Observation of a mixed close-packed structure in superionic water

Domain: physics
Published: 2025-12-07
Editorial outcome: minor_revision

Abstract
The study of superionic (SI) water has been a highly active research area since its theoretical prediction. Despite significant experimental and computational efforts, its melting curve and the stability of different oxygen lattices remain debated, impacting our understanding of SI ice’s peculiar transport properties. Experimental results at lower pressures show disagreement, whereas data at higher pressures are scarce due to the extreme challenges of such experiments. In this work, we present ultrafast X-ray diffraction results of water compressed by multiple shocks to pressures up to ~180 GPa. At pressures exceeding 150 GPa and temperatures around 2500 K, our diffraction patterns challenge the pure FCC-SI phase model, providing experimental evidence of the mixed close-packed superionic phase predicted by advanced ab initio calculations. At lower pressures, we observe simultaneous signatures of BCC and FCC structures within a pressure-temperature range consistent with some static-compression experiments, helping to resolve contradictory results in literature. These insights offer new constraints on the stability domains of SI phases and reveal detailed structural features, such as stacking faults. Our results advance the structural understanding of high-pressure SI ice to a level approaching that of ice I polymorphs, with potential implications for water-rich interiors of giant planets.

Introduction
Under extreme pressures and temperatures, water transforms into a variety of exotic phases, including the superionic (SI) phase, characterised by a crystalline oxygen lattice permeated by highly mobile protons. This phase, with its hybrid solid-liquid properties, has profound implications for fundamental physics and chemistry, material science, and planetary science, particularly in understanding the interiors of ice giants like Uranus and Neptune. However, despite decades of research, the phase diagram of superionic water remains poorly understood, with considerable controversy surrounding competing phases, precise phase boundaries, and transformation mechanisms. As water is the archetype of hydrogen-rich compounds, its phase behaviour has inspired continuous investigation, leading to the discovery of at least 19 (partially) crystalline phases under different thermodynamic conditions1. Therefore, addressing uncertainties on the water SI behaviour is critical for a general understanding of such an exotic state.

Early theoretical work suggested a uniform superionic phase inheriting the body-centered cubic (BCC) oxygen lattice of ice VII and X2–4. This belief was seriously questioned after thermodynamic integration-based free energy calculations predicted that a face-centred cubic (FCC) oxygen lattice is thermodynamically stable and could potentially affect transport properties5. Adding to the complexity, theoretical studies have proposed the existence of competing phases within the SI regime. Ab initio calculations by Sun et al.6 and Cheng et al.7 suggest that lower symmetry close-packed structures, such as stacking-disordered phases, may compete with or bridge the pure SI-BCC, SI-FCC, or SI-HCP phases. Characterising these faulted phases is essential, as dislocations can significantly influence viscosity, thus potentially affecting the interior dynamics of ice giants8. Interestingly, stacking disorder has also been observed in the water ice Ih and Ic polymorphs, forming ice Isd, which might be encountered in various environments in the Solar System, including the outermost ice-rich layer of icy moons9,10.

Experimentally probing the SI phase at the extreme pressures (>100 GPa) and temperatures (>2000 K) of planetary interiors poses significant challenges and the aforementioned predictions remain unconfirmed. Even at relatively low pressures, where diamond anvil cell (DAC) experiments allow robust phase determination, discrepancies abound. For instance, Prakapenka et al.11 observed the SI-FCC phase at pressures as low as 30 GPa, while Weck et al.12 found no evidence of SI-FCC below 50 GPa. The phase boundaries reported in these studies diverge significantly, with Prakapenka et al. observing transition temperatures that are 600 K higher than those reported by Weck et al. at 60 GPa. Most recently, Forestier et al.13 have characterised the BCC-FCC phase boundary and successfully recovered metastable FCC ice, thereby enabling a precise determination of the superionic transition onset. At higher pressures, the situation becomes even more complicated due to the experimental limitations of static compression methods. Dynamic compression techniques, such as laser-driven shock loading, have extended the exploration of the SI phase to higher pressures, but results remain inconsistent. Millot et al.14 reported the FCC-SI phase at 160 GPa and 3200 K, whereas Gleason et al.15, using a similar technique, observed a BCC phase under similar pressures and ~2700 K. The BCC phase would remain stable at 200 GPa and 3200 K up to temperatures exceeding 5000 K. In general, dynamic compression experiments face inherent challenges in achieving high-quality in-situ X-ray diffraction data under extreme conditions, which hindered the ability to unambiguously identify crystalline phases and to resolve microstructural features such as mixed phases or stacking faults.

In this work, we present ultrafast X-ray diffraction measurements of water compressed by reverberating shocks, achieving an unprecedented level of resolution for dynamic compression experiments. By exploring a broad region of the phase diagram within the predicted superionic regime, our data complement and refine interpretations from previous experiments. Our results at P≥150 (10) GPa and T≥2450 (135) K reveal a predominantly FCC lattice, consistent with the findings of Millot et al.14. However, the observed diffraction patterns deviate from the pure SI-FCC model and require the inclusion of stacking disorder to fully explain the data. This provides experimental evidence of a mixed close-packed phase in the superionic regime, as predicted by advanced ab initio calculations6,7. At lower pressures, our results supply critical data for the refinement of the high-pressure phase diagram of water, addressing the complex mechanisms underlying the SI phase transitions.

Results
The results reported here were obtained by coupling laser-driven compression with high-brilliance X-ray probes from free-electron lasers (Fig. 1). The thermodynamic conditions of the samples were constrained using optical diagnostics routinely applied in dynamic compression studies and correlated with time-resolved X-ray diffraction measurements (Fig. 2). This experimental configuration allows us to reach extreme pressures and temperatures while simultaneously capturing the structural evolution of the material on sub-nanosecond timescales. Comprehensive details of the experimental setup, diagnostic configuration, and analysis procedures are given in the ‘Methods’ section and Supplementary Information. Figure 3 presents representative diffraction patterns recorded under different compression conditions. Additional data are reported in the Supplementary Information. The diffracted signal from diamonds is minimised mainly by using single crystal windows instead of polycrystalline samples (see Supplementary Information Fig. S6). Moreover, diamond contributions are readily distinguishable from those originating from the water ice, due to their distinct textures: the diamond features appear as characteristic ‘textured’, while the water ice produces very ‘powder-like’ patterns. This distinction is particularly clear in the data shown in the lower panel of Fig. 3, where the feature near 4.2 Å−1 cannot be attributed to any diamond reflections but it is instead consistent with the 200 reflection of BCC ice (further details are in the Supplementary Information Section 4.1).

Results for the maximum compression achieved in our campaigns, at P~180 GPa and temperature of 2500–3000 K, are shown in Fig. 3a. Similar diffraction patterns were observed for pressures exceeding 150 GPa in the same temperature range, recorded across four independent experiments in the two campaigns (see Fig. 4b and Supplementary Information Fig. S12). These data clearly lack peaks associated with the BCC phase, ruling out its stability in this region of the phase diagram15. Instead, reflections corresponding to the 111, 200, and 220 planes of the FCC phase are observed, in line with earlier results obtained under comparable conditions14. However, the unprecedented spectral resolution of our data reveals that a single FCC phase is insufficient to fully explain the diffraction patterns. Considering contributions from the diamond windows, liquid signal, or grain-size effects do not allow a comprehensive understanding of the data (see Supplementary Information Section 6). Instead, the patterns are best explained by a model combining FCC and lower symmetry structures, such as hexagonal close-packed (HCP). The broadening of the HCP-related peaks relative to those of the FCC phase suggests a highly complex and disordered structure (see Supplementary Information Fig. S10). This observation suggests that the features are better explained by stacking disorder (SD) between FCC- and HCP-like sequences rather than by the coexistence of distinct phases16–18. This interpretation is reinforced by the remarkable agreement between our data and the stacking faults (SF) refinement, as illustrated in Figs. 3a and 4b. Further evidence for this scenario is also provided by the strong similarity between our diffraction pattern and machine-learning-potential-based simulations of mixed stacking, shown as yellow and blue curves in Fig. 4. The stacking disorder results from variations in the stacking sequence of the atomic planes, which can alternate locally between structures (see Fig. 4c inset). This behaviour in SI water has been predicted by theoretical simulations, which suggest that within specific pressure and temperature ranges—typically between 100 GPa and 800 GPa, and temperatures from 2000 K to 5500 K—lower symmetry SI stacked structures can compete with the FCC-SI phase6,7. In these predictions, the oxygen sublattice may alternate between different stacking configurations, predominantly HCP (AB), FCC (ABC), and dhcp (ABAC), leading to stacking defects within the crystals. Furthermore, in their simulations, Sun et al.6 report that these different structures are so similar that they can transform from one configuration to another. As a result, they were unable to distinguish between these structures, referring to a single ‘close-packed’ (CP-SI) phase. This assumption also holds for our data, and the FCC/HCP stacking fault phases were used to model the data. The stacking disorder is quantified by a factor α, representing the probability of observing an HCP sequence among three random layers. A perfect FCC crystal corresponds to α=0 (ABCABC sequence), while a perfect HCP crystal has α=1 (ABABAB sequence)16,17 (see Fig. 4c). In our dataset, α values range from 0.25 to 0.32, indicating a predominant FCC phase with 25–32% HCP contributions (see Supplementary Information section 5 for more details). These results suggest significant local stacking disorder within the lattice. However, the refinement assuming simple stacking faults does not fully capture all the experimental features. Small but systematic discrepancies are observed between the calculated and experimental patterns, notably near Q~3.4 Å−1 and around the (200)/(110) reflections (see Fig. 4b and Supplementary Information Fig. S12). This suggests that the local structure is more complex than a simple FCC-HCP stacking sequence, and that more sophisticated models may be required to accurately reproduce the diffraction features.

From our results, we cannot discriminate whether the observed stacking disorder arises from the strong uniaxial, high-strain-rate compression associated with shock-driven pressure, as for example observed in copper and gold19,20, or if it represents an intrinsic feature of superionic ice.

Given that stacking-disordered ice I (referred to as ice Isd) may be very widespread10, this structural feature could be a fundamental characteristic of water ice, suggesting that even at high pressures its behaviour is far more complex than previously understood. Kinetic effects similar to those observed in ice Ic21, where disordered cubic and hexagonal stacking sequences gradually lose cubic disorder, transforming over time into the more stable hexagonal phase, may also influence the detection of stacking fault, hindering its observation in static measurements13. In this case, the fast temporal scale of our experiment (ns) provides an optimal means to capture such details in the transformation dynamics.

Varying the laser energy and probing time, we can explore other regions of the phase diagram. Between ~70 and 120 GPa and at temperature ranging from 2000 to 2500 K, the diffraction patterns exhibit a notable change. In addition to the 111, 200 and 220 reflections of the FCC structure, we found characteristic peaks of the BCC structure, as shown in Fig. 3b and the Supplementary Information. Differently from the higher pressure data discussed above, here the width of the BCC and FCC peaks is similar (see Supplementary Information Fig. S8). Remarkably, both structures also exhibit the same density within the measurement uncertainties (see Supplementary Information Table S1). This observation is consistent with previous studies11,12,22, some of which12 attributed the coexistence of the two phases to a temperature gradient. However, since the occurrence of similar temperature and pressure gradients seems unlikely in our experiments, this finding raises the possibility that BCC-SI and FCC-SI structures may coexist locally, prompting further discussion on the underlying mechanisms and kinetics. Indeed, in this pressure/temperature range ab initio-based calculations find very similar chemical potentials μ for the two phases and even μ bcc=μ fcc in the hatched area in Fig. 5. This provides a plausible explanation for the simultaneous observation of BCC- and FCC-like signatures in our experimental data within this region of the P-T diagram (Fig. 5). In addition, they predict that the BCC arrangement remains kinetically stabilised, while the FCC structure is thermodynamically favoured7.

At even lower laser energies and longer delays, our diffraction data provide unequivocal evidence for the presence of a sole BCC structure. As shown in Fig. 3c and Supplementary Information Fig. S7 up to four distinct peaks are clearly observed. VISAR-calibrated hydrodynamic simulations place the experimental conditions between ~25 and 50 GPa and at temperature ranging from 900 to 1300 K. The densities inferred from the XRD (2.153(9), 2.207(2), and 2.371(8) g/cm3, respectively) at the simulated temperatures are consistent with BCC-SI (see Supplementary Information Fig. S17). These findings are in line with data available in literature11,12,22,23.

In Fig. 5, our data ensemble is plotted in the P-T diagram alongside previous results. To ensure clarity and effectively convey our findings, we present only a subset of existing experimental and theoretical results. A more comprehensive comparison with additional data is provided in the Supplementary Information (Fig. S18).

Our measurements corroborate the BCC-FCC phase boundary established by Weck et al.12, Forestier et al.13 and Millot et al.14, in disagreement with the higher-temperature transition proposed by Prakapenka et al.11, Husband et al.22 and Gleason et al.15 (Supplementary Information Fig. 18). For pressures and temperatures between 148(10)-180(20) GPa and 2445(150)-2700(350) K we identify a mixed close-packed structure, providing experimental support for the complex phase behaviour predicted by ab initio calculations of Cheng et al.7 and Sun et al.6. The high resolution of our XRD data allows us to revisit previous shock compression results. We unravel HCP stacking disorder within the FCC-SI phase previously identified by Millot et al.14, while contesting the stability of the BCC-SI in this P-T regime as suggested by Gleason et al.15. If intrinsic, the observed stacking disorder would affect the transport properties that govern internal structure, rheology, and magnetic field generation in ice-rich planets8,24. If instead shock-induced, its significance would depend on the lifetime of the metastable faulted phase, a question that our work cannot resolve, but that future investigations should address, given the growing recognition of giant impacts as a key hypothesis for shaping planetary interiors25,26.

Our work thus provides new critical constraints on water’s high-density phase behaviour. It demonstrates a high-pressure high-temperature diagram, shaped by mixed structural phases and kinetic effects, that is significantly more intricate than previously established. The complexity of the superionic regime mirrors the rich phase behaviour of solid ice and reinforces how water, a seemingly simple and ubiquitous molecule, continues to reveal exceptional and unexpected physical properties, with potential impacts on astrophysics and planetary science.

Methods
Shock-compression experiment setup and diagnostics
Two separate experimental campaigns were performed at the Linac Coherent Light Source (LCLS)27 and at the European X-ray Free-Electron Laser Facility (EuXFEL)28. In both campaigns, we used the high-energy nanosecond laser pulses, available at MEC and HED-HiBEF end-stations, respectively, to compress water to the SI regime and probed the atomic structure via ultrafast in-situ X-ray diffraction (XRD) using the XFEL beam. The water layer (30–60 μm thick) was confined between a diamond ablator (30 μm thick) and a diamond window (60 μm thick). A sketch of the target assembly is presented in the inset of Fig. 1a. A shock wave is generated in the diamond ablator by focusing a 10 ns square pulse operating at 527 nm into 250 or 300 μm phase-plate smoothed spots. The peak pressure was varied by changing the total energy delivered to the target, in the range of a few 1012 W/cm2. At these laser intensities elastic and plastic waves are generated in the diamond ablator and reverberate between the diamonds with higher shock impedance. The water layer thus undergoes a multi-step compression. This configuration allowed to probe the regime where the SI phase is predicted to be stable, which is not possible for single shock compression due to the steep rise in the temperature.

To obtain the generated pressure and temperature conditions, we used optical data combined with hydrodynamic simulations (see Fig. 2 and Fig. S1 in the Supplementary Information). Two line-imaging Velocity Interferometer System for Any Reflectors (VISARs) were used to record the compression history of each shot, by measuring the shock arrival and exit times in water, the velocity of the diamond window free surface, and, for some shots, the ablator/water interface velocity. Uncertainties in the VISAR velocity measurements were obtained from the standard deviation of the fringe-shift values within the analysed region. When only timing measurements were used, the uncertainties were derived from the combined errors in timings and sample thickness.

Hydrodynamic simulations
These measurements were then used to calibrate hydrodynamic simulations and infer the thermodynamic state in the water layer. For each experiment, simulations were run using the experimental laser temporal profile, optimising the intensity and the target thickness within uncertainties to better reproduce measured timings and velocities (see Supplementary Information). The simulated pressure and temperature conditions at the X-ray probing time were considered to be representative of the compressed water layer. The reported error bars on the inferred thermodynamic conditions account for the variability among numerical simulations that yield agreement with the experimental data within the VISAR uncertainties. Simulations were done using the hydrodynamic code ESTHER29. For diamond, we used the SESAME 7830 equation of state, implemented with mechanical properties issued from ref. 30. This enables us to account for the strong diamond elastic precursor, crucial for the description of the water compression history. For water, we implemented the recent ‘AQUA’ equation of state31. In the pressure and temperature range of the experiment, ‘AQUA’ shows a very good agreement with the most established ab initio calculations (see Supplementary Information Fig. S3), which corroborates the simulated conditions in water, especially for temperatures, where other widespread EOS, e.g., SESAME tables 7150 or 7154, differ substantially (Supplementary Information Fig. S3).

X-ray diffraction
In-situ XRD was performed using quasi-monochromatic (dE/E=0.2 – 0.5%) 9.5 keV and 18 keV X-ray pulses at the LCLS and EuXFEL, respectively. In both cases, X-ray pulses of ~50 fs duration and ~20–30 μm spot diameter were used, much smaller than the flat-compressed region to minimise pressure and temperature gradients. By varying the timing between the X-ray beam and the drive laser, we probed different stages of the compression history. This approach, combined with different drive laser energies, allowed us to probe a large range of pressure and temperature conditions of the water high-pressure phase diagram.

Molecular dynamics simulations
Classical molecular dynamics simulations have been performed with the open-source LAMMPS code (stable release version 3 March 2020)32 interfaced with n2p233 to employ a machine learning potential based on density functional theory (DFT) for water. The previously developed water potential was generated by Cheng et al.7 using a PBE DFT dataset that covers the entire thermodynamic range relevant for this work. Our simulations were carried out for fluid, solid, and superionic phases with BCC, FCC, Pbcm, and mixed structures at various thermodynamic conditions. The number of water molecules considered in the simulation box varied for each studied phase (e.g., up to 13824 molecules in the mixed phase). The simulations were performed in the NpT ensemble, and each calculation was run for 100 ps with a timestep size of 0.25 fs. Every 1000 time steps, a snapshot of all the atomic positions was collected, and the XRD spectrum was calculated for this configuration using the atomic simulation environment (ASE). The final XRD spectrum for each thermodynamic condition was obtained by averaging the 400 snapshots. The simulation input files, i.e., initial structures for each phase and MD parameters, can be found in the repository associated with Cheng et al.7.

Data availability
Data recorded for the experiment at the European XFEL will be openly available at https://doi.org/10.22003/XFEL.EU-DATA-004463-00 once the data embargo of the experiment campaign 4463 has been lifted (2026-10-30). Before the end of the data embargo, all relevant raw data will be available from the authors upon request. Source data are provided with this paper and available in the repository https://doi.org/10.6084/m9.figshare.3062437734. Additional data available upon reasonable request. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The target of this study, superionic ice, has recently been the subject of intense research competition, owing to its dual nature—possessing characteristics of both solids and liquids—as well as its significance in planetary science. The notable strength of this work lies in the use of XFEL and VISAR, which has enabled an unprecedentedly precise determination of temperature and pressure conditions, along with the acquisition of high-quality X-ray diffraction data. Remarkably, under such extreme high-temperature and high-pressure conditions, diffraction patterns are typically “spotty,” yet in this study, the authors obtained homogeneous and ideal “smooth” powder patterns. This has been crucial for the high-precision analysis of ice with stacking disorder, and for the first time, has allowed a truly crystallographic discussion of superionic ice.\nI am not a specialist in dynamic compression, and thus I am not in a position to evaluate the validity of the techniques employed in this work, such as VISAR. If the temperature and pressure in this study are indeed determined with higher accuracy and reliability than previous estimates, I have no doubt that the paper will eventually merit publication in Nature Communications. I therefore leave the judgment on this matter to other reviewers with expertise in dynamic compression.\nThat said, there remain several issues of concern that should be carefully addressed prior to publication.\n________________________________________\nInterpretation of the phase diagram\nThe primary concern lies in the authors’ claim that FCC and HCP phases coexist as stable phases in the phase diagram. \nFrom the viewpoint of classical thermodynamics, as is evident from the Gibbs phase rule (F – C + P = 2), for a single-component system (C = 1), coexistence of two phases (P = 2) leaves one degree of freedom (F = 1), meaning that such coexistence is typically allowed only along a phase boundary line in the phase diagram. However, in Fig. 5, a certain P–T region is depicted as a coexistence region of bcc and fcc phases.\nIndeed, as the authors argue, under such high-temperature conditions the free energy difference between bcc and fcc phases may be extremely small, and non-equilibrium coexistence of the two phases may occasionally occur. Nevertheless, only thermodynamically stable phases can have their own stability fields in a phase diagram, and it is inconsistent with classical thermodynamics for two phases of different symmetry to both be stable within the same P–T region.\nThe only possible exception (or out of the classical thermodynamic) would be if the bcc and fcc structures were intermixed on a scale smaller than ~100 nm, with no long-range periodicity maintained—essentially a nanostructured mixture. Such a nanoscale “phase separation” would not constitute a macroscopically homogeneous thermodynamic phase, and should perhaps be regarded differently in the context of a phase diagram. How to treat such nanoscopic phase-separated states in phase diagrams may be an open question for future condensed matter physics.\nA similar argument applies to the mixed-stacking SI phase. At such high temperatures, it is entirely plausible that stacking disorder could be energetically more favorable than either pure fcc or hcp phases, due to the entropic gain. Stacking disorder, by its nature, lacks long-range periodicity along a given axis and therefore has no macroscopic symmetry; changes in the stacking probability α do not alter the overall symmetry. Consequently, one might interpret it as a single homogeneous phase. Indeed, in the case of ice Isd, Lupi et al. (2018, Nature 551, 241–245; doi:10.1038/nature24279) have pointed out that ice Isd could be more stable than either pure Ih or Ic under certain temperatures and domain sizes.\nIn light of the above, the discussion of Fig. 5 and the purported coexistence of bcc and fcc phases should be reconsidered.\n________________________________________\nMinor points\n• The manuscript discusses possible implications for “giant impacts.” Could the authors support this statement with examples—preferably from the metallurgy literature—where stacking disorder affects mechanical properties? Relevant citations would be welcome. Furthermore, given that ice produced under shock compression is likely to revert rapidly to ambient-pressure phases, do its mechanical or transport properties still hold planetary significance during its brief lifetime? Overly speculative implications may risk diminishing the paper’s value and should be carefully considered.\n• In Fig. S5, the CeO₂ profile does not fit well. Both the high-Q and low-Q regions appear to deviate in peak positions (with asymmetric residuals at high-Q). This may be improved by optimizing the peak shift parameters and the profile function. Although fixing U = 0 seems reasonable given the limited 2θ range, V should likely be treated as a variable. Also, in the caption to Fig. S5, “CeCO₂” should be corrected to “CeO₂,” and X and Y may not be asymmetric parameters but Lorentzian terms; please confirm.\n• Throughout the manuscript, reflection indices, hkl, should not be enclosed in parentheses (see the introduction to International Tables for Crystallography, Vol. A).\n• The terms “stacking fault(s)” and “stacking disorder” should be used distinctly. “Stacking fault(s)” refers to individual defect(s) on one or plural layer(s) with differently aligned to surrounding layers, whereas “stacking disorder” refers to the state of a material containing many such faults, may be defined by detectable using diffraction methods . In most cases in this manuscript, “stacking disorder” would be the more accurate term rather than “stacking falut(s)”.\n• P-T conditions may be included in the legend in respective figures as well as run numbers.\n• Several figure references appear as “Fig. ??.” Please check and correct them.\n• On page 2 of the Supplementary Materials, “ice VII/VII*/X” is written. What does VII* mean? (Is it a typographical error for VII’?) Since the manuscript does not explain VII’, a definition should be provided.", "reviewer_2": "This paper describes a series of x-ray diffraction measurements of ice subjected to extremes of temperature and pressure via shock compression techniques. The approach is to confine the samples using diamond windows and then subject them to a shock wave triggered by a 10ns laser pulse with a very high energy density. This creates a rather complex environment described as “elastic and plastic waves [reverberating] between the diamonds” . While the sample responds to this, it is probed using a single 50fs pulse from a free electron laser (generated by either the LCSL or XFEL facilities in two separate experiments). Thus, it effectively takes a rapid snapshot, capturing the instantaneous state of the sample. By controlling the exact delay between shock and x-ray, the experimental team thus aim to select the P/T conditions of the measurement (although determining these relies on some ancillary hydrodynamic simulations).\n\nOne concern I had in interpreting the diffraction data is that they correspond to signal from a gauge volume that is has a 20-30um diameter and thickness equal to that of the sample. Correspondingly, although radial gradients are minimized by the small beam diameter, any axial gradient will be averaged. This seems like an important point, but I did not see this explicitly addressed in the manuscript. If I correctly read the right-hand panel of Fig 2, it looks like there is measurable step in density around 2/3 of the way through the sample of around 5%. Which, if I look at the equations of state in Fig S17, would map on to a 10-15 GPa pressure differential. The authors state that “temperature and pressure gradients seem unlikely in [their] experiments” , but then in the SI it is mentioned that “where the sample thickness was significant, a contribution from the liquid phase was observed” . This surely indicates that there are significant gradients within the sample if liquid and solid phases co-exist. How do the authors reconcile this?\n\nOf course, the data themselves contain information on any strain gradient. The authors helpfully included data and refinement details from a CeO2 standard, so the instrument resolution can be accounted for. With this information, substantial sample broadening is clearly present in all the datasets shown in Fig 3, although it is difficult to quantitatively assess this just by looking at the figures and any strain could also be convolved with particle size effects.\n\nThe diffraction profiles observed in the higher P/T region of the phase diagram do convincingly show both FCC and HCP features. And it seems reasonable to interpret these in terms of stacking faults. But I some concerns regarding the quality of the Lebail fits. All the patterns in Fig 3 seem to contain Bragg features that are much sharper than simulated peaks. These are easily discernable in the difference curves, circled in the image below:\nThese sharp features appear more-or-less exactly at the expected positions for the respective crystalline phases, so could be explained by a handful of fault-free crystallites, or extended fault-free domains within crystallites with correspondingly sharper peaks. Having a distribution of domain sizes seems to be rather likely.\n\nThere are also significant discrepancies between fit and data in the SF refinements. This is most notable in Fig 4b below. There are clearly “lumps” in the data that are not fitted by the model in the region from 2.7 to 3.5 Å−1. I think this is probably indicating that the peak profile model is not adequate to capture the complexities of the real system. It seems plausible that different directions in the lattice could retain coherence over longer distances than others. Usually, the profile parameters in refinement are parameterized in a way that would not capture hkl-dependent effects on peak broadening. I wondered if simple individual peak fits might describe the data better. And these might contain interesting information, for example, I was curious if either the HCP or FCC component had measurably and consistently sharper peaks than the other.\n\nA final thing I noticed was significant mismatch between observed and calculated peak positions for the FCC 220/HCP 110 reflection. This is clearly seen in the figure above where the calculated position is well to the left of the observed reflection (which is also much broader than the calculation). Since crystallites contributing this reflection have a different orientation relative to the load axis (and x-ray beam), could this be the result of anisotropic compressibility of the lattice? It seems like this should at least be commented on. In summary, it is impressive that it is possible to extract data of this quality from such extreme conditions. As intimated above, I think there may be quite a lot of complexity in the local structure and it seems as though more sophisticated models may be needed to accurate describe the diffraction features."}, "author_response": "Response to Reviewers : manuscript NCOMMS-25-52819-T\nWe would like to begin by sincerely thanking the referees for their thoughtful comments and detailed suggestions, which have been very helpful in further refining our diffraction analysis. We are also grateful for their positive assessment of our study. Acquiring diffraction data of this quality under shock-compression conditions is far from routine, and it is especially encouraging that expert referees have recognized the significance of our results. Their observations that “it is impressive that it is possible to extract data of this quality from such extreme conditions” andthatthishas“for the first time, allowed a truly crystallographic discussion of superionic ice” arebothmotivatingand validating for our work.\nIn the following, we provide detailed point-by-point responses to all referees’ comments and questions (our responses appear in blue). For clarity, all significant changes made in the revised manuscript are also highlighted in blue.\nResponse to the reviewers\nReviewer 1\nThe target of this study, superionic ice, has recently been the subject of intense research competition, owing to its dual nature-possessing characteristics of both solids and liquids—as well as its significance in planetary science. The notable strength of this work lies in the use of XFEL and VISAR, which has enabled an unprecedentedly precise determination of temperature and pressure conditions, along with the acquisition of high-quality X-ray diffraction data. Remarkably, under such extreme high-temperature and high-pressure conditions, diffraction patterns are typically “spotty,” yet in this study, the authors obtained homogeneous and ideal “smooth” powder patterns. This has been crucial for the high-precision analysis of ice with stacking disorder, and for the first time, has allowed a truly crystallographic discussion of superionic ice. I am not a specialist in dynamic compression, and thus I am not in a position to evaluate the validity of the techniques employed in this work, such as VISAR. If the temperature and pressure in this study are indeed determined with higher accuracy and reliability than previous estimates, I have no doubt that the paper will eventually merit publication in Nature Communications. I therefore leave the judgment on this matter to other reviewers with expertise in dynamic compression. That said, there remain several issues of concern that should be carefully addressed prior to publication.\nReviewer Comment1.1 — Interpretation of the phase diagram The primary concern lies in the authors’ claim that FCC and HCP (BCC) phases coexist as stable phases in the phase diagram. From the viewpoint of classical thermodynamics, as is evident from the Gibbs phase rule (F – C + P = 2), for a single-component system (C = 1), coexistence of two phases (P = 2) leaves one degree of freedom (F = 1), meaning that such coexistence is typically allowed only along a phase boundary line in the phase diagram. However, in Fig. 5, a certain P–T region is depicted as a coexistence region of bcc and fcc phases.\nIndeed, as the authors argue, under such high-temperature conditions the free energy difference between bcc and fcc phases may be extremely small, and non-equilibrium coexistence of the two phases may occasionally occur. Nevertheless, only thermodynamically stable phases can have their 1 own stability fields in a phase diagram, and it is inconsistent with classical thermodynamics for two phases of different symmetry to both be stable within the same P-T region.\nThe only possible exception (or out of the classical thermodynamic) would be if the bcc and fcc structures were intermixed on a scale smaller than 100 nm, with no long-range periodicity maintained—essentially a nanostructured mixture. Such a nanoscale “phase separation” would not constitute a macroscopically homogeneous thermodynamic phase, and should perhaps be regarded differently in the context of a phase diagram. How to treat such nanoscopic phase-separated states in phase diagrams may be an open question for future condensed matter physics.\nA similar argument applies to the mixed-stacking SI phase. At such high temperatures, it is entirely plausible that stacking disorder could be energetically more favorable than either pure fcc or hcp phases, due to the entropic gain. Stacking disorder, by its nature, lacks long-range periodicity along a given axis and therefore has no macroscopic symmetry; changes in the stacking probability α do not alter the overall symmetry. Consequently, one might interpret it as a single homogeneous phase. Indeed, in the case of ice Isd, Lupi et al. (2018, Nature 551, 241–245; doi:10.1038/nature24279) have pointed out that ice Isd could be more stable than either pure Ih or Ic under certain temperatures and domain sizes. In light of the above, the discussion of Fig. 5 and the purported coexistence of bcc and fcc phases should be reconsidered.\nReply:\nWe are very grateful to the Reviewer for raising this important point.\nInourshockexperiments,weobservecoexistenceofthebccandfccstructuresinapressure/temperature\nregime, where, according to theoretical predictions, the free-energy difference between the two phases\nbecomes extremely small. We therefore interpret this domain not as a stability field in the thermo-\ndynamic sense, but as a region of metastable or non-equilibrium coexistence reflecting the structures\nexperimentally detected. Metastability appears to be a common feature of high-pressure ice: in static\ncompression studies, for example, the fcc phase is known to persist metastably over the bcc phase at\nroom temperature, despite not being the lowest free-energy phase (Forestier et al. PRL. 2025)\nWe fully agree with the Reviewer that, in light of this discussion, Figure 5 should not be regarded as\na phase diagram in the strict thermodynamic sense, since we cannot determine the equilibrium stability\nof the indicated phases. Rather, it should be viewed as a P–T diagram (experimental observation\nmap) showing the structures identified in our measurements. This approach is standard in high-pressure\nresearch, including studies using static compression in diamond anvil cells.\nTo clarify this point, we have revised both the caption of the figure and the main text to explicitly\nstate that Figure 5 represents a P-T diagram (experimental observation map), rather than a thermody-\nnamic equilibrium phase diagram, and that the “region of ”BCC + FCC” corresponds to experimentally\nobserved, likely metastable, coexistence.\nReviewer Comment1.2 — The manuscript discusses possible implications for “giant impacts”.\nCould the authors support this statement with examples—preferably from the metallurgy litera-\nture—where stacking disorder affects mechanical properties? Relevant citations would be welcome.\nFurthermore,giventhaticeproducedundershockcompressionislikelytorevertrapidlytoambient-\npressurephases, doitsmechanicalortransportpropertiesstillholdplanetarysignificanceduringits\nbrief lifetime? Overly speculative implications may risk diminishing the paper’s value and should\nbe carefully considered.\nReply: As emphasized on p. 8 of the manuscript, our experiments clearly reveal the presence of\nstacking disorder. However, the available data do not allow us to determine whether this disorder is\n2\nan intrinsic feature of the superionic phase or instead arises from the strong uniaxial, high–strain-rate\ncompression associated with shock loading. Both scenarios are interesting from a fundamental point of\nview and potentially relevant for planetary science. If the disorder is intrinsic to superionic ice, its conse-\nquences for giant planets would be directly linked to the physical properties of the faulted ice phase. In\nmetallurgy, stacking disorder is well known to influence strength and transport properties of metals and\nalloys (e.g., Su et al., Mater. Sci. Eng. A 803, 140696 (2021)). In planetary science, it is well known\nthat crystalline defects play a central role in controlling rheological properties (e.g., Karato et al., Prog.\nEarth Planet. Sci. 12, 27 (2025); Sumita & Bergman, in Treatise on Geophysics, Elsevier, 2007) and\nsimulations at extreme conditions are striving to include them (e.g., Li & Scandolo, Phys. Rev. B 109,\n184108 (2024)). Stacking disorder is also known to affect viscosity and thermal conductivity, including\nin ice at low pressures (e.g., F. Matusalem et al, PNAS 119, e2203397119 (2022), Johari & Andersson,\nJ. Chem. Phys. 143, 054505 (2015)). If intrinsic, such effects could therefore directly impact models\nof planetary interiors, transport properties, and even dynamo generation. Alternatively, if the disorder is\nshock-induced, it could provide important insight into the physics of planetary collisions, which cannot\nbe reproduced by static compression alone. For instance, shocked feldspars or silicates in meteorites\nform maskelynite or diaplectic glass (e.g., Rubin, Icarus 257, 221–229 (2015)), while static experiments\nyield only crystalline phases. Similarly, iron alloys form equilibrium crystalline structures under static\nhigh-pressure experiments but can adopt metastable phases under shock (e.g., Boettger & Wallace,\nPhys. Rev. B 55, 5 (1997)). By analogy, shock-induced stacking disorder in superionic ice could rep-\nresent a transient but relevant state during giant impacts. Whether such a metastable phase persists\nlong enough to influence impact dynamics depends on its lifetime relative to hydrodynamic timescales,\na point our current experiments cannot resolve. Given the increasing recognition of giant impacts as a\nkey hypothesis for shaping planetary interiors (e.g.,e.g., Helled et al., Space Sci. Rev. 216, 38 (2020);\nLiu et al., Nature 572, 355–357 (2019); Mu¨ller et al., A&A638, A124 (2020); Esteves et al., Icarus\n429, 116428 (2025); Kegerreis et al., Astrophys. J. 861, 52 (2018); Reinhardt et al., MNRAS 492,\n5336–5353 (2020)), we believe it important to highlight this possibility. We note that longer-duration\nshock studies at these extreme conditions could be challenging, so that theoretical work and numerical\nsimulations will be essential to address this aspect.\nWe agree with the referee that this aspect was not adequately discussed in the first version of the\nmanuscript, and we have now changed the sentence at old line 197:\nThe identified stacking faults may significantly influence the mechanical and transport properties\nof superionic water [8], particularly under the extreme conditions associated with giant impacts. As a\nresult, our understanding of ice-rich planetary interiors—such as those expected in Uranus and Nep-\ntune—could be substantially revised, given their dependence on properties like viscosity and thermal\nconductivity.\nwith\nIf intrinsic, the observed stacking disorder would affect the transport properties that govern internal\nstructure, rheology and magnetic field generation in ice-rich planets [8,Johari & Andersson, J. Chem.\nPhys. 143, 054505 (2015)]. If instead shock-induced, its significance would depend on the lifetime of\nthe metastable faulted phase, a question that our work cannot resolve, but that future investigations\nshould address, given the growing recognition of giant impacts as a key hypothesis for shaping planetary\ninteriors (e.g., Helled et al., Space Sci. Rev. 216, 38 (2020); Liu et al., Nature 572, 355–357 (2019)).\nlow-Q regions appear to deviate in peak positions (with asymmetric residuals at high-Q). This may\nbe improved by optimizing the peak shift parameters and the profile function. Although fixing U\n= 0 seems reasonable given the limited 2θ range, V should likely be treated as a variable. Also,\nin the caption to Fig. S5, “CeCO2” should be cor- rected to “CeO2,” and X and Y may not be\nasymmetric parameters but Lorentzian terms; please confirm.\nReply: We thank the Reviewer for pointing this out. We have repeated the refinement of the CeO\n2\ncalibrant profile, this time keeping U fixed at 0 but allowing V and W to vary, together with the Zero\n2θ shift. The rest of the refinement procedure remained unchanged.\nThis adjustment significantly improved the agreement between observed and calculated profiles\nacross the entire Q-range, including both low-Q and high-Q regions. The final refined profile parameters\nare: U = 0.0,V = 0.012011,W = 0.004258,X = 0.058403,Y = 0.057187,Z = −0.0026\nzero\nThe fit is now much better than in the previous version, and the asymmetric residuals at high-\nQ are largely suppressed. We also corrected the caption typo (“CeCO ” −→ “CeO ”) as suggested.\n2 2\nFinally, we confirm that in the Thompson–Cox–Hastings pseudo-Voigt function used in FullProf, X and\nY are Lorentzian terms (size and strain broadening, respectively), not asymmetry parameters. This new\ncalibration does not affect the main findings of our work. The main changes concern lattice parameters,\ngrain sizes and micro strains, but such adjustments lie within the error bars.\nFigure 1: Refinement of the CeO calibrant profile using the Thompson–Cox–Hastings function.\nare V = 0.0120, W = 0.0043, X = 0.058, Y = 0.057, Z = -0.0026. The fit quality is significantly\nzero\nimproved compared to the previous versio\nReviewer Comment1.4 — Throughout the manuscript, reflection indices, hkl, should not be\nenclosedinparen-theses(seetheintroductiontoInternationalTablesforCrystallography, Vol. A).\nReply: In the revised manuscript, all reflection indices hkl have been reformatted to follow the conven-\ntion of the International Tables for Crystallography, Vol. A, i.e., without parentheses.\nReviewerComment1.5 — Theterms“stackingfault(s)”and“stackingdisorder”shouldbeused\ndistinctly. “Stackingfault(s)”referstoindividualdefect(s)ononeorplurallayer(s)withdifferently\n4\nalignedtosurroundinglayers,whereas“stackingdisorder”referstothestateofamaterialcontaining\nmany such faults, may be defined by detectable using diffraction methods . In most cases in this\nmanuscript, “stacking disorder” would be the more accurate term rather than “stacking falut(s)”\nReply: Wethanktherefereeforthisimportantremark. Intherevisedversionofthemanuscript,wenow\ndistinguish carefully between stacking fault(s), which refer to local defects in the stacking sequence, and\nstacking disorder, which describes the overall disordered state of the structure as observed in diffraction.\nSince our experimental data reveal mainly average structural effects rather than isolated planar defects,\nwe agree that ’stacking disorder’ is the most appropriate term in most contexts. We have therefore\nsystematically revised the manuscript to adopt this terminology.\nReviewer Comment1.6 — P-T conditions may be included in the legend in respective figures\nas well as run numbers\nReply: In the main text figures, the P–T conditions and run numbers are already included. We have\nupdated the supplementary figures accordingly, to ensure that the P–T conditions and run numbers are\nsystematically visible.\nReviewer Comment1.7 — Several figure references appear as “Fig. ??.” Please check and\ncorrect them\nReply: Theoccurrencesof “Fig. ??”wereduetomissingorbrokencross-referencesinthemanuscript.\nhave checked carefully check all figure references and correct them to ensure consistency throughout\nthe text.\nReviewer Comment1.8 — On page 2 of the Supplementary Materials, “ice VII/VII*/X” is\nwritten. What does VII* mean? (Is it a typographical error for VII’ ?) Since the manuscript does\nnot explain VII’, a definition should be provided\nReply: The notation VII∗ was indeed a typographical error, and it referred to ice VII′, as defined in\nthe articles of Hernandez and Caracas, Phys. Rev. Lett. (2016) and Queyroux et al. Phys. Rev. Lett.\n(2020). These papers are now cited as a reference for such phase.\nReviewer 2\nThis paper describes a series of x-ray diffraction measurements of ice subjected tovextremes of\ntemperatureandpressureviashockcompressiontechniques. Theapproachistoconfinethesamples\nusing diamond windows and then subject them to a shock wave triggered by a 10ns laser pulse\nwith a very high energy density. This creates a rather complex environment described as “elastic\nand plastic waves [reverberating] between the diamonds” . While the sample responds to this, it is\nprobed using a single 50fs pulse from a free electron laser (generated by either the LCSL or XFEL\nfacilities in two separate experiments). Thus, it effectively takes a rapid snapshot, capturing the\ninstantaneous state of the sample. By controlling the exact delay between shock and x-ray, the\nexperimentalteamthusaimtoselecttheP/Tconditionsofthemeasurement(althoughdetermining\nthese relies on some ancillary hydrodynamic simulations).\ncorrespond to signal from a gauge volume that is has a 20-30um diameter and thickness equal to\nthat of the sample. Correspondingly, although radial gradients are minimized by the small beam\ndiameter, any axial gradient will be averaged. This seems like an important point, but I did not\nsee this explicitly addressed in the manuscript. If I correctly read the right-hand panel of Fig 2, it\nlooks like there is measurable step in density around 2/3 of the way through the sample of around\n5%. Which, if I look at the equations of state in Fig S17, would map on to a 10-15 GPa pressure\ndifferential. The authors state that “temperature and pressure gradients seem unlikely in [their]\nexperiments” , but then in the SI it is mentioned that “where the sample thickness was significant,\na contribution from the liquid phase was observed” . This surely indicates that there are significant\ngradients within the sample if liquid and solid phases co-exist. How do the authors reconcile this?\nReply: Wethanktherefereeforraisingthisimportantpoint. Indeed,ascorrectlynoted,thesmallX-ray\nbeam diameter minimizes radial gradients, while the averaging across the sample thickness may include\ncontributionsfromanyaxialgradients. ThisissueisdiscussedinSection3oftheSupplementaryMaterial,\nbut we are happy to provide further clarification here. The presence and importance of axial gradients\ndepend strongly on the hydrodynamic history of the shock compression and the probing time. To\nreach a representative description of these effects, we combined hydrodynamic simulations with optical\ndiagnostics (see Section 2 of the Supplementary Material, Fig. 2 and Fig. S1). As the referee observes,\nFig. 2 does display a density “step”. This feature arises in the LCLS data because reverberations from\nthe 30 µm diamond ablator overtake the first shock waves within the water layer (Fig.S1 a,b,c). As\na result, a steep gradient forms, with a colder region produced by more numerous reverberations, and\na hotter region resulting from fewer compressions. In this case, two sets of distinct thermodynamic\nconditions are generated (Fig. S4a,b). Although both regions contribute to the diffraction patterns, we\nfind that the hotter part is close to (or above) melting (Fig. S4a,b and Fig. S18) and primarily adds a\nliquidbackgroundsignal. Bycontrast,thecrystallinediffractionsignaloriginatespredominantlyfromthe\ncolder region. This interpretation is consistent with the need to include a substantial liquid background\ncomponent when refining the LCLS diffraction data (Section 4 of the Supplementary Material). In\ncontrast, for the EuXFEL experiments, we used thinner water layers (∼25–30 µm), which prevented\nthe overtaking of shock reverberations and produced much more uniform conditions (Fig. S1d and\nFig. S4c,d). The resulting diffraction data are consistent with those from the “cold” part of the LCLS\nexperiments and show similar features under comparable conditions (see Fig. S8 and Fig. S9). This\nprovides further support for our interpretation.\nFinally, regarding the quoted sentence “temperature and pressure gradients seem unlikely in [their]\nexperiments”: we agree that, when read in isolation, this wording may appear ambiguous. In its context\n(line 169 of the old version of the manuscript), the full sentence reads: “However, since the occurrence\nof similar temperature and pressure gradients seems unlikely in our experiments, this finding raises the\npossibility of a genuine coexistence of BCC-SI and FCC-SI phases, prompting further discussion on the\nunderlyingmechanismsandkinetics.”Thissentencewasspecificallyincludedtoaddressthesimultaneous\nobservation of BCC and FCC structures in both static and dynamic experiments. Our intention was not\nto deny the presence of gradients in the shock-compression data, but rather to emphasize that such\ngradients are unlikely to match those in static experiments.\nReviewer Comment2.2 — Of course, the data themselves contain information on any strain\ngradient. The authors helpfully included data and refinement details from a CeO2 standard, so the\ninstrument resolution can be accounted for. With this information, substantial sample broadening\n6\nis clearly present in all the datasets shown in Fig 3, although it is difficult to quantitatively assess\nthis just by looking at the figures and any strain could also be convolved with particle size effects.\nReply: Thebroadeiningofthelinesassociatedtothegrainsizeeffectshasbeenevaluatedanddiscussed\ninthesupplementarymaterial, section6.5. Wefindthatthegrainsizebroadeninghasanegligebleeffect\non the widths of the collected diffraction peaks.\nReviewer Comment2.3 — The diffraction profiles observed in the higher P/T region of the\nphase diagram do convincingly show both FCC and HCP features. And it seems reasonable to\ninterpret these in terms of stacking faults. But I some concerns regarding the quality of the Lebail\nfits. All the patterns in Fig 3 seem to contain Bragg features that are much sharper than simulated\npeaks. These are easily discernable in the difference curves, circled in the image below:\nThese sharp features appear more-or-less exactly at the expected positions for the respective\ncrystallinephases,socouldbeexplainedbyahandfuloffault-freecrystallites,orextendedfault-free\ndomains within crystallites with correspondingly sharper peaks. Having a distribution of domain\nsizes seems to be rather likely.\nReply: We appreciate the referee’s detailed comments and for highlighting the differences observed\nin the curves of Fig. 3. We agree that it is indeed plausible that our data reflect a distribution of\ndomain sizes. However, we believe it is difficult to infer the origin of such a distribution solely from\nthe available data. As the referee points out, similarly sharp features are also visible in the BCC phase\n(Fig. 3c), where the data can be explained without invoking faults. We therefore consider it more\nlikely that these features arise from the nucleation process associated with compression. Experimentally,\nsharper features than those obtained from calculated refinements have also been reported for shocked\nSiO (see Suplementary Fig. 2. of A.E. Gleason et.al., Nat. Com. 2015), which may support our\n2\ninterpretation. Nevertheless, we fully agree with the referee that future studies should examine these\nfeatures in greater detail.\nReviewer Comment2.4 — There are also significant discrepancies between fit and data in the\nSF refinements. This is most notable in Fig 4b below. There are clearly “lumps” in the data that\nare not fitted by the model in the region from 2.7 to 3.5 ˚A−1. I think this is probably indicating\nthat the peak profile model is not adequate to capture the complexities of the real system. It seems\nplausible that different directions in the lattice could retain coherence over longer distances than\nothers.\nReply: We fully agree with the referee that the local structure may exhibit significant complexity\nthat our simple intrinsic stacking model does not capture. As we explicitly state in Section 5 of the\nSupplementary Material, “the actual microstructure is likely more complex than a simple FCC–HCP\nlayer alternation.” Nonetheless, we chose not to overinterpret our data by attempting to construct a\nmore complex structural model.\nReviewer Comment2.5 — Usually, the profile parameters in refinement are parameterized in\na way that would not capture hkl-dependent effects on peak broadening. I wondered if simple indi-\nvidual peak fits might describe the data better. And these might contain interesting information,\nfor example, I was curious if either the HCP or FCC component had measurably and consistently\nsharper peaks than the other.\nrepresentative reflections (HCP: 100, 002, 101; FCC: 111, 200), as summarized in Table 1. On average,\nthe HCP reflections are broader (mean FWHM ∼ 0.27 ˚A−1) compared to the FCC reflections (mean\nQ\nFWHM ∼ 0.10 ˚A−1). However, the broadening within HCP is not uniform: HCP(100) is comparably\nQ\nsharp to FCC(111), whereas HCP(002) and HCP(101) are substantially broader. We also observed that\nthe HCP peaks tend to have a shape factor η ≈ 0.5, indicating a more Gaussian-like profile, whereas the\nFCC peaks are more Lorentzian (η ≈ 1). These results are consistent with what we already presented in\nthe Supplementary Materilas (see SM section 6.2), where separate FCC and HCP fits revealed similar\ntrends, in line with disorder-dominated broadening.\nFigure 2: XRD pattern (Run 111) with cumulative peak fits from individual pseudo-Voigt compo-\nnents (FCC in blue, HCP in green). SF simulation with faulting probability α = 0.3 (blue line) is\nshown for comparison.\nPhase Reflection (hkl) FWHM (˚A−1) η\nQ\nHCP 100 0.12999 0.5\nFCC 111 0.06629 1.0\nHCP 002 0.26538 0.5\nHCP 101 0.42035 0.5\nFCC 200 0.1305 1.0\nTable 1: Individual pseudo-Voigt fits in Q-space for representative reflections. The pseudo-Voigt\nshape parameter η weights Lorentz vs Gauss (η = 1 Lorentz, η = 0 Gauss).\nand calculated peak positions for the FCC 220/HCP 110 reflection. This is clearly seen in the\nfigure above where the calculated position is well to the left of the observed reflection (which is also\nmuch broader than the calculation). Since crystallites contributing this reflection have a different\norientation relative to the load axis (and x-ray beam), could this be the result of anisotropic\ncompressibility of the lattice? It seems like this should at least be commented on. In summary, it\nis impressive that it is possible to extract data of this quality from such extreme conditions. As\nintimated above, I think there may be quite a lot of complexity in the local structure and it seems\nas though more sophisticated models may be needed to accurate describe the diffraction features.\nReply: We agree with the referee that a noticeable misfit is observed for the FCC(220)/HCP(110)\nreflection. Although this discrepancy is visible in Fig. 3a & 4b, its direction and magnitude vary\nbetween runs (e.g. Run 111 vs Run 250), indicating that it is not a systematic effect. We do not\nbelieve that anisotropic compressibility is the main cause of this misfit. If lattice anisotropy were\nresponsible, one would expect a consistent offset of the FCC(220)/HCP(110) reflection in the same\ndirection across all runs, which is not observed (e.g. Run 111 vs Run 250 show opposite trends).\nOur mixed-stacking machine-learning-potential (MLP) simulations reproduce the FCC(220)/HCP(110)\npositions much more accurately, yielding lattice spacings in closer agreement with experiment. This\nsuggests that the residual discrepancies in the simplified refinements reflect limitations of the refinement\nparameterization, which does not fully capture the microstructural complexity of the system. As already\ndiscussed in the Supplementary and emphasized by the referee, more sophisticated models will likely be\nneeded to describe the diffraction features in detail.\n11"}, {"round": 2, "reviewer_comments": {"reviewer_1": "I confirm that all the concerns I raised in the previous review have been appropriately corrected. In addition, it appears that the comments from other experts on the dynamic compression experiments have been adequately addressed. There are no further points raised by me and I can conclude that the manuscript can be ready to be published.", "reviewer_2": "The authors have provided detailed responses to the questions I had on the original manuscript. I thought the additional analysis they report in their response to my comments: showing that the FWHM of HCP is on average broader than FCC is interesting and I thought they might consider adding this to their SM? Beyond this, I'm happy to recommend publication."}, "author_response": ""}]

Decision letter