features directly account for the anomalously large volume reductions observed experimentally, which far exceed those of the plastic ice at 700 K. While simulations predict the onset of superionicity at ~ 1300 K higher than the experimental temperature, the progressive increase in compressibility from 0.29 Å 3 /GPa for ice VII to 0.32 Å 3 /GPa for plastic ice and then to 0.38 Å 3 /GPa for superionic phase reproduces the experimental trend. This correspondence confirms that ice at 900 K between 14.3 and 25 GPa has indeed entered the water-like superionic regime, underscoring both its structural distinctiveness and its exceptional compressibility as defining physical properties. Strikingly, its stability below ~ 18 GPa contradicts prior expectations of complete melting at this temperature 30 , revealing that the melting boundary of ice requires revision (Fig. 3 ). Based on the P-V relation in our experiments, the water-like superionic phase persists up to ~ 25 GPa at 900 K, beyond which a distinct volume discontinuity appears along the isotherm, signaling a potential transformation (Fig. 1 ). A similar transition has been predicted in AIMD simulations at 26.6 GPa and 1300 K, associated with changes in hydrogen dynamics (Fig. 2 ). The calculated dissociation fraction of hydrogen bonds, both in angle and length, decreases from ~ 60 Å 2 at 24.3 GPa to ~ 48 Å 2 at 26.6 GPa. For the water-like superionic phase, the MSD reaches ~ 30 Å 2 at 20.9 and 24.3 GPa, drops to ~ 20 Å 2 at 26.6 GPa, and then ~ 4 Å 2 at 30.2 GPa. At 26.6 and 30.2 GPa, simulated trajectories show that hydrogen atoms can move freely within and diffuse periodically in the lattice, leaving their original positions along with oxygen atoms and coordinating with others. With increasing pressure, the hydrogen-bond network becomes increasingly ordered and begins to obey the Bernal–Fowler ice rules, marking the transition from a water-like to an ice-like superionic state. During the transition from the water-like to the ice-like superionic state, the oxygen framework remains largely static, while the suppression of hydrogen rotational freedom destabilizes the water-like phase and triggers the structural transition. The resulting abrupt volume decrease reflects the sudden loss of proton diffusivity and restricted hydrogen motion. From our 900 K experiments, the ice-like superionic state is restricted to a limited pressure range of 27.8–31.2 GPa, where it exhibits anomalously high compressibility comparable to that of the water-like superionic phase (Fig. 1 ). Above 31.2 GPa, however, the compressibility of ice at 900 K converges with that of 700 K ice at similar pressures, suggesting a transition from the superionic to the plastic state. AIMD simulations further confirm that superionicity vanishes by 43.3 GPa at 1300 K, where the dissociation fraction of hydrogen bonds becomes indistinguishable from that of covalent bonds, consistent with the emergence of a plastic phase (Fig. 2 ). Because only selected pressures were sampled in our calculations, the precise phase boundary between superionic ice and the plastic state cannot yet be fully resolved. Instead, we identify this transition mainly from the abrupt change in compressibility in the 900 K experiments and by comparison with the 700 K results. Experimentally, the onset of plasticity at 900 K occurs at 33.8 GPa, which is at a much higher pressure than at 700 K (Fig. 3 ). The upward shift in the formation of plastic ice with temperature indicates that proton mobility, rather than thermal energy alone, primarily governs the stability of the plastic phase, as hydrogen rotational motion within the lattice counteracts thermal facilitation of proton hopping. Toward a Refined Understanding of High P-T Ice Our high P-T XRD experiments combined with AIMD simulations provide a major advance in understanding the high-pressure behavior of ice across 8–80 GPa and 500–900 K (Table 1). Contrary to previous interpretations that ice in this regime retains the ice-VII structure and only enters the superionic state above 890 K, our results uncover a well-defined sequence of thermally driven transitions: ice-VII transforms into plastic ice at 700 K, which then evolves into distinct superionic states at 900 K 30–32,37,40 . By integrating experimental constraints with theoretical insights, we resolve the microscopic mechanisms driving these transformations and establish the crystal structures and hydrogen-bonding characteristics of each high-temperature phase (Fig. 3 ). At 700 K, plastic ice undergoes a pressure-induced transition from ice-VII to a distinct ice-VII’ form, controlled by changes in hydrogen mobility. At 900 K, the phase behavior becomes markedly more complex. We identify a superionic phase emerging at 14.3–31.2 GPa that violates the ice rules, and most importantly, demonstrate for the first time the existence of a water-like superionic state between 14.3 and 25 GPa, where protons exhibit liquid-like diffusivity within a crystalline oxygen framework. Notably, this water-like superionic ice remains solid throughout this pressure range, contrary to previous expectations that ice would melt below ~ 18 GPa at 900 K. With increasing pressure, this unusual state transforms into an ice-like superionic phase. Both superionic forms display anomalously high compressibility, far exceeding that of ice-VII and plastic ice, showing that ice at extreme conditions attains densities much greater than previously predicted. Building on the anomalously high density and compressibility of plastic and superionic ice revealed here, these properties have profound implications for water transport within Earth’s mantle and for the internal structures of icy planets. In the Earth, for relatively cold subducting slabs such as Fuji-Tonga, temperatures at depths of 600 km remain below ~ 1000 K 57,58 . Although previous studies suggested that water released at these depths would exist as a fluid, our results indicate instead that it is likely stabilized in the plastic or superionic state 59 – 61 . At comparable pressures and temperatures, plastic ice is ~ 5% denser than ice VII, while the superionic state is ~ 2% denser than plastic ice. Such enhanced densities facilitate more efficient downward transport of water into the deep mantle and are expected to influence the physical and chemical properties of deeper mantle regions. Beyond Earth, many ice-rich exoplanets, such as sub-Neptunes identified by the Kepler and Eddington missions, host thick ice layers surrounding rocky cores 62 , 63 . Our findings suggest that these ice mantles are composed not of conventional ice VII, but of the denser and more compressible plastic and superionic phases. The revised phase diagram and associated density profiles thus provide essential constraints for modeling the internal structure, thermal evolution, and convective dynamics of icy planets, offering a more realistic framework for understanding water-rich planetary interiors. Declarations Acknowledgments We thank GSECARS, APS, ANL for providing the X-ray diffraction facility for the study. ZM acknowledges support China National Science Foundation (42241117, 42272036 and 42425202) and National Key R&D Program of China (2024YFF0807500). XRD data were collected at 13-BM-C of the Advanced Photon Source (APS), Argonne National Laboratory and BL15U1 of Shanghai Synchrotron Radiation Facility. APS is supported by DOE-BES, under Contract No. DE-AC02-06CH11357. Author Contributions ZM and XL designed the project. LL designed and performed the experiments. ZZ performed the AIMD simulations. LL and ZZ are responsible for integrating and interpreting the experimental and theoretical simulation results. YY, YZ, DZ and NS take participate in the experiments. All authors discussed the implications and participated in writing the manuscript. Additional information Peer review information Nature Physics thanks anonymous reviewers for their contribution to the peer review of this work. Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Correspondence and requests for materials should be addressed to Z. Mao ( [email protected] ). Competing financial interests The authors declare no competing finance interests. References Lammer, H., Bredehöft, J., Coustenis, A., Khodachenko, M., Kaltenegger, L. What makes a planet habitable? The Astronomy and astrophysics review 17 , 181-249 (2009). Gross, J., Filiberto, J. & Bell, A. S. Water in the martian interior: Evidence for terrestrial MORB mantle-like volatile contents from hydroxyl-rich apatite in olivine–phyric shergottite NWA 6234. Earth Planet. Sci. Lett. 369 , 120-128 (2013). Jacobsen, S. D. & Smyth, J. R. Effect of water on the sound velocities of ringwoodite in the transition zone. 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Lai, X., Zhu, F., Zhang, D., Tkachev, S., Prakapenka, V. B. Thermal equation of state of ice-VII revisited by single-crystal X-ray diffraction. Am. Miner. 108 , 1530-1537 (2023). Sugimura, E., Komabayashi, T., Hirose, K., Sata, N., Ohishi, Y. Simultaneous high-pressure and high-temperature volume measurements of ice VII and its thermal equation of state. Phys. Rev. B 82 , 134103 (2010). Liu, L.-g. Compression of ice VII to 500 kbar. Earth Planet. Sci. Lett. 61 , 359-364 (1982). Hemley, R., Jephcoat, A., Mao, H., Zha, C., Finger, L. Static compression of H 2 O-ice to 128 GPa (1.28 Mbar). Nature 330 , 737-740 (1987). Wolanin, E., Pruzan, P., Chervin, J., Canny, B., Gauthier, M. Equation of state of ice VII up to 106 GPa. Phys. Rev. B 56 , 5781 (1997). Bezacier, L., Journaux, B., Perrillat, J.-P., Cardon, H., Hanfland, M. Equations of state of ice VI and ice VII at high pressure and high temperature. J. Chem. Phys. 141 , 104505 (2014). Frank, M. R., Runge, C. E., Scott, H. P., Maglio, S. J., Olson, J. Experimental study of the NaCl–H 2 O system up to 28 GPa: Implications for ice-rich planetary bodies. Phys. Earth Planet. Inter. 155 , 152-162 (2006). Methods High pressure and temperature X-ray diffraction 0.5 mol/L NaCl-H 2 O aqueous solution was loaded into a BX90 DACs equipped with a pair of diamonds with 200 µm culet. Rhenium was used as the gasket material, which was pre-indented to a thickness of 25-30 μm. A hole of 100 μm was drilled in the pre-indented area by using a laser drilling machine. The hole acted as the sample chamber and a small gold foil were loaded into the chamber as the pressure calibrant. The DACs were closed quickly after finishing the loading of NaCl-H 2 O solution to avoid the evaporation of the water. The pressure medium was served by the NaCl-H 2 O mixture itself. The diamond anvil placed in the piston end was heated by the Pt-Rh heater. Temperature measurement was conducted using K-type thermocouple attached to surface of diamond, situated at a distance of approximately 500 μm from the culet 64 , 65 . X-ray diffraction experiment was performed from 8 to ~80 GPa along different isotherms at 500 K, 700 K, 900 K and 1000 K at beamline 13-BM-C (GSECARS) of the Advanced Photon Source (APS) at Argonne National Laboratory and BL15U1 of Shanghai Synchrotron Radiation Facility (SSRF) (Extended Data Fig. 1). A monochromatic X-ray beam with wavelength of 0.4344 Å for APS and 0.6199 for SSRF was used. X-ray diffraction patterns were recorded with a MarCCD detector with an exposure time of ~20 s. Instrument parameter of the CCD were calibrated by a LaB 6 standard. The diffraction peaks of NaCl could not be observed over the entire pressure range. Computational methods First-principles calculations, including structure optimizations and AIMD simulations, were conducted by using the Vienna ab initio Simulation Package (vasp 6) 66 . The generalized gradient approximation 67 parametrized by Perdew, Burke, and Ernzerhof 68 was specified as exchange-correlation functional. The pseudopotential for the electron-ion interactions was described by the projector augmented plane-wave approximation 69 , where valence electrons 1s 1 and 2s 2 2p 4 were considered for elements H and O, respectively. The plane-wave cutoff energy was set to 500 eV, and the Monkhorst-Pack 𝑘-spacing value was selected as 0.03 2𝜋/Å and 0.05 2𝜋/Å for structure optimizations and ab initio molecular dynamics simulations, respectively. The initio structures, which are 3 × 3 × 3 supercells (162 atoms), for ab initio molecular dynamics simulations are from structure optimizations at 12, 16, 20, 26 and 40 GPa. We adopted the NVT ensemble at temperatures 500 K, 900 K and 1300K, lasting for 6 ps with a time step of 0.5 fs, and we allowed 1 ps for thermalization and then extracted data from the last 5 ps. Although AIMD based on density functional theory captures water dissociation with high fidelity and is widely used in the simulations of superionic ice 42 , 53 , 70 , 71 , a quantitative description for the dissociation of both covalent and hydrogen bonds is still lacking. Bond length is the natural principle of identification for dissociation covalent and hydrogen bonds, but increasing pressure could significantly affect the bond length of covalent and hydrogen bonds. Therefore, it is difficult to quantitative describe the bond networks of ice in its high P-T phase diagram. To address this gap, we proposed a theoretical workflow as shown in Extended Data Fig. 8 and Supplementary Information section 2 based on ice rules. In our workflow, only one hypothesis is used to generate the bond networks of ice below the pressure of hydrogen-bond symmetrization: every H atom has and only has one covalent bond and one hydrogen bond in ice. Therefore, the number of covalent and hydrogen bonds of each O atom is quantitatively given, based on which the dissociation fractions of covalent and hydrogen bonds were defined to identify the dynamics phases of ice. Tables Table . 1 | Qualitative bonding and compressibility across ice phases . Phase Covalent O-H H-bond Molecular bond-angle (H-O-H) H-bond geometry (O-H···O) Key characteristics Compressibility Ice-VII Intact Intact Retained Maintain four-fold coordination (Bernal-Fowler rules) Covalent bond is much shorter than hydrogen bond Low (difficult to compress) VII’ Transition between covalent and hydrogen bond Retained Maintain four-fold coordination Proton hopping, the dissociation fraction between covalent and H-bond is almost same Moderately low (harder than ice-VII) X Symmetrization of covalent and hydrgen bond Retained Maintain four-fold coordination H-bond symmetrization, covalent ≈ H-bond length Lowest (hardest to compress) Plastic ice Intact Slightly disrupted Retained Slightly disrupted, dynamically four-fold coordination Molecular reorientation Moderately high (easier than ice VII) Water-like superionic Broken Strongly disrupted Dissociated Strongly disrupted, not maintain four-fold coordination Free H diffusion Highest (easiest to compress) Ice-like superionic Broken Strongly disrupted Dissociated Strongly disrupted, dynamically maintain four-fold coordination Confined H diffusion Moderately high Additional Declarations There is NO Competing Interest. Supplementary Files supplementarymaterialv2.docx The definition of each phase of ice at high P-T and details of the workflow Data.rar Experimental and theoretical calculation data ExtendedData.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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09:20:22","extension":"png","order_by":21,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":56116,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8118931/v1/da88ab203389f7e4f8d1f62d.png"},{"id":97666671,"identity":"32de7e3b-4c5b-4bb4-a004-27adb21cba10","added_by":"auto","created_at":"2025-12-08 09:21:50","extension":"png","order_by":22,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":43297,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8118931/v1/4f5d0b0d2c114226b33ec4a3.png"},{"id":97666284,"identity":"729d18e0-7d8a-48d1-84c9-4de259fcf354","added_by":"auto","created_at":"2025-12-08 09:20:51","extension":"png","order_by":23,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":92828,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8118931/v1/70d1c7ead355bc55b5d4c24c.png"},{"id":97667671,"identity":"5bb3e0c1-b2aa-4dd1-ac32-9c40b3f82040","added_by":"auto","created_at":"2025-12-08 09:24:03","extension":"png","order_by":24,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":79095,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-8118931/v1/49fefb5926632de37bf4dbab.png"},{"id":97666389,"identity":"5ee03626-2dcd-4000-930c-f2f974b7f440","added_by":"auto","created_at":"2025-12-08 09:21:05","extension":"png","order_by":25,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":72183,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-8118931/v1/e5cfcc158fc16adda0836fdd.png"},{"id":97409934,"identity":"e834512e-7c10-4d17-8aa5-e3ad301fbed5","added_by":"auto","created_at":"2025-12-04 05:07:36","extension":"xml","order_by":26,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":162761,"visible":true,"origin":"","legend":"","description":"","filename":"NCOMMS25923160structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8118931/v1/f58869cdcf8099e7648eefd5.xml"},{"id":97666078,"identity":"508aa3cf-1641-45b4-b73b-82be186d2051","added_by":"auto","created_at":"2025-12-08 09:20:22","extension":"html","order_by":27,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":177378,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8118931/v1/302fb48980972e24ddfb204f.html"},{"id":97667516,"identity":"c007ed0e-8000-4df4-919b-fe71da66b6e6","added_by":"auto","created_at":"2025-12-08 09:23:41","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":194490,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eEquation of state of ice at high P-T. a\u003c/strong\u003e, Unit-cell volume. \u003cstrong\u003eb\u003c/strong\u003e, Volume difference (Δ\u003cem\u003eV\u003c/em\u003e) between high P-T data and equation state of ice VII at 300 K. A comparison of \u003cem\u003eV\u003c/em\u003e and Δ\u003cem\u003eV\u003c/em\u003e with published datasets are shown in Extended Data Fig. 2, Table 1 and ref. \u003ca href=\"#_ENREF_30\" title=\"Queyroux, 2020 #985\"\u003e\u003csup\u003e30\u003c/sup\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca href=\"#_ENREF_72\" title=\"Fei, 1993 #984\"\u003e\u003csup\u003e72-75\u003c/sup\u003e\u003c/a\u003e.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8118931/v1/b57c7b01daddc192ed8f8a6f.png"},{"id":97666614,"identity":"d758bcf3-3124-49ab-931a-76a688dc3a01","added_by":"auto","created_at":"2025-12-08 09:21:42","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":228093,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eAIMD results of high P-T ice. a\u003c/strong\u003e, Calculated unit-cell volume of ice from AIMD along selected isotherms. \u003cstrong\u003eb\u003c/strong\u003e, The difference relative to the 0 K equation of state of ice-VII. \u003cstrong\u003ec\u003c/strong\u003e, Representative mean square displacement (MSD) of hydrogen. All the MSD results are provided in Extended Data Fig. 4. \u003cstrong\u003ed-e\u003c/strong\u003e, Mean bond angle and length. The results were calculated using the distribution in Extended Data Figs. 5 and 6. \u003cstrong\u003ef\u003c/strong\u003e, The dissociation fraction, the value of covalent and hydrogen bond is almost same at 500 K so the symbols and lines are overlap. \u003cstrong\u003eg-h\u003c/strong\u003e, The distribution of hydrogen bond angle and length. \u003cstrong\u003ei\u003c/strong\u003e-\u003cstrong\u003ek\u003c/strong\u003e Typical trajectory of the H and O atoms for different phases. All the trajectory are shown in Extended Data Fig. 7. SI: superionic. Grey: 0 K; green: 500 K; orange: 900 K; red: 1300 K.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8118931/v1/c9600115c03d56aa98657a09.png"},{"id":97409907,"identity":"13f06089-1754-4687-8d93-8dea05594f00","added_by":"auto","created_at":"2025-12-04 05:07:36","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":151180,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePhase relations and hydrogen-bond dynamics of ice. a\u003c/strong\u003e, Phase diagram of ice at high P-T.\u003cstrong\u003e b\u003c/strong\u003e, Schematic diagram of ice across different isotherms. SI: superionic.\u003cstrong\u003e \u003c/strong\u003eAlong 300 K and 500 K isotherm, ice-VII undergoes a phase transition to ice VII’ and then ice X. At 700 K, plastic ice VII appears adjacent to the melting curve and then transforms to plastic ice VII’. The plastic phase and VII’ are characterized by molecular rotation and proton hopping, respectively. For plastic VII’, proton hopping is enhanced with increasing pressure, while molecular rotation becomes weaker. Because molecule rotation prohibits the transition from plastic VII to plastic VII’, the onset pressure of plastic VII-VII’ at 900 K is higher than that of VII-VII’ at 700 K. At 900 K, photon diffusivity is observed in both water-like and ice-like superionic phases. The difference between these two phases is shown in Extended Data Fig. 2 and Table 1. The diffusivity decreases with pressure, transitioning first to plastic VII and then to plastic VII’. Circles and diamonds are the experimental results in this study and literature\u003ca href=\"#_ENREF_32\" title=\"Rescigno, 2025 #1034\"\u003e\u003csup\u003e32\u003c/sup\u003e\u003c/a\u003e, respectively. The phase boundary of superionic phase and melting curve are from the ref \u003ca href=\"#_ENREF_30\" title=\"Queyroux, 2020 #985\"\u003e\u003csup\u003e30\u003c/sup\u003e\u003c/a\u003e\u003csup\u003e,\u003c/sup\u003e\u003ca href=\"#_ENREF_40\" title=\"Prakapenka, 2021 #264\"\u003e\u003csup\u003e40\u003c/sup\u003e\u003c/a\u003e.\u0026nbsp;\u0026nbsp;\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8118931/v1/0d1fa0b0e2069390eb6f5eac.png"},{"id":100365967,"identity":"2e3a5619-3705-4cf5-8266-283ce2ef090f","added_by":"auto","created_at":"2026-01-16 07:55:47","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1323382,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8118931/v1/5e0cc8ad-155f-4cce-8b47-d32098f43f69.pdf"},{"id":97409903,"identity":"2e3342f5-0d6c-44ed-a095-660b43dbff94","added_by":"auto","created_at":"2025-12-04 05:07:36","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":22632,"visible":true,"origin":"","legend":"The definition of each phase of ice at high P-T and details of the workflow","description":"","filename":"supplementarymaterialv2.docx","url":"https://assets-eu.researchsquare.com/files/rs-8118931/v1/1d38ee13574ce4e4077cf0f2.docx"},{"id":97409936,"identity":"f4f48f7d-f6fd-45d5-8d41-26a0588737ba","added_by":"auto","created_at":"2025-12-04 05:07:41","extension":"rar","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":281108036,"visible":true,"origin":"","legend":"Experimental and theoretical calculation data","description":"","filename":"Data.rar","url":"https://assets-eu.researchsquare.com/files/rs-8118931/v1/99ecc51f2db5625ead192258.rar"},{"id":97409909,"identity":"80ca1f2d-1dc8-4d2e-b1ea-664824f60137","added_by":"auto","created_at":"2025-12-04 05:07:36","extension":"docx","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":1384890,"visible":true,"origin":"","legend":"","description":"","filename":"ExtendedData.docx","url":"https://assets-eu.researchsquare.com/files/rs-8118931/v1/bc488f91d687cec9195d6ed5.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Highly Compressible Plastic and Superionic Ice","fulltext":[{"header":"Main","content":"\u003cp\u003eUnderstanding water at extreme pressures and temperatures (P-T) conditions is central to condensed-matter physics and to the evolution of icy planets and moons\u003csup\u003e\u003cspan additionalcitationids=\"CR2 CR3 CR4 CR5\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. More than 20 crystalline ice phases have been identified, underscoring the structural complexity of water at high pressures\u003csup\u003e\u003cspan additionalcitationids=\"CR8 CR9 CR10 CR11 CR12 CR13 CR14\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. This complexity is evident even at 300 K, where ice-VII transitions to ice VII\u0026rsquo; (~\u0026thinsp;40\u0026ndash;50 GPa) and then ice X (~\u0026thinsp;60\u0026ndash;100 GPa), driven by progressive hydrogen-bond disorder (proton hopping) and eventual bond symmetrization\u003csup\u003e\u003cspan additionalcitationids=\"CR17 CR18 CR19 CR20 CR21 CR22 CR23\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. These transformations remain consistent with the Bernal-Fowler rules, in which each oxygen is covalently bonded to two hydrogens and hydrogen-bonded to two others, forming a four-fold network\u003csup\u003e\u003cspan additionalcitationids=\"CR26\" citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. By contrast, at elevated pressures and temperatures of planetary interiors, hydrogen positions are difficult to resolve experimentally, dynamic hydrogen-bond rearrangements strongly influence lattice compressibility, and the stability of intermediate ice phases remains largely unconstrained\u003csup\u003e\u003cspan additionalcitationids=\"CR29 CR30 CR31 CR32 CR33 CR34 CR35\" citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. Defining the stability, structure, and properties of ice in these regimes is therefore essential for modeling the thermodynamics and dynamics of icy worlds.\u003c/p\u003e\u003cp\u003eUnder the P-T conditions relevant to the shallow interiors of icy planets, recent neutron scattering experiments revealed the formation of plastic phase at ~\u0026thinsp;3\u0026ndash;8 GPa and 450\u0026ndash;600 K\u003csup\u003e32\u003c/sup\u003e. In this state, water molecules are orientationally disordered while the oxygen sublattice remains crystalline, and hydrogen bonds satisfy the Bernal-Fowler rules dynamically rather than statically\u003csup\u003e\u003cspan additionalcitationids=\"CR34 CR35\" citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. Plastic ice is predicted to be highly compressible and to exhibit low thermal conductivity\u0026mdash;properties that would strongly influence the structure and heat transport of icy worlds\u0026mdash;yet these predictions remain untested\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e,\u003cspan additionalcitationids=\"CR38\" citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e. Its stability beyond ~\u0026thinsp;8 GPa and 600 K, and whether it connects to higher-temperature phases, are poorly constrained. At higher pressures and temperatures, water adopts superionic forms in which hydrogen ions diffuse through the oxygen lattice, first in a body-center-cubic (bcc) arrangement (~\u0026thinsp;18 GPa, 890 K) and then possibly in a face-center-cubic structure (\u0026gt;\u0026thinsp;29 GPa, 1300 K)\u003csup\u003e37,40\u003c/sup\u003e. Whether the plastic and superionic states are connected through a continuous sequence of transformations or represent distinct regimes of dense ice remains a central open question for completing the phase diagram of water under planetary conditions\u003csup\u003e\u003cspan additionalcitationids=\"CR42 CR43\" citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eResolving this question requires moving beyond phase boundaries to the microscopic mechanisms that govern stability and physical properties\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e,\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e,\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e. In particular, how hydrogen-bond dynamics govern the transition from orientational disorder to full ionic diffusion, and how these processes affect key properties such as compressibility and thermal transport, are unresolved issues\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e,\u003cspan additionalcitationids=\"CR48 CR49 CR50 CR51 CR52 CR53\" citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e. This lack of understanding limits our ability to assess the behavior of water under planetary conditions and to model the interiors of ice-rich worlds. To address these gaps, we combined high P-T experiments with \u003cem\u003eab\u003c/em\u003e initio molecular dynamics (AIMD) simulations to systematically probe hydrogen-bond dynamics and lattice compressibility across 8\u0026ndash;80 GPa and 500\u0026ndash;900 K. By directly linking hydrogen-bond rearrangements to compressibility anomalies, we show how the onset of molecular reorientation drives the transitions from ice-VII to plastic ice, and ultimately to the superionic state. This integrated experimental-computational framework delineates the stability fields and physical properties of these phases, providing essential constraints for planetary interior models\u003csup\u003e\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\n\u003ch3\u003eConsistent Ice-VII Transition Behavior at 300 and 500 K\u003c/h3\u003e\n\u003cp\u003eWe first tracked the structural evolution of ice at 500 K using synchrotron X-ray diffraction (XRD) up to ~\u0026thinsp;73.5 GPa. To better mimic planetary conditions, where water ice rarely occurs in a pure form, we synthesized samples containing 0.5 mol.% NaCl. The pressure-volume relation of ice-VII at 500 K was established and compared with previous 300 K data (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and Extended Data Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e)\u003csup\u003e56\u003c/sup\u003e. To quantify the thermal effect, we defined the volume offset as Δ\u003cem\u003eV\u003c/em\u003e\u0026thinsp;=\u0026thinsp;\u003cem\u003eV\u003c/em\u003e\u003csub\u003eT\u003c/sub\u003e - \u003cem\u003eV\u003c/em\u003e\u003csub\u003e300 K\u0026minus;ice\u0026minus;VII\u003c/sub\u003e, \u003cem\u003eV\u003c/em\u003e\u003csub\u003eT\u003c/sub\u003e is the measured volume at a given temperature, T, and \u003cem\u003eV\u003c/em\u003e\u003csub\u003e300 K\u0026minus;ice\u0026minus;VII\u003c/sub\u003e is the volume calculated from equation of state of ice-VII at the same pressure. Between 8 and 30 GPa, Δ\u003cem\u003eV\u003c/em\u003e remains nearly constant (~\u0026thinsp;0.9 \u0026Aring;\u0026sup3;), consistent with the stability of ice-VII in this regime. From ~\u0026thinsp;30 GPa, however, we observe an obvious volume drop at 500 K. This anomaly resembles the ice-VII to ice-X transition previously reported at ~\u0026thinsp;50 GPa at 300 K, but the reduced transition pressure and the broad intermediate regime between 30 and 60 GPa leave its microscopic origin uncertain. Above ~\u0026thinsp;60 GPa, the volume difference becomes nearly pressure-independent, suggestive of ice-X (Extended Data Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eBecause XRD cannot directly resolve hydrogen positions at these conditions, the underlying mechanism of the compressibility anomalies cannot be identified from experiments alone. We therefore turned to AIMD simulations, which are particularly suited to capturing hydrogen dynamics while the oxygen sublattice remains in a bcc framework (Table\u0026nbsp;1 and Supplementary Information section 1). The simulations reveal that at 500 K, hydrogen atoms develop increasing mean square displacements (MSD), rising from ~\u0026thinsp;0.1 \u0026Aring;\u003csup\u003e2\u003c/sup\u003e at 13.9\u0026ndash;22 GPa to ~\u0026thinsp;0.2 \u0026Aring;\u003csup\u003e2\u003c/sup\u003e at 27.2 GPa (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and Extended Data Fig.\u0026nbsp;4). This behavior marks the onset of orientational disorder and hydrogen-bond rearrangement, consistent with the transitional VII\u0026rsquo; state inferred from the experiments. In this pressure range, protons frequently hop between neighboring oxygens, resulting a jump in dissociation fraction from 0 to 2.2%, shortening hydrogen bonds from 1.6 \u0026Aring; at 22 GPa to 1.54 \u0026Aring; at 27.2 GPa and enhancing compressibility, which provide microscopic explanation for the observed volume anomaly (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). These findings demonstrate that hydrogen-bond symmetrization and dynamic disorder govern the VII-VII\u0026rsquo;-X sequence.\u003c/p\u003e\u003cp\u003eThe microscopic picture provided by AIMD also clarifies how our observations relate to previously reported high-temperature phases\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e,\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e,\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e,\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e. Neutron diffraction indicates that a plastic phase is stabilized between 4\u0026ndash;7 GPa at 500 K\u003csup\u003e32\u003c/sup\u003e. Since our experiments began at 8 GPa, just above this field, ice may undergo a plastic to ice-VII transition near 7\u0026ndash;8 GPa, a possibility that awaits direct confirmation. Beyond this boundary, the structural sequence we observed at 500 K mirrors that at 300 K, but shifted systematically to lower pressures. This systematic offset highlight how elevated temperature promotes hydrogen-bond disorder and symmetrization, thereby narrowing the stability of ordered ice phases.\u003c/p\u003e\u003cp\u003e\u003cb\u003e700 K Ice: Emergence of Plastic Phase with Distinct Hydrogen Dynamics\u003c/b\u003e\u003c/p\u003e\u003cp\u003eSynchrotron XRD measurements at 700 K reveal a markedly different compressional response compared with that at 300 and 500 K. Using the unit-cell volume of ice-VII at 300 K as a reference, we also calculated the Δ\u003cem\u003eV\u003c/em\u003e at each pressure. Within 13.7\u0026ndash;37.4 GPa, Δ\u003cem\u003eV\u003c/em\u003e decreases sharply with compression, from 1.6 \u0026Aring;\u0026sup3; at 13.7 GPa to only 0.6 \u0026Aring;\u0026sup3; at 37.4 GPa (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). From 25 GPa, the volume at 700 K not only converges with that at 500 K but, surprisingly, falls below it at higher pressures. Above ~\u0026thinsp;50 GPa, the volumes at 700 and 500 K become nearly indistinguishable. These results demonstrate that ice at 700 K is markedly more compressible than ice-VII at lower temperatures, exhibiting a behavior that cannot be explained by the conventional stability field of ice-VII and pointing towards the emergence of distinct structural states under compression.\u003c/p\u003e\u003cp\u003eFurther AIMD simulations indicate that the anomalous compressibility observed experimentally in ice at 700 K and 13.7\u0026ndash;37.4 GPa originates from the formation of plastic ice. In simulations, the plastic phase emerges at slightly higher temperature (~\u0026thinsp;900 K) and lower pressure, reflecting the systematic temperature and pressure offset inherent to AIMD. In our AIMD calculations, hydrogen exhibits substantial local mobility at 15.5 and 22.8 GPa, with MSD rapidly increasing from 0 to ~\u0026thinsp;0.2\u0026ndash;0.3 \u0026Aring;\u003csup\u003e2\u003c/sup\u003e between 1 and 5 ps, roughly twice the value at 500 K (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and Extended Data Fig.\u0026nbsp;4). Covalent and hydrogen-bond lengths and angles remain largely similar to those at 500 K, indicating that the ice rules are locally preserved (Table\u0026nbsp;1 and Supplementary Information section 1). Bond-breaking analysis shows covalent-bond dissociation fraction increases from 0 at 500 K to ~\u0026thinsp;10% at 900 K, accompanied by a marked extension of hydrogen trajectories. And MSD undergoes a three- to four-fold increase from 500 to 900 K. These indicate that hydrogen transitions from broken covalent bonds to molecular rotation, rather than undergoing the covalent-hydrogen-bond transition observed for ice VII\u0026rsquo;. We identify this phase as plastic ice VII. The disrupted hydrogen-bond network and enhanced rotational freedom of water molecules make this phase highly compressible, while the average bcc oxygen framework is retained (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and Extended Data Fig.\u0026nbsp;4). Although plastic ice VII appears at slightly higher temperature in simulations than in experiments, its high compressibility shown in AIMD simulations provides a direct mechanistic explanation for the enhanced volume reduction observed experimentally at 13.7\u0026ndash;37.4 GPa and 700 K.\u003c/p\u003e\u003cp\u003eAt 27.9 GPa in AIMD calculations, covalent-hydrogen-bond transitions occur as the dissociation fractions of hydrogen and covalent bonds become comparable (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Correspondingly, the hydrogen MSD at 5 ps decreases from ~\u0026thinsp;0.4 \u0026Aring;\u0026sup2; at 15.5 GPa to ~\u0026thinsp;0.3 \u0026Aring;\u0026sup2; at 27.9 GPa, reflecting a gradual reduction in rotational motion. This reduced hydrogen mobility leads to lower molecular rotation and decreased compressibility, indicating the formation of a more constrained hydrogen-bond network and marking the transition to a less compressible plastic ice VII\u0026rsquo;. Importantly, this transition from highly compressible plastic ice VII to less compressible VII\u0026rsquo; in AIMD simulations corresponds closely to the experimentally observed change in ice compressibility at 700 K and 37.4 GPa, confirming that the structural evolution of the plastic phase governs the compressibility trend (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eOur combined experimental and AIMD results uncover that plastic ice not only exists at 700 K but also undergoes a distinct transition from plastic ice VII to plastic ice VII\u0026rsquo;. This sequence exhibits remarkably high compressibility, with plastic ice VII at 37.4 GPa reaching densities exceeding those of ice-VII under the same pressure at 500 K. The plastic phase persists to higher pressures than previously reported\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e,\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e, revealing an unexpectedly extended stability field and a pronounced structural response to compression.\u003c/p\u003e\u003cp\u003e\u003cb\u003e900 K Ice: From Water-like Superionic to Plastic Phases\u003c/b\u003e\u003c/p\u003e\u003cp\u003eSynchrotron XRD measurements at 900 K, using the unit-cell volume of ice-VII at 300 K as a reference, reveal that ice becomes even more compressible than the plastic phase observed at 700 K (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Between 14.3 and 22.3 GPa, the volume reduction is markedly enhanced, exceeding that of 700 K ice over the same pressure range. At ~\u0026thinsp;25 GPa, a distinct volume discontinuity is observed along the isotherm, indicating a first-order phase transition. Beyond this transition, the compressibility decreases, and the pressure-volume relation between 33.8 and 50 GPa closely resembles that of 700-K plastic ice at the same pressure range. At higher pressures above ~\u0026thinsp;50 GPa, compressibility is further reduced, consistent with the behavior of the less compressible plastic ice VII\u0026rsquo;. These experimental observations point to the emergence of new structural states at 900 K below ~\u0026thinsp;30 GPa, distinct from both ice VII and the plastic phase observed at lower temperatures.\u003c/p\u003e\u003cp\u003eAIMD simulations at ~\u0026thinsp;1300 K, accounting for the systematic temperature offset relative to experiments, provide a mechanistic explanation for these experimental observations at 900 K. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, calculations of hydrogen-bond yield a full width at half maximum (FWHM) of ~\u0026thinsp;0.6 \u0026Aring; (~\u0026thinsp;40\u0026deg;) at 20.9 and 24.3 GPa. These values indicate that hydrogen-bond lengths (angles) are strongly perturbed relative to ice VII with FWHM of ~\u0026thinsp;0.3 \u0026Aring; (~\u0026thinsp;20\u0026deg;) (Extended Data Figs.\u0026nbsp;5 and 6). The regularities of hydrogen-bond distribution vanish, and the phase violates the ice rules (Table\u0026nbsp;1 and Supplementary Information section 1). Hydrogen atoms from the MSD and atomic trajectories exhibit liquid-like diffusivity while oxygen atoms remain vibrating at their lattice sites, revealing the emergence of a water-like superionic phase (Extended Data Fig.\u0026nbsp;7). The onset of water-like superionic phase is marked by extremely high MSD up to ~\u0026thinsp;30 \u0026Aring;\u003csup\u003e2\u003c/sup\u003e at 5 ps and dissociation fraction up to ~\u0026thinsp;60%, unequivocally signaling a breakdown of the Bernal-Fowler ice rules. This phase is distinguished by a disrupted hydrogen-bond network and markedly increased proton mobility. These features directly account for the anomalously large volume reductions observed experimentally, which far exceed those of the plastic ice at 700 K. While simulations predict the onset of superionicity at ~\u0026thinsp;1300 K higher than the experimental temperature, the progressive increase in compressibility from 0.29 \u0026Aring;\u003csup\u003e3\u003c/sup\u003e/GPa for ice VII to 0.32 \u0026Aring;\u003csup\u003e3\u003c/sup\u003e/GPa for plastic ice and then to 0.38 \u0026Aring;\u003csup\u003e3\u003c/sup\u003e/GPa for superionic phase reproduces the experimental trend. This correspondence confirms that ice at 900 K between 14.3 and 25 GPa has indeed entered the water-like superionic regime, underscoring both its structural distinctiveness and its exceptional compressibility as defining physical properties. Strikingly, its stability below ~\u0026thinsp;18 GPa contradicts prior expectations of complete melting at this temperature\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e, revealing that the melting boundary of ice requires revision (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eBased on the P-V relation in our experiments, the water-like superionic phase persists up to ~\u0026thinsp;25 GPa at 900 K, beyond which a distinct volume discontinuity appears along the isotherm, signaling a potential transformation (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). A similar transition has been predicted in AIMD simulations at 26.6 GPa and 1300 K, associated with changes in hydrogen dynamics (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The calculated dissociation fraction of hydrogen bonds, both in angle and length, decreases from ~\u0026thinsp;60 \u0026Aring;\u003csup\u003e2\u003c/sup\u003e at 24.3 GPa to ~\u0026thinsp;48 \u0026Aring;\u003csup\u003e2\u003c/sup\u003e at 26.6 GPa. For the water-like superionic phase, the MSD reaches\u0026thinsp;~\u0026thinsp;30 \u0026Aring;\u003csup\u003e2\u003c/sup\u003e at 20.9 and 24.3 GPa, drops to ~\u0026thinsp;20 \u0026Aring;\u003csup\u003e2\u003c/sup\u003e at 26.6 GPa, and then ~\u0026thinsp;4 \u0026Aring;\u003csup\u003e2\u003c/sup\u003e at 30.2 GPa. At 26.6 and 30.2 GPa, simulated trajectories show that hydrogen atoms can move freely within and diffuse periodically in the lattice, leaving their original positions along with oxygen atoms and coordinating with others. With increasing pressure, the hydrogen-bond network becomes increasingly ordered and begins to obey the Bernal\u0026ndash;Fowler ice rules, marking the transition from a water-like to an ice-like superionic state. During the transition from the water-like to the ice-like superionic state, the oxygen framework remains largely static, while the suppression of hydrogen rotational freedom destabilizes the water-like phase and triggers the structural transition. The resulting abrupt volume decrease reflects the sudden loss of proton diffusivity and restricted hydrogen motion.\u003c/p\u003e\u003cp\u003eFrom our 900 K experiments, the ice-like superionic state is restricted to a limited pressure range of 27.8\u0026ndash;31.2 GPa, where it exhibits anomalously high compressibility comparable to that of the water-like superionic phase (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Above 31.2 GPa, however, the compressibility of ice at 900 K converges with that of 700 K ice at similar pressures, suggesting a transition from the superionic to the plastic state. AIMD simulations further confirm that superionicity vanishes by 43.3 GPa at 1300 K, where the dissociation fraction of hydrogen bonds becomes indistinguishable from that of covalent bonds, consistent with the emergence of a plastic phase (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Because only selected pressures were sampled in our calculations, the precise phase boundary between superionic ice and the plastic state cannot yet be fully resolved. Instead, we identify this transition mainly from the abrupt change in compressibility in the 900 K experiments and by comparison with the 700 K results. Experimentally, the onset of plasticity at 900 K occurs at 33.8 GPa, which is at a much higher pressure than at 700 K (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The upward shift in the formation of plastic ice with temperature indicates that proton mobility, rather than thermal energy alone, primarily governs the stability of the plastic phase, as hydrogen rotational motion within the lattice counteracts thermal facilitation of proton hopping.\u003c/p\u003e\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eToward a Refined Understanding of High P-T Ice\u003c/h2\u003e\u003cp\u003eOur high P-T XRD experiments combined with AIMD simulations provide a major advance in understanding the high-pressure behavior of ice across 8\u0026ndash;80 GPa and 500\u0026ndash;900 K (Table\u0026nbsp;1). Contrary to previous interpretations that ice in this regime retains the ice-VII structure and only enters the superionic state above 890 K, our results uncover a well-defined sequence of thermally driven transitions: ice-VII transforms into plastic ice at 700 K, which then evolves into distinct superionic states at 900 K\u003csup\u003e30\u0026ndash;32,37,40\u003c/sup\u003e. By integrating experimental constraints with theoretical insights, we resolve the microscopic mechanisms driving these transformations and establish the crystal structures and hydrogen-bonding characteristics of each high-temperature phase (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eAt 700 K, plastic ice undergoes a pressure-induced transition from ice-VII to a distinct ice-VII\u0026rsquo; form, controlled by changes in hydrogen mobility. At 900 K, the phase behavior becomes markedly more complex. We identify a superionic phase emerging at 14.3\u0026ndash;31.2 GPa that violates the ice rules, and most importantly, demonstrate for the first time the existence of a water-like superionic state between 14.3 and 25 GPa, where protons exhibit liquid-like diffusivity within a crystalline oxygen framework. Notably, this water-like superionic ice remains solid throughout this pressure range, contrary to previous expectations that ice would melt below ~\u0026thinsp;18 GPa at 900 K. With increasing pressure, this unusual state transforms into an ice-like superionic phase. Both superionic forms display anomalously high compressibility, far exceeding that of ice-VII and plastic ice, showing that ice at extreme conditions attains densities much greater than previously predicted.\u003c/p\u003e\u003cp\u003eBuilding on the anomalously high density and compressibility of plastic and superionic ice revealed here, these properties have profound implications for water transport within Earth\u0026rsquo;s mantle and for the internal structures of icy planets. In the Earth, for relatively cold subducting slabs such as Fuji-Tonga, temperatures at depths of 600 km remain below ~\u0026thinsp;1000 K\u003csup\u003e57,58\u003c/sup\u003e. Although previous studies suggested that water released at these depths would exist as a fluid, our results indicate instead that it is likely stabilized in the plastic or superionic state\u003csup\u003e\u003cspan additionalcitationids=\"CR60\" citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e\u003c/sup\u003e. At comparable pressures and temperatures, plastic ice is ~\u0026thinsp;5% denser than ice VII, while the superionic state is ~\u0026thinsp;2% denser than plastic ice. Such enhanced densities facilitate more efficient downward transport of water into the deep mantle and are expected to influence the physical and chemical properties of deeper mantle regions. Beyond Earth, many ice-rich exoplanets, such as sub-Neptunes identified by the Kepler and Eddington missions, host thick ice layers surrounding rocky cores\u003csup\u003e\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e,\u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e\u003c/sup\u003e. Our findings suggest that these ice mantles are composed not of conventional ice VII, but of the denser and more compressible plastic and superionic phases. The revised phase diagram and associated density profiles thus provide essential constraints for modeling the internal structure, thermal evolution, and convective dynamics of icy planets, offering a more realistic framework for understanding water-rich planetary interiors.\u003c/p\u003e\u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u0026nbsp;\u003c/strong\u003eWe thank GSECARS, APS, ANL for providing the X-ray diffraction facility for the study. ZM acknowledges support China National Science Foundation (42241117, 42272036 and 42425202) and National Key R\u0026amp;D Program of China (2024YFF0807500). XRD data were collected at 13-BM-C of the Advanced Photon Source (APS), Argonne National Laboratory and BL15U1 of Shanghai Synchrotron Radiation Facility. APS is supported by DOE-BES, under Contract No. DE-AC02-06CH11357.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u0026nbsp;\u003c/strong\u003eZM and XL designed the project. LL designed and performed the experiments. ZZ performed the AIMD simulations. LL and ZZ are responsible for integrating and interpreting the experimental and theoretical simulation results. YY, YZ, DZ and NS take participate in the experiments. All authors discussed the implications and participated in writing the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u003c/strong\u003e Peer review information Nature Physics thanks anonymous reviewers for their contribution to the peer review of this work. Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Correspondence and requests for materials should be addressed to Z. Mao ([email protected]).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting financial interests\u0026nbsp;\u003c/strong\u003eThe authors declare no competing finance interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eLammer, H., Bredeh\u0026ouml;ft, J., Coustenis, A., Khodachenko, M., Kaltenegger, L. What makes a planet habitable? \u003cem\u003eThe Astronomy and astrophysics review\u003c/em\u003e \u003cstrong\u003e17\u003c/strong\u003e, 181-249 (2009).\u003c/li\u003e\n\u003cli\u003eGross, J., Filiberto, J. \u0026amp; Bell, A. S. Water in the martian interior: Evidence for terrestrial MORB mantle-like volatile contents from hydroxyl-rich apatite in olivine\u0026ndash;phyric shergottite NWA 6234. \u003cem\u003eEarth Planet. Sci. Lett.\u003c/em\u003e \u003cstrong\u003e369\u003c/strong\u003e, 120-128 (2013).\u003c/li\u003e\n\u003cli\u003eJacobsen, S. D. \u0026amp; Smyth, J. R. 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The hole acted as the sample chamber and\u0026nbsp;a small gold foil\u0026nbsp;were loaded into the chamber as the pressure calibrant. The DACs were closed quickly after finishing the loading of NaCl-H\u003csub\u003e2\u003c/sub\u003eO solution to avoid the evaporation of the water.\u0026nbsp;The pressure medium was served by the NaCl-H\u003csub\u003e2\u003c/sub\u003eO mixture itself.\u0026nbsp;The diamond anvil placed in the piston end was heated by the Pt-Rh heater. Temperature measurement was conducted using K-type thermocouple attached to surface of diamond, situated at a distance of approximately 500 μm from the culet\u003csup\u003e64\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e65\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eX-ray diffraction experiment was performed from 8 to ~80 GPa\u0026nbsp;along different isotherms at 500 K, 700 K, 900 K and 1000 K\u0026nbsp;at beamline 13-BM-C\u0026nbsp;(GSECARS) of the Advanced Photon Source (APS) at Argonne National Laboratory\u0026nbsp;and BL15U1 of Shanghai Synchrotron Radiation Facility (SSRF) (Extended Data Fig. 1). A monochromatic X-ray beam with wavelength of 0.4344\u0026nbsp;Å\u0026nbsp;for APS and 0.6199 for SSRF\u0026nbsp;was used. X-ray diffraction patterns were recorded with a MarCCD detector with an exposure time of ~20 s. Instrument parameter of the CCD were calibrated by a LaB\u003csub\u003e6\u003c/sub\u003e standard. The diffraction peaks of NaCl could not be observed over the entire pressure range.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eComputational methods\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFirst-principles calculations, including structure optimizations and AIMD simulations, were conducted by using the Vienna \u003cem\u003eab initio\u003c/em\u003e Simulation Package (vasp 6)\u003csup\u003e66\u003c/sup\u003e. The generalized gradient approximation\u003csup\u003e67\u003c/sup\u003e parametrized by Perdew, Burke, and Ernzerhof\u003csup\u003e68\u003c/sup\u003e was specified as exchange-correlation functional.\u0026nbsp;The\u0026nbsp;pseudopotential for the electron-ion interactions was described by the projector augmented plane-wave approximation\u003csup\u003e69\u003c/sup\u003e,\u0026nbsp;where valence electrons\u0026nbsp;1s\u003csup\u003e1\u003c/sup\u003e and 2s\u003csup\u003e2\u003c/sup\u003e2p\u003csup\u003e4\u003c/sup\u003e were considered for elements H and O, respectively. The plane-wave cutoff energy was set to 500 eV, and the Monkhorst-Pack\u0026nbsp;𝑘-spacing value was selected as\u0026nbsp;0.03 2𝜋/Å\u0026nbsp;and 0.05 2𝜋/Å for\u0026nbsp;structure optimizations\u0026nbsp;and\u0026nbsp;\u003cem\u003eab initio\u003c/em\u003e molecular dynamics simulations, respectively.\u0026nbsp;The initio structures, which are\u0026nbsp;3 × 3 × 3 supercells (162\u0026nbsp;atoms), for\u0026nbsp;\u003cem\u003eab initio\u003c/em\u003e molecular dynamics simulations\u0026nbsp;are from\u0026nbsp;structure optimizations\u0026nbsp;at 12, 16, 20, 26 and 40 GPa.\u0026nbsp;We adopted the NVT ensemble\u0026nbsp;at temperatures 500 K, 900 K and 1300K, lasting for\u0026nbsp;6\u0026nbsp;ps with a time step of\u0026nbsp;0.5\u0026nbsp;fs, and we allowed\u0026nbsp;1\u0026nbsp;ps for thermalization and then extracted data from the last\u0026nbsp;5\u0026nbsp;ps.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAlthough AIMD based on density functional theory captures water dissociation with high fidelity and is widely used in the simulations of superionic ice\u003csup\u003e42\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e53\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e70\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e71\u003c/sup\u003e, a quantitative description for the dissociation of both covalent and hydrogen bonds is still lacking. Bond length is the natural principle of identification for dissociation covalent and hydrogen bonds, but increasing pressure could significantly affect the bond length of covalent and hydrogen bonds. Therefore, it is difficult to quantitative describe the bond networks of ice in its high P-T phase diagram. To address this gap, we proposed a theoretical workflow as shown in Extended Data Fig. 8 and Supplementary Information section 2 based on ice rules. In our workflow, only one hypothesis is used to generate the bond networks of ice below the pressure of hydrogen-bond symmetrization: every H atom has and only has one covalent bond and one hydrogen bond in ice. Therefore, the number of covalent and hydrogen bonds of each O atom is quantitatively given, based on which the dissociation fractions of covalent and hydrogen bonds were defined to identify the dynamics phases of ice.\u003c/p\u003e"},{"header":"Tables","content":"\u003cp\u003e\u003cstrong\u003eTable\u003c/strong\u003e\u003cstrong\u003e.\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e1\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;| Qualitative bonding and compressibility across ice phases\u003c/strong\u003e\u003cstrong\u003e.\u003c/strong\u003e\u003c/p\u003e\n\u003cdiv\u003e\n \u003ctable border=\"1\" cellspacing=\"4\" cellpadding=\"0\" width=\"722\" class=\"fr-table-selection-hover\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 62px;\"\u003e\n \u003cp\u003ePhase\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003eCovalent O-H\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 99px;\"\u003e\n \u003cp\u003eH-bond\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003eMolecular bond-angle (H-O-H)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 158px;\"\u003e\n \u003cp\u003eH-bond geometry (O-H\u0026middot;\u0026middot;\u0026middot;O)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 129px;\"\u003e\n \u003cp\u003eKey characteristics\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003eCompressibility\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 62px;\"\u003e\n \u003cp\u003eIce-VII\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003eIntact\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 99px;\"\u003e\n \u003cp\u003eIntact\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003eRetained\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 158px;\"\u003e\n \u003cp\u003eMaintain four-fold coordination (Bernal-Fowler rules)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 129px;\"\u003e\n \u003cp\u003eCovalent bond is much shorter than hydrogen bond\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003eLow (difficult to compress)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 62px;\"\u003e\n \u003cp\u003eVII\u0026rsquo;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" style=\"width: 171px;\"\u003e\n \u003cp\u003e\u0026nbsp;Transition between covalent and hydrogen bond\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003eRetained\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 158px;\"\u003e\n \u003cp\u003eMaintain four-fold coordination\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 129px;\"\u003e\n \u003cp\u003eProton hopping, the dissociation fraction between covalent and H-bond is almost same\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003eModerately low (harder than ice-VII)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 62px;\"\u003e\n \u003cp\u003eX\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" style=\"width: 171px;\"\u003e\n \u003cp\u003eSymmetrization of covalent and hydrgen bond\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003eRetained\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 158px;\"\u003e\n \u003cp\u003eMaintain four-fold coordination\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 129px;\"\u003e\n \u003cp\u003eH-bond symmetrization, covalent \u0026asymp; H-bond length\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003eLowest (hardest to compress)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 62px;\"\u003e\n \u003cp\u003ePlastic ice\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003eIntact\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 99px;\"\u003e\n \u003cp\u003eSlightly disrupted\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003eRetained\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 158px;\"\u003e\n \u003cp\u003eSlightly disrupted, dynamically four-fold coordination\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 129px;\"\u003e\n \u003cp\u003eMolecular reorientation\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003eModerately high (easier than ice VII)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 62px;\"\u003e\n \u003cp\u003eWater-like superionic\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003eBroken\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 99px;\"\u003e\n \u003cp\u003eStrongly disrupted\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003eDissociated\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 158px;\"\u003e\n \u003cp\u003eStrongly disrupted, not maintain four-fold coordination\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 129px;\"\u003e\n \u003cp\u003eFree H diffusion\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003eHighest (easiest to compress)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 62px;\"\u003e\n \u003cp\u003eIce-like superionic\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 69px;\"\u003e\n \u003cp\u003eBroken\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 99px;\"\u003e\n \u003cp\u003eStrongly disrupted\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003eDissociated\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 158px;\"\u003e\n \u003cp\u003eStrongly disrupted, dynamically maintain four-fold coordination\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 129px;\"\u003e\n \u003cp\u003eConfined H diffusion\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003eModerately high\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-8118931/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8118931/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eWater is a key constituent of icy planets and exhibits a remarkable diversity of crystalline ice phases that govern their internal structure and evolution. Recent discoveries of plastic and superionic ice before melting have greatly extended the stability range of solid ice, yet experimental observations remain limited. In particular, the relationships, physical properties, and transition mechanisms of these phases remain largely unexplored. Here, we combine in situ synchrotron X-ray diffraction and \u003cem\u003eab\u003c/em\u003e initio molecular dynamics at 8\u0026ndash;80 GPa and 500\u0026ndash;900 K to link hydrogen-bond dynamics to lattice response. Ice at 500 K follows the same ice-VII sequence as at 300 K, but at 700 K it transforms into a plastic phase at 13.7\u0026ndash;37.4 GPa via rapid molecular reorientation. At 900 K, fast proton diffusion stabilizes a water-like superionic state at 14.3\u0026ndash;25 GPa, which converts to an ice-like superionic state at 31.2 GPa. Disruption of hydrogen bonds and enhanced molecular freedom markedly increase compressibility from ice-VII to plastic and superionic ice, with plastic ice-VII at 13.7\u0026ndash;37.4 GPa and 700 K even denser than ice-VII at 500 K. These findings redefine the high-pressure ice phase diagram and reveal how microscopic mechanisms govern both phase transitions and the exceptional compressibility of plastic and superionic ice, shedding light on the interiors and thermal evolution of icy planets.\u003c/p\u003e","manuscriptTitle":"Highly Compressible Plastic and Superionic Ice","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-04 05:07:31","doi":"10.21203/rs.3.rs-8118931/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"703cf8be-9866-4b44-8b1b-f9d6743dbce7","owner":[],"postedDate":"December 4th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":59021621,"name":"Earth and environmental sciences/Solid Earth sciences/Mineralogy"},{"id":59021622,"name":"Earth and environmental sciences/Planetary science/Giant planets"},{"id":59021623,"name":"Physical sciences/Physics/Condensed-matter physics/Structure of solids and liquids"},{"id":59021624,"name":"Earth and environmental sciences/Planetary science/Mineralogy"}],"tags":[],"updatedAt":"2026-01-13T08:06:20+00:00","versionOfRecord":[],"versionCreatedAt":"2025-12-04 05:07:31","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8118931","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8118931","identity":"rs-8118931","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}