{"id":"cb256428-9f61-4596-bc4c-2075fadfef21","arxiv_id":"2411.15683","paper_version":3,"verdict":"REJECT","confidence":"MODERATE","novelty_score":4.0,"correctness_risk":"high","formal_verification":"none","parameter_count":2,"one_line_summary":"At 20 GPa, Mg2IrH6 and Mg2RhH6 are predicted to have superconducting transition temperatures of 96.3 K and 80.2 K, respectively.","lead":"This paper uses first-principles calculations to predict that two Mg-based hydrides, Mg2IrH6 and Mg2RhH6, superconduct above 77 K at 20 GPa. It screens fifteen atomic substitutions in the X2MH6 family for stability and superconducting properties.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Thermodynamic stability at 20 GPa is supported only by zero-pressure OQMD decomposition energies, so the claimed high-Tc compounds may decompose under pressure.","rationale":"The reader's verdict identifies the abstract/body mismatch, which is a genuine reporting error that justifies rejection as submitted. However, the most load-bearing concern for the physical central claim is thermodynamic stability at 20 GPa. The paper's own evidence for stability is Table 1, built from OQMD zero-pressure energies, with no pressure correction. For Mg2IrH6, the margin is only -0.041 eV, so pressure effects could easily reverse stability. For Mg2RhH6 the margin is larger (-1.434 eV), but the reference phases, including H2, are still evaluated at the wrong pressure. The authors had access to Dolui et al.'s 20 GPa convex hull for Mg2IrH6 but did not report pressure-dependent hulls for the substituted compositions; the formation-energy screen in Section 3 likewise omits pressure corrections. Consequently, the list of ten metallic compounds, including the two high-Tc candidates, is not established at 20 GPa. A pressure-corrected decomposition-energy calculation would settle this specific issue. This is exactly the reader's weakest_assumption, so I agree with the reader's identification. Since the stability gap reinforces the REJECT verdict, the verdict should remain unchanged; if pressure-corrected stability is later supplied, the paper could be reconsidered after also addressing the abstract inconsistency and missing convergence tests.","tokens_in":10390,"tokens_out":5288,"duration_ms":49603,"concrete_test":"Recompute the decomposition enthalpy Ed at 20 GPa for the ten X2MH6 phases and all decomposition products in Table 1 using the same VASP/PAW-PBE settings as the relaxations (550 eV cutoff, 2π × 0.02 Å⁻¹ k-spacing), performing variable-cell relaxations at a fixed 20 GPa pressure for every phase, and evaluating H = E + PV at 20 GPa rather than zero-pressure E. For the hydrogen reference, use the stable high-pressure phase of H2 at 20 GPa. If any Ed in Table 1 becomes positive, the corresponding compound is not thermodynamically stable at 20 GPa and should be removed from the list of candidate superconductors; if all remain negative, this objection is resolved.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim that Mg2IrH6 and Mg2RhH6 superconduct at 96.3 K and 80.2 K under 20 GPa presupposes that these phases are thermodynamically stable at 20 GPa. The only quantitative stability evidence for the substituted compounds is Table 1, which lists decomposition energies Ed obtained from Open Quantum Materials Database (OQMD) energies. OQMD stores zero-pressure, zero-temperature DFT total energies; there is no PV term. At 20 GPa the relevant quantity is H = E + PV, and the PV contribution differs by phase. Hydrides are especially sensitive: the reference H2 phase at 0 GPa is a molecular gas/solid, whereas at 20 GPa dense solid hydrogen phases are relevant. Using the 0 GPa H2 energy in reactions such as 'Mg2IrH6 = Mg2IrH5 + 1/2 H2' can shift Ed by amounts comparable to the reported values; Mg2IrH6 has Ed = -0.041 eV, so a shift of a few hundredths of an eV per atom would place it above the 20 GPa hull. The manuscript relies on Dolui et al.'s 20 GPa convex hull for Mg2IrH6, but for the other nine compounds it does not provide a pressure-dependent hull; the formation-energy screen in Section 3 uses average elemental energies without pressure correction. Thus the screening funnel that selects the ten metallic candidates, including the two high-Tc phases, is not actually demonstrated at 20 GPa. If either compound is thermodynamically unstable at 20 GPa, the headline claim loses its physical basis regardless of the quality of the electron-phonon calculation.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper uses first-principles calculations to study X2MH6-type hydrides (X = Be, Mg, Ca, Sr, Ba; M = Co, Rh, Ir) at 20 GPa, obtained by substituting atoms in the previously predicted ambient-pressure superconductor Mg2IrH6. After screening for mechanical, thermodynamic, and dynamical stability and excluding a non-metallic phase, the authors report ten stable metallic compounds. Allen-Dynes modified McMillan electron-phonon calculations give superconducting critical temperatures for four of them, with Mg2IrH6 and Mg2RhH6 at 96.3 K and 80.2 K, respectively, at 20 GPa; Mg2CoH6 and Ca2IrH6 are calculated at 45.8 K and 2.2 K. The paper attributes the higher Tc values to mid-frequency hydrogen phonons, van Hove singularities near the Fermi level, and complex Fermi-surface topology. The arXiv abstract, however, states different headline numbers (ten superconducting compounds and three Tc values above 100 K), which is inconsistent with the full text and Table 2.","tokens_in":10621,"tokens_out":6174,"duration_ms":52852,"significance":"If the thermodynamic stability at 20 GPa is confirmed, the prediction of Tc above 77 K at a modest pressure of 20 GPa would be a meaningful step because most high-Tc hydrides require hundreds of GPa. The electron-phonon protocol is standard (VASP relaxation, PHONOPY phonons, Quantum ESPRESSO EPC, Allen-Dynes McMillan with mu* = 0.1), and the paper makes falsifiable predictions for specific compounds. The two headline compounds were, however, already predicted to be ambient-pressure superconductors (Dolui et al. reported 160 K for Mg2IrH6; Sanna et al., Cerqueira et al. reported 77 K and 59.4 K, respectively), so the incremental contribution is the 20 GPa extension and the substitutional screening. The mechanistic discussion of mid-frequency hydrogen phonons and van Hove singularities is plausible and useful, but the significance is currently conditional on a missing pressure-dependent stability proof and on resolving the abstract/text inconsistency.","major_comments":[{"comment":"The thermodynamic stability of the ten X2MH6 compounds at 20 GPa is not established by the evidence presented. The decomposition energies Ed in Table 1 are obtained from OQMD total energies, which are zero-pressure, zero-temperature DFT energies without a PV term. At 20 GPa the relevant quantity is H = E + PV, and the PV contributions differ between the X2MH6 phase and the decomposition products; for hydrides the reference H2 phase at 20 GPa is a dense solid, not the 1/2 H2 gas used in several pathways. For Mg2IrH6, Ed = -0.041 eV is so small that pressure corrections of a few hundredths of an eV can change its sign, so the claim that Mg2IrH6 and Mg2RhH6 are stable at 20 GPa, which underpins the reported Tc values, is not demonstrated. A 20 GPa convex-hull calculation with pressure-corrected reference phases is required.","section":"Section 3, Table 1"},{"comment":"The formation-energy screen used to select the twelve mechanically stable structures is based on average elemental energies E(X), E(M), and E(H) without pressure corrections, so it does not constitute a thermodynamic stability criterion at 20 GPa. The paper cites Dolui et al.'s 20 GPa convex hull only for Mg2IrH6; for the other nine compounds no pressure-dependent hull is provided, so the screening funnel that produces the ten metallic candidates, including the two high-Tc phases, is not actually demonstrated at the working pressure.","section":"Section 3, Eq. (1)"},{"comment":"The arXiv abstract and the full-text abstract report mutually inconsistent central results. The arXiv abstract states that ten of the eleven stable compounds exhibit superconducting transition temperatures and that three compounds (Mg2CoH6, Mg2RhH6, and Mg2IrH6) exceed 100 K, while the full-text abstract, Table 2, and Conclusion state that only four compounds are superconductors, with Mg2CoH6 at 45.8 K and Ca2IrH6 at 2.2 K, and only Mg2IrH6 and Mg2RhH6 exceeding 77 K. This contradiction must be resolved before the paper can be evaluated, since it changes the headline claim of the study.","section":"Abstract and Conclusion"}],"minor_comments":[{"comment":"The statement that applied pressure enhances Tc relative to previous reports should be reconciled with the fact that Dolui et al. reported Tc = 160 K for Mg2IrH6 at 0 GPa, whereas the present work reports 96.3 K at 20 GPa; if the comparison is intended only against Refs. [38,39], that should be stated explicitly.","section":"Table 2"},{"comment":"The sensitivity of the Allen-Dynes Tc to the Coulomb pseudopotential mu* is not discussed; a short analysis with mu* in the 0.1-0.16 range would strengthen the quantitative claim.","section":"Section 2"},{"comment":"The description of the AIMD simulation at 200 K does not specify the ensemble or whether the 20 GPa pressure was applied; please clarify.","section":"Section 3, Figure 3"},{"comment":"The superconducting figure of merit S is used without giving its defining formula or numerical values for the pressure term; a one-sentence definition would make the comparison with MgB2 (S = 1) reproducible.","section":"Section 3"}],"recommendation":"major_revision","confidential_remarks":"The stability screening is the load-bearing weakness: Table 1 uses zero-pressure OQMD energies for a 20 GPa claim, and Eq. (1) is not pressure-corrected. This is fixable by recomputing decomposition enthalpies at 20 GPa with appropriate reference states. There is also a version-control problem between the arXiv abstract and the full text that must be corrected. I do not see grounds for rejection if the stability question is answered positively, but the current manuscript does not support the headline claim as written."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Short version: the body of this paper is a standard and honest first-principles screen, but the submission package is not coherent — the arXiv abstract describes a different, stronger set of results than the full-text abstract and Table 2. Beyond that packaging problem, the main scientific gap is that stability at 20 GPa is asserted without a pressure-dependent phase diagram.\n\nWhat is new: the substitution screen over X2MH6 (X = Ca, Sr, Ba; M = Co, Rh, Ir) at 20 GPa, the computed Tc for Mg2IrH6 (96.3 K) and Mg2RhH6 (80.2 K) at that pressure, and the mechanistic discussion of mid-frequency H phonons, van Hove singularities, and Fermi-surface topology. The two headline compounds were already predicted to superconduct near ambient pressure by Dolui, Sanna, and Cerqueira, and the paper says so. The EPC calculation is not circular: mu* = 0.1 is a standard assumption, not a fit to the target Tc.\n\nSoft spots, in order. First, the abstract mismatch. The arXiv abstract says ten superconducting compounds with three Tc above 100 K; the full-text abstract and Table 2 say four superconductors, max 96.3 K. That is a clear self-contradiction, and it has to be fixed before anything else. Second, Table 1's decomposition energies come from zero-pressure OQMD energies. At 20 GPa the relevant quantity is H = E + PV, and the PV terms differ by phase. For Mg2IrH6 the paper leans on Dolui et al.'s 20 GPa hull, but for Mg2RhH6 and the other candidates there is no pressure-corrected hull; the reported Ed is -0.041 eV for Mg2IrH6, so even a small pressure shift matters. Third, there are no convergence tests for the EPC grid or for mu*; Tc values without sensitivity estimates should be read as qualitative. Minor: a couple of reference DOIs are wrong (refs [1] and [3]).\n\nThe body's calculations look standard and the qualitative ranking is plausible. The stability gap is real but partially covered for the headline compound. I would not accept the paper as is, but I would send it to a referee rather than desk-reject: the substitution screen is a legitimate extension, and the mechanism analysis is useful to the hydride community. A revision that fixes the abstract, adds a pressure-dependent hull for at least the two high-Tc compounds, and reports a mu* sweep would make this a solid contribution. For my own work, I would not cite the quantitative Tc values yet.","headline":"A useful but incrementally new hydride screen; the submission is marred by an abstract that contradicts its own Table 2 and by missing 20 GPa stability evidence.","tokens_in":11262,"tokens_out":5692,"would_cite":false,"duration_ms":45884,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"At 20 GPa, Mg2IrH6 and Mg2RhH6 are predicted to superconduct at 96.3 K and 80.2 K.","keywords":["high-temperature superconductors","electron-phonon coupling","first-principles calculations","hydrogen-rich metallic compounds","X2MH6 structure","Mg2IrH6","Mg2RhH6","van Hove singularity"],"falsifier":"Compute the decomposition enthalpy of each X2MH6 compound at 20 GPa directly by relaxing all decomposition products under that pressure; if any decomposition energy Ed becomes positive, that compound is not stable at 20 GPa, and the superconducting candidates list would need revision. Alternatively, synthesize Mg2IrH6 or Mg2RhH6 at 20 GPa and measure the resistance drop to check for transitions near 96 K and 80 K.","tokens_in":10107,"feed_emoji":"⚡","tokens_out":10227,"duration_ms":79181,"temperature":0.7,"pith_summary":"This paper asks whether the X2MH6 family of ternary hydrides—already known to contain an ambient-pressure superconductor—can be pushed past the liquid-nitrogen temperature with only a modest pressure. Using first-principles electronic-structure and phonon calculations, it predicts that at 20 GPa the compounds Mg2IrH6 and Mg2RhH6 become conventional electron-phonon superconductors with critical temperatures of 96.3 K and 80.2 K. If those numbers hold, these would be hydride superconductors that operate above 77 K without megabar pressures, making them far more plausible for applications than typical hydrogen-rich superconductors. The paper also proposes a mechanism for why some members of the family superconduct and others do not: mid-frequency hydrogen vibrations, a van Hove singularity near the Fermi level, and hydrogen-derived states at the Fermi level all cooperate in the high-temperature members.","feed_headline":"Two hydrides pass the 77 K superconductivity line at 20 GPa","feed_subtitle":"First-principles calculations put Mg2IrH6 at 96.3 K and Mg2RhH6 at 80.2 K, above liquid nitrogen's 77 K.","key_machinery":"The central object is the X2MH6 structure, a face-centred cubic phase built around an MH6 octahedral unit, which carries the superconductivity through metal-hydrogen vibrations. The critical temperature is obtained from the Allen-Dynes modified McMillan equation, Tc = (omega_log/1.20) exp[-1.04(1+lambda)/(lambda(1-0.62 mu*) - mu*)], which converts the electron-phonon coupling constant lambda and the logarithmic average phonon frequency omega_log into Tc; mu* is the Coulomb pseudopotential, set to 0.1. The argument's mechanism is that in Mg2IrH6 and Mg2RhH6 the mid-frequency phonons (15-40 THz), which are dominated by hydrogen motion, contribute the largest share of the coupling (51.5% for Mg2RhH6), and the electronic structure places a van Hove singularity and substantial hydrogen weight at the Fermi level. The screening pipeline that selects the compounds is mechanical stability via the Born criteria, thermodynamic stability via negative formation and decomposition energies from a materials database, dynamical stability via the absence of imaginary phonons, and metallicity from band-structure calculations.","core_discovery":"The central claim is that applying 20 GPa to the face-centred cubic X2MH6 structure, with X chosen from Be, Mg, Ca, Sr, Ba and M from Co, Rh, Ir, produces ten stable metallic hydrides, four of which superconduct. Among them Mg2IrH6 and Mg2RhH6 reach computed critical temperatures of 96.3 K and 80.2 K using the Allen-Dynes modified McMillan equation with a Coulomb pseudopotential mu* = 0.1, while Mg2CoH6 and Ca2IrH6 fall below 77 K at 45.8 K and 2.2 K. These values place the two Mg compounds above the liquid-nitrogen line at a pressure of only 20 GPa, much gentler than the roughly 150 GPa needed for most hydrogen-rich high-temperature superconductors. The paper attributes the difference to the strength of mid-frequency (15-40 THz) hydrogen phonons in the electron-phonon coupling, to a van Hove singularity (a sharp feature in the electronic density of states) sitting at the Fermi level in Mg2IrH6, and to a more complex, multi-layered second-band Fermi surface in the two high-temperature members.","pith_inferences":["If the zero-pressure decomposition energies used to build the phase diagrams shift under compression, the ten stable compounds could change; re-running the thermodynamic screen with pressure-corrected enthalpies would test the chemical space directly.","The paper's mechanism suggests a design rule for this family: keep hydrogen-derived states at the Fermi level and place a van Hove singularity near it, using lighter alkaline-earth cations whose empty states lie high in energy; this could be screened computationally across more X/M combinations.","All Tc values are computed with mu* = 0.1; recomputing with other mu* values, or measuring the isotope effect, would show how sensitive the predicted 96.3 K and 80.2 K are to the Coulomb pseudopotential.","The family's ambient-pressure members previously predicted at high Tc could be revisited at intermediate pressures between 0 and 20 GPa to map where the superconducting dome peaks."],"forward_implications":["At 20 GPa, Mg2IrH6 and Mg2RhH6 are predicted to superconduct at 96.3 K and 80.2 K, above the 77 K liquid-nitrogen line and at a pressure far lower than the megabar range typical of hydrogen-rich superconductors.","Substituting Group II and Group IX elements preserves the X2MH6 motif and yields ten stable metallic hydrides, four of them superconducting, so the family offers a tunable chemical space for hydride superconductivity.","Mid-frequency hydrogen phonons (15-40 THz) are the dominant contributors to the electron-phonon coupling in the two high-temperature members; compounds that lack this mid-frequency contribution have markedly lower critical temperatures.","The superconducting figure of merit S for Mg2IrH6 (2.20) and Mg2RhH6 (1.83) exceeds the value S=1 assigned to MgB2, indicating that these materials are comparatively attractive when pressure is part of the cost.","Molecular dynamics at 200 K shows the two high-temperature structures remain stable over 10 ps, so they are plausible targets for synthesis and measurement at attainable conditions."],"supporting_citations":[{"why":"Prior work predicted ambient-pressure superconductivity in Mg2IrH6 and identified the X2MH6 motif as the starting point for substitution.","marker":"[19]"},{"why":"Predicted that other MH6-octahedra hydrides such as Mg2RhH6 and Li2CuH6 are promising ambient-pressure superconductors, motivating this substitution study.","marker":"[20]"},{"why":"Supplies the Allen-Dynes modified McMillan equation used to compute every critical temperature in the paper.","marker":"[29]"},{"why":"Provides the phase-energy data used to build the decomposition pathways in Table 1.","marker":"[35]"},{"why":"Supplies the formation-energy accuracy assessment for the database used in the thermodynamic stability screen.","marker":"[36]"},{"why":"Reports ambient-pressure Tc values for Mg2IrH6 and Mg2RhH6 that the paper compares against its 20 GPa results.","marker":"[38]"},{"why":"Reports an independent set of ambient-pressure Tc values for the same compounds, used as a second baseline.","marker":"[39]"},{"why":"Explains why Mg2IrH6 superconducts while Ca2IrH6 does not through d-orbital back-donation, which the paper invokes to rationalize its Ca2IrH6 result.","marker":"[42]"}],"fun_headline_variants":["Two hydrides break liquid-nitrogen barrier at just 20 GPa","Mg2IrH6 superconducts at 96 K under only 20 GPa","At 20 GPa, Mg2IrH6 and Mg2RhH6 top 77 K","96 K and 80 K superconductors at a gentle 20 GPa"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that the decomposition energies used to judge thermodynamic stability at 20 GPa come from a database computed at zero pressure and stay valid when pressure is applied; if compression reshuffles those phase energies, the set of stable compounds and hence the list of superconductors could change.","fun_headline_variants_meta":{"raw":{"variants":["Two hydrides break liquid-nitrogen barrier at just 20 GPa","Mg2IrH6 superconducts at 96 K under only 20 GPa","At 20 GPa, Mg2IrH6 and Mg2RhH6 top 77 K","96 K and 80 K superconductors at a gentle 20 GPa"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000825,"raw_usage":{"total_tokens":3664,"prompt_tokens":1061,"completion_tokens":2603,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":677,"completion_tokens_details":{"reasoning_tokens":2513}},"tokens_in":677,"tokens_out":2603,"duration_ms":16932,"temperature":1.0,"reasoning_tokens":2513,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-12T14:01:15.912271+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Compute the decomposition enthalpy of each X2MH6 compound at 20 GPa directly by relaxing all decomposition products under that pressure; if any decomposition energy Ed becomes positive, that compound is not stable at 20 GPa, and the superconducting candidates list would need revision. Alternatively, synthesize Mg2IrH6 or Mg2RhH6 at 20 GPa and measure the resistance drop to check for transitions near 96 K and 80 K.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Prior work predicted ambient-pressure superconductivity in Mg2IrH6 and identified the X2MH6 motif as the starting point for substitution."},{"cited_title":"Today Phys","cited_arxiv_id":null,"evidence_quote":"Predicted that other MH6-octahedra hydrides such as Mg2RhH6 and Li2CuH6 are promising ambient-pressure superconductors, motivating this substitution study."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Reports ambient-pressure Tc values for Mg2IrH6 and Mg2RhH6 that the paper compares against its 20 GPa results."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Explains why Mg2IrH6 superconducts while Ca2IrH6 does not through d-orbital back-donation, which the paper invokes to rationalize its Ca2IrH6 result."}],"review_version":1}