{"id":"1d56d49c-b15d-470e-ae13-3b3632c071d3","arxiv_id":"2411.19014","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":4.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":1,"one_line_summary":"Actinium borohydride AcBH7 is predicted to be dynamically stable at 70 GPa and to superconduct at 122 K, based on density functional theory and electron-phonon coupling calculations.","lead":"Using structure prediction and first-principles calculations, the authors predict that the compound AcBH7 becomes a superconductor at 122 K under 70 GPa, joining a family of ternary hydrides that superconduct below megabar pressures. A generalist reader may care because it identifies a new candidate material for high-temperature superconductivity that could in principle be synthesized at pressures lower than many known hydrides.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Missing 70 GPa thermodynamic stability check leaves AcBH7's 122 K prediction contingent on metastability.","rationale":"The paper's own data show that AcBH7 is thermodynamically stable at 200 GPa but not at 100 GPa. The 70 GPa prediction is an extrapolation where only phonon stability is checked, and dynamic stability does not guarantee synthesizability; many predicted hydrides are dynamically stable yet decompose into more stable phases. Since the paper's synthesis motivation and the comparison to LaBeH8 and LaB2H8 implicitly assume a quenchable or directly stable phase, the missing 70 GPa convex hull is the most load-bearing gap. The reader's weakest assumption identifies exactly this issue, and I agree. The proposed test is straightforward—it uses the same methodology as Fig. 1—and would settle the concern. Secondary issues such as anharmonicity of soft phonons or the sensitivity of the Eliashberg Tc to the Coulomb pseudopotential are real but are secondary to the thermodynamic viability of the phase.","tokens_in":10877,"tokens_out":5377,"duration_ms":88405,"concrete_test":"Compute the 70 GPa convex hull for the Ac-B-H system using the same CALYPSO and VASP setup as in Fig. 1, including all binary and ternary phases identified at 100 and 200 GPa plus newly searched structures at 70 GPa, with zero-point energy corrections. If P-3m1 AcBH7 lies on the hull, the stability objection is resolved; if it is above the hull, the 122 K claim applies only to a metastable phase and the central assertion should be revised.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim, that P-3m1 AcBH7 has a Tc of 122 K at 70 GPa, requires that this phase be realizable at that pressure. The paper's Fig. S13 confirms dynamical stability at 70 GPa, but the thermodynamic convex hull is computed only at 100 and 200 GPa (Fig. 1). At 100 GPa AcBH7 is absent from the hull; it becomes stable only at 200 GPa. No enthalpy or free-energy comparison at 70 GPa against AcH, BH, elemental phases, or other Ac-B-H compounds is provided. The phase could decompose or transform upon decompression to 70 GPa. The comparison to LaBeH8 and LaB2H8, which were synthesized as thermodynamically stable phases at their respective pressures, is only valid if AcBH7 is likewise thermodynamically stable at 70 GPa. This missing stability check is the least secure condition underpinning the headline result.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript uses CALYPSO structure searches combined with DFT and linear-response phonon calculations to explore the ternary Ac-B-H system at 100 and 200 GPa, reporting several thermodynamically stable compounds. Among them, the P-3m1 AcBH7 phase is found to be dynamically stable down to 70 GPa upon decompression, and Eliashberg calculations give a superconducting critical temperature of about 122 K (ADM estimate: 86 K). The authors compare this with the synthesized ternary hydrides LaBeH8 (110 K at 80 GPa) and LaB2H8 (106 K at 90 GPa) and attribute the high Tc to H-derived states at the Fermi level and to B-H vibrational modes in BH6 units.","tokens_in":11084,"tokens_out":8727,"duration_ms":81648,"significance":"If the phase is experimentally realizable, the prediction would extend ternary hydride superconductivity to an actinium borohydride at submegabar pressure and provide a concrete synthesis target. The paper follows a standard computational pipeline, reports both Allen-Dynes and Eliashberg Tc estimates, and explicitly checks phonon stability with denser q meshes. The comparison with isostructural LaBH7 and with Ce- and Th-substituted analogues is also useful. However, the central claim currently rests on a phase that is only dynamically, not thermodynamically, stable at 70 GPa; this is the main weakness that needs to be addressed.","major_comments":[{"comment":"The headline prediction of 122 K at 70 GPa lacks a thermodynamic stability analysis at that pressure. The convex hull is computed only at 100 and 200 GPa (Fig. 1), and at 100 GPa AcBH7 is not on the hull; it becomes stable only at 200 GPa. The CALYPSO searches were also performed only at 100 and 200 GPa. Thus, the P-3m1 AcBH7 structure at 70 GPa is presently only a dynamically stable metastable phase: it could decompose into AcH, BH, or other Ac-B-H compounds, or transform to a different structure, upon decompression. I request a full convex-hull calculation at 70 GPa (and ideally at intermediate pressures such as 80 and 90 GPa) against all relevant elemental, binary, and ternary phases, or at least an enthalpy comparison of P-3m1 AcBH7 with the most plausible decomposition products. Without this, the comparison with the experimentally synthesized equilibrium phases LaBeH8 and LaB2H8 is not justified.","section":"Results and Discussion; Fig. 1; Fig. S13"},{"comment":"The reported Tc of 122 K for AcBH7 at 70 GPa is obtained from the Eliashberg equations, whereas the Allen-Dynes formula gives only 86 K for the same λ = 1.98 and ωlog = 600.86 K. The large difference shows that the result is sensitive to the strong-coupling regime, yet the paper does not discuss the convergence of the electron-phonon coupling constant with respect to the k- and q-point meshes, nor the sensitivity of Tc to the Coulomb pseudopotential μ* in the standard 0.10–0.13 range. Because 122 K is presented as the central quantitative prediction, please provide convergence checks for λ and a Tc versus μ* curve (or at least the Tc value at μ* = 0.13) to establish the robustness of the estimate.","section":"Computational Details; Table I"}],"minor_comments":[{"comment":"The paper states that nine stable compounds are uncovered, but the list in the Conclusions contains ten chemical formulas, including AcB2H8, while the Introduction lists nine compounds without AcB2H8. Please reconcile the count and clarify whether AcB2H8 is being counted as a new compound or as a previously known phase.","section":"Abstract and Conclusions"},{"comment":"The Abstract says that seven Ac-B-H compounds were found to become superconductive, but only six compounds (AcB2H8, AcB2H14, AcBH16, AcBH7, AcBH8, AcB2H13) have EPC or Tc results reported in the text and Table I. Please either provide the missing Tc values or correct the count.","section":"Abstract and throughout"},{"comment":"There are several typographical errors, including 'idea candidates' instead of 'ideal candidates' and 'submegar' instead of 'submegabar'; a careful proofreading pass is recommended.","section":"Abstract"},{"comment":"The text says 'another four stoichiometries AcBH7, AcBH8, AcB2H8 and AcB2H14 start to locate at the convex hull at 200 GPa,' but because AcB2H8 was previously reported, the phrasing should clarify which of these are newly predicted and which are known phases.","section":"Results and Discussion"}],"recommendation":"major_revision","confidential_remarks":"The main technical issue is the absence of a 70 GPa convex hull, which is fixable with standard calculations and does not require a new methodology. The internal inconsistencies in the numbers of stable compounds and superconductors should also be corrected. If the hull calculation confirms thermodynamic stability at 70 GPa, the paper would be a solid contribution; if not, the claim should be explicitly downgraded to a metastable-phase prediction."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Quick take: this is a competent CALYPSO+DFT+EPC study that finds a new P-3m1 AcBH7 phase and predicts a 122 K Tc at 70 GPa, slightly above the synthesized LaBeH8 and LaB2H8 at similar pressures. The main result is new — the earlier Ac-B-H paper [67] did not report AcBH7 — and the phonon and EPC calculations are internally consistent. That part deserves credit.\n\nThe soft spot is exactly the one the stress test flags. The convex hull is computed at 100 and 200 GPa only; AcBH7 is off the hull at 100 GPa and on it at 200 GPa. The 70 GPa claim is supported by dynamical stability (no imaginary phonons) but no enthalpy comparison against AcH, BH, elements, or other Ac-B-H phases at 70 GPa. Without that, the 122 K superconductor is metastable at best. The comparison to LaBeH8 and LaB2H8, both synthesized as thermodynamic phases, is weakened by the same gap.\n\nMinor issues: the paper says \"nine\" stable compounds but lists ten in the conclusion; mu* is fixed at 0.10 with no sensitivity scan; and the structural and EPC details are in an unpublished SM, so the numbers cannot be fully checked from the preprint. These are minor but worth fixing.\n\nThe central idea is plausible, the work is honest and standard, and the internal consistency checks are adequate. It is a prediction, not a proof, and the load-bearing assumption—that the phase survives decompression thermodynamically—needs explicit verification.\n\nI would send it to peer review. A referee should ask for the 70 GPa convex hull and a mu* sensitivity analysis; those are reasonable requests and the paper would be substantially stronger for them. I would only bring it to a reading group if the topic is ternary hydride superconductors specifically.","headline":"Solid ternary-hydride prediction with a genuinely new AcBH7 phase; the headline Tc rests on dynamic stability only, so the missing 70 GPa thermodynamic check is the real soft spot.","tokens_in":11776,"tokens_out":2017,"would_cite":true,"duration_ms":31698,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":["74.70.Ad","74.25.Kc"],"model":"deepseek-v4-flash","headline":"The paper predicts that the ternary hydride AcBH7 superconducts at 122 K under 70 GPa, a pressure already reached in experiments on LaBeH8 and LaB2H8.","keywords":["ternary hydride superconductor","actinium borohydride","high-pressure superconductivity","electron-phonon coupling","Eliashberg equations","structure prediction","BH6 octahedron","submegabar pressure"],"falsifier":"A convex-hull computation at 70 GPa that places $P\\bar{3}m1$ AcBH7 above the equilibrium mixture of Ac, B, H and the known binary or ternary phases would falsify the synthesizability claim, as would a diamond-anvil synthesis attempt at 70 GPa that recovers no AcBH7 phase.","tokens_in":10704,"feed_emoji":"🧲","tokens_out":6477,"duration_ms":51463,"temperature":0.7,"pith_summary":"The paper predicts that the ternary hydride AcBH7, in a hexagonal $P\\bar{3}m1$ structure, is dynamically stable at 70 GPa and becomes superconducting at 122 K, a value obtained by solving the Eliashberg equations. If correct, it would be a new high-temperature hydride superconductor stabilized at submegabar pressure, comparable to LaBeH8 (110 K at 80 GPa) and LaB2H8 (106 K at 90 GPa), both of which have been synthesized. The authors map the Ac-B-H phase diagram at 100 and 200 GPa, identify nine thermodynamically stable compounds, and trace the enhanced superconductivity of AcBH7 to soft phonon modes and to the hydrogen-rich BH6 octahedral units that dominate the Fermi-level density of states. The result gives experimentalists a specific ternary composition and structure to target in high-pressure synthesis.","feed_headline":"AcBH7 predicted to superconduct at 122 K under 70 GPa","feed_subtitle":"The actinium borohydride would match LaBeH8 and LaB2H8, two ternary hydrides already made below megabar pressure.","key_machinery":"The central object is the $P\\bar{3}m1$ AcBH7 phase and its BH6 octahedral units, arranged on a hexagonal lattice, with additional hydrogen atoms not bonded to boron. The argument runs through three calculated quantities: the convex hull of the Ac-B-H system at 100 and 200 GPa, the phonon spectrum that establishes dynamical stability, and the Eliashberg spectral function $\\alpha^2F(\\omega)$, whose moments give $\\lambda$ and $\\omega_{\\log}$ and feed the Allen-Dynes-modified McMillan equation or a full Eliashberg solution. The softening of four phonon modes under decompression, one dominated by Ac motion and three by B-H and H vibrations, is the mechanism that raises $\\lambda$ to 1.98 and $T_c$ to 122 K.","core_discovery":"The central claim is that $P\\bar{3}m1$ AcBH7, a layered hexagonal phase in which each boron sits in a BH6 octahedron, remains dynamically stable at 70 GPa and has a superconducting critical temperature of 122 K, obtained by solving the Eliashberg equations with Coulomb pseudopotential $\\mu^* = 0.10$. At 200 GPa the same phase has $T_c$ of only 43 K; decompression to 70 GPa softens four phonon modes, raises the electron-phonon coupling $\\lambda$ from 0.67 to 1.98, and increases the hydrogen share of the Fermi-level DOS to 56%. The paper argues that the strong B-H interaction inside the BH6 units is what stabilizes the structure at submegabar pressure and drives the high $T_c$. It also compares with isostructural LaBH7, CeBH7, and ThBH7 to show that the H-dominated DOS at the Fermi level, not the total DOS, is the decisive factor for achieving high-temperature superconductivity.","pith_inferences":["A natural extension is to compute the convex hull at 70 GPa and at intermediate pressures, which the paper does not do; this would determine the true synthesis window and could reveal whether even lower pressures raise or suppress $T_c$.","The systematic comparison with La, Ce, and Th substitutions suggests the metal site is a tunable parameter, so substituting other light metals into the BH7 framework might lower the stabilization pressure below 70 GPa while keeping a high hydrogen contribution at the Fermi level.","The BH6 octahedron may act as a reusable chemical pre-compression module, implying that other ternary systems containing BH6 units could be searched deliberately rather than scanning all stoichiometries blindly.","If the 122 K prediction is confirmed, measurements of the isotope effect and upper critical field on the same structure would give experimental fingerprints to verify the phonon-mediated mechanism."],"forward_implications":["If correct, AcBH7 joins LaBeH8 and LaB2H8 as a ternary hydride with $T_c$ above 100 K at pressures below 100 GPa, giving experimentalists a specific target composition and crystal structure.","The predicted pressure dependence, 43 K at 200 GPa rising to 122 K at 70 GPa, means that tuning pressure alone could nearly triple the superconducting critical temperature in the same compound.","The BH6 motif and the H-dominated Fermi-level DOS identified here can be used to screen other actinide and rare-earth borohydrides computationally before synthesis.","The dynamical stability down to 70 GPa makes AcBH7 compatible with existing laser-heated diamond-anvil synthesis techniques used for other ternary hydrides."],"supporting_citations":[{"why":"Provides the experimental benchmark of LaBeH8 with $T_c$ = 110 K at 80 GPa, the synthesized ternary hydride that AcBH7 is compared against.","marker":"[33]"},{"why":"Provides the second experimental benchmark, LaB2H8 with $T_c$ = 106 K at 90 GPa, for submegabar ternary hydride superconductivity.","marker":"[39]"},{"why":"Introduces the isostructural LaBH7 phase and its dynamical stability at 85 GPa, which the paper uses to contrast the role of the metal atom.","marker":"[31]"},{"why":"Supplies the earlier actinium hydride predictions (AcH10, AcH12, AcH16) that motivate choosing actinium for the ternary search.","marker":"[40]"},{"why":"Gives the Allen-Dynes modified McMillan equation used for the ADM estimates of $T_c$.","marker":"[60]"},{"why":"Gives the Eliashberg equations used for the full $T_c$ estimate of 122 K for AcBH7.","marker":"[68]"},{"why":"Reports previous work on AcBH4, AcBH6, AcBH8, and AcB2H8, which the paper builds on and shows its new structures are more energetically stable.","marker":"[67]"}],"fun_headline_variants":["AcBH7 predicted to hit 122 K at 70 GPa","Submegabar high-Tc in AcBH7: 122 K at 70 GPa","Actinium borohydride AcBH7: superconductivity at 122 K","New ternary hydride AcBH7: Tc = 122 K at 70 GPa","AcBH7: high-temperature superconductor at 70 GPa"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The 122 K result at 70 GPa assumes the $P\\bar{3}m1$ AcBH7 phase found at 200 GPa survives decompression without transforming into another phase or decomposing; the paper checks only phonon stability at 70 GPa, not thermodynamic stability against competing mixtures at that pressure.","fun_headline_variants_meta":{"raw":{"variants":["AcBH7 predicted to hit 122 K at 70 GPa","Submegabar high-Tc in AcBH7: 122 K at 70 GPa","Actinium borohydride AcBH7: superconductivity at 122 K","New ternary hydride AcBH7: Tc = 122 K at 70 GPa","AcBH7: high-temperature superconductor at 70 GPa"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000706,"raw_usage":{"total_tokens":3208,"prompt_tokens":994,"completion_tokens":2214,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":610,"completion_tokens_details":{"reasoning_tokens":2107}},"tokens_in":610,"tokens_out":2214,"duration_ms":13406,"temperature":1.0,"reasoning_tokens":2107,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-12T10:38:07.537390+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A convex-hull computation at 70 GPa that places $P\\bar{3}m1$ AcBH7 above the equilibrium mixture of Ac, B, H and the known binary or ternary phases would falsify the synthesizability claim, as would a diamond-anvil synthesis attempt at 70 GPa that recovers no AcBH7 phase.","supporting_citations":[{"cited_title":"Belli and I","cited_arxiv_id":null,"evidence_quote":"Provides the experimental benchmark of LaBeH8 with $T_c$ = 110 K at 80 GPa, the synthesized ternary hydride that AcBH7 is compared against."},{"cited_title":"Dolui, L","cited_arxiv_id":null,"evidence_quote":"Provides the second experimental benchmark, LaB2H8 with $T_c$ = 106 K at 90 GPa, for submegabar ternary hydride superconductivity."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the earlier actinium hydride predictions (AcH10, AcH12, AcH16) that motivate choosing actinium for the ternary search."},{"cited_title":"It contains the charges transfer, the electronic properties and phonon dispersions of other compounds at different pres- sure and structrue information of all the compounds, etc","cited_arxiv_id":null,"evidence_quote":"Gives the Allen-Dynes modified McMillan equation used for the ADM estimates of $T_c$."},{"cited_title":"Li, W.-H","cited_arxiv_id":null,"evidence_quote":"Gives the Eliashberg equations used for the full $T_c$ estimate of 122 K for AcBH7."},{"cited_title":"Yang, W.-C","cited_arxiv_id":null,"evidence_quote":"Reports previous work on AcBH4, AcBH6, AcBH8, and AcB2H8, which the paper builds on and shows its new structures are more energetically stable."}],"review_version":1}