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arxiv: 2604.20716 · v1 · submitted 2026-04-22 · ❄️ cond-mat.supr-con

Stabilization of a non-superconducting, orthorhombic phase by over-hydrogenating LaFeSiH

M. F. Hansen , C. Lepoittevin , J.-B. Vaney , P. Boullay , V. Nassif , A. Sulpice , H. Mayaffre , M.-H. Julien
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S. Tenc\'e P. Toulemonde
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Pith reviewed 2026-05-09 22:53 UTC · model grok-4.3

classification ❄️ cond-mat.supr-con
keywords LaFeSiHover-hydrogenationorthorhombic phasesemiconductor-like behavioriron-based superconductorshydrogen dopingchemical flexibilitystructural distortion
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The pith

Over-hydrogenation of LaFeSi produces an orthorhombic semiconductor phase that reverts to superconductivity upon hydrogen release.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows that high-pressure thermal decomposition with ammonia borane over-hydrogenates LaFeSi to LaFeSiH1.6, yielding an orthorhombic structure with excess hydrogen at a second site located by neutron diffraction. This phase displays semiconductor-like resistivity, unlike metallic LaFeSi or tetragonal superconducting LaFeSiH. Mild heating near 100 °C releases the extra hydrogen and converts the material back to the superconducting tetragonal form. The work demonstrates how chemical composition can access a high-doping regime in the layered LaFeSiX family.

Core claim

High-pressure thermal decomposition of hydrogen-rich precursors using ammonia borane produces over-hydrogenated LaFeSiH1+x (x ~ 0.6) with an orthorhombic structure. Chemical analysis and neutron diffraction establish a second hydrogen site, and resistivity measurements show semiconductor-like behavior. Upon hydrogen release near 100 °C the phase transforms into tetragonal superconducting LaFeSiH1+δ.

What carries the argument

The second interstitial hydrogen site in orthorhombic LaFeSiH1.6, which induces structural distortion and switches the electronic ground state from superconducting to semiconductor-like.

If this is right

  • High hydrogen doping levels become accessible in the LaFeSiX family, allowing stabilization of new structural phases.
  • Hydrogen release at low temperature provides a reversible route between semiconducting and superconducting states.
  • The chemical flexibility of LaFeSiX (X = H, O, F) enables systematic investigation of superconductivity in Fe-based silicides across doping regimes.
  • Structural distortion from the second H site offers a handle to tune electronic ground states without changing the metal framework.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same high-pressure over-hydrogenation route could be tested on other iron-based superconductors to reach doping levels outside conventional synthesis limits.
  • The semiconductor-like gap may arise from the orthorhombic distortion splitting bands; optical or spectroscopic measurements could confirm this mechanism.
  • Mapping the occupancy and local environment of the second H site may clarify how interstitial hydrogen suppresses magnetism or electron pairing in this structure type.

Load-bearing premise

The excess hydrogen is uniformly incorporated into a single-phase orthorhombic structure whose transport properties are not dominated by impurities, defects or synthesis strain.

What would settle it

Resistivity data on a phase-pure orthorhombic LaFeSiH1.6 sample (confirmed by neutron diffraction and chemical analysis) that shows metallic conductivity or superconductivity below the LaFeSiH critical temperature would falsify the claim that over-hydrogenation stabilizes a non-superconducting phase.

Figures

Figures reproduced from arXiv: 2604.20716 by A. Sulpice, C. Lepoittevin, H. Mayaffre, J.-B. Vaney, M. F. Hansen, M.-H. Julien, P. Boullay, P. Toulemonde, S. Tenc\'e, V. Nassif.

Figure 1
Figure 1. Figure 1: FIG. 1. X-ray diffraction patterns, in red and orange respec [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 3
Figure 3. Figure 3: displays a representative fit constrained to equal [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Zone axis ED patterns of o-LaFeSiH [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. The electrical resistance of LaFeSi (precursor), [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. (a): The X-ray diffraction patterns collected at different temperatures during the thermal decomposition of o [PITH_FULL_IMAGE:figures/full_fig_p006_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. Thermogravimetric analysis, differential thermal [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7. (a) [PITH_FULL_IMAGE:figures/full_fig_p007_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8. Rietveld fit of the neutron powder diffraction pattern [PITH_FULL_IMAGE:figures/full_fig_p008_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9 [PITH_FULL_IMAGE:figures/full_fig_p008_9.png] view at source ↗
Figure 11
Figure 11. Figure 11: FIG. 11. Magnetization data (in the low T range) measured [PITH_FULL_IMAGE:figures/full_fig_p011_11.png] view at source ↗
Figure 10
Figure 10. Figure 10: FIG. 10. Resistivity data (in the low T range), under mag [PITH_FULL_IMAGE:figures/full_fig_p011_10.png] view at source ↗
Figure 13
Figure 13. Figure 13: FIG. 13 [PITH_FULL_IMAGE:figures/full_fig_p012_13.png] view at source ↗
Figure 12
Figure 12. Figure 12: FIG. 12 [PITH_FULL_IMAGE:figures/full_fig_p012_12.png] view at source ↗
Figure 14
Figure 14. Figure 14: FIG. 14. The [PITH_FULL_IMAGE:figures/full_fig_p013_14.png] view at source ↗
Figure 15
Figure 15. Figure 15: FIG. 15. The electrical resistance of LaFeSi and derived compounds as a function of temperature and measured at different [PITH_FULL_IMAGE:figures/full_fig_p013_15.png] view at source ↗
read the original abstract

Chemical composition provides a powerful route to tune the electronic ground state of iron-based superconductors and other quantum materials, yet access to highly doped phases remains limited. Here we demonstrate that high-pressure thermal decomposition of hydrogen-rich precursors enables over-hydrogenation of LaFeSi. Using anthracene, we synthesize tetragonal superconducting LaFeSiH, including a single hydrogen site, while ammonia borane yields a structurally distorted over-hydrogenated phase, LaFeSiH1+x, with an orthorhombic structure. Chemical analysis reveal excess hydrogen (x ~ 0.6), implying a second H site in LaFeSiH1.6 whose localization and occupancy are determined by neutron diffraction. In contrast to metallic LaFeSi and superconducting LaFeSiH, orthorhombic LaFeSiH1.6 exhibits semiconductor-like behavior. Upon hydrogen release near 100 {\deg}C, it transforms into tetragonal superconducting LaFeSiH1+{\delta} ({\delta} << 0.6). These results establish the chemical flexibility of the layered LaFeSiX (X = H, O, F) family and provide access to a high hydrogen-doping regime, creating new opportunities to investigate superconductivity in Fe-based silicides.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper reports synthesis of over-hydrogenated LaFeSiH_{1.6} via high-pressure decomposition of ammonia borane, yielding an orthorhombic phase with excess hydrogen (x ~ 0.6) occupying a second site as determined by neutron diffraction and chemical analysis. This phase shows semiconductor-like resistivity, in contrast to metallic LaFeSi and superconducting tetragonal LaFeSiH; upon heating near 100°C it releases hydrogen and reverts to the superconducting tetragonal phase. The work claims this demonstrates chemical flexibility in the LaFeSiX family and access to a high-doping regime.

Significance. If the single-phase character and intrinsic transport properties hold, the result would be significant for expanding the phase space of iron-based silicides, providing a route to non-superconducting states at high hydrogen content and new opportunities to tune superconductivity. The precursor-dependent synthesis approach is a clear experimental strength.

major comments (2)
  1. [Neutron diffraction and structural analysis] Neutron diffraction section: quantitative Rietveld statistics (R_wp, χ², phase fractions) and explicit upper limits on secondary phases or disorder at the second H site are not reported. This is load-bearing for the central claim, as the semiconductor-like behavior is attributed to the uniform orthorhombic LaFeSiH1.6 structure rather than impurities or defects.
  2. [Transport measurements] Transport data section: resistivity curves lack error bars, full temperature dependence details, and controls (e.g., grain-size variation or defect annealing) to rule out synthesis-induced artifacts dominating the upturn. This directly affects the contrast with metallic LaFeSi and superconducting LaFeSiH.
minor comments (2)
  1. [Abstract and results] Notation for hydrogen content alternates between LaFeSiH1+x and LaFeSiH1.6 without clear definition of the range; consistent use would aid clarity.
  2. [Figures] Figure captions for diffraction and resistivity plots should explicitly state sample masses, measurement conditions, and any background subtraction to improve reproducibility.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive review and positive assessment of the significance of our results on over-hydrogenated LaFeSiH1.6. We address each major comment below and have prepared revisions to the manuscript where appropriate.

read point-by-point responses

  1. Referee: [Neutron diffraction and structural analysis] Neutron diffraction section: quantitative Rietveld statistics (R_wp, χ², phase fractions) and explicit upper limits on secondary phases or disorder at the second H site are not reported. This is load-bearing for the central claim, as the semiconductor-like behavior is attributed to the uniform orthorhombic LaFeSiH1.6 structure rather than impurities or defects.

    Authors: We agree that quantitative Rietveld statistics are necessary to rigorously support the single-phase character and the occupancy of the second hydrogen site. In the revised manuscript we will add the requested R_wp, χ², and phase-fraction values from the neutron diffraction refinements, together with explicit upper limits on secondary phases and on disorder at the second H site. These additions will directly address the concern that the observed transport properties could arise from impurities rather than the orthorhombic LaFeSiH1.6 structure itself. revision: yes

  2. Referee: [Transport measurements] Transport data section: resistivity curves lack error bars, full temperature dependence details, and controls (e.g., grain-size variation or defect annealing) to rule out synthesis-induced artifacts dominating the upturn. This directly affects the contrast with metallic LaFeSi and superconducting LaFeSiH.

    Authors: We acknowledge that error bars and expanded temperature-range details will improve the presentation. These will be added to the revised figures and text. The reversible hydrogen-release experiment, in which the orthorhombic phase transforms back to the known superconducting tetragonal LaFeSiH1+δ upon mild heating, already provides a strong internal control against synthesis artifacts. Additional grain-size or annealing controls would require new sample batches and are beyond the scope of the present study; we therefore regard the existing reversibility evidence as sufficient to support the intrinsic nature of the semiconductor-like behavior. revision: partial

Circularity Check

0 steps flagged

No circularity; purely experimental synthesis and characterization

full rationale

The manuscript reports direct experimental results from high-pressure synthesis, chemical analysis, neutron diffraction for structure and H-site occupancy, and resistivity measurements showing semiconductor-like behavior in the orthorhombic phase. No equations, theoretical derivations, fitted parameters renamed as predictions, or load-bearing self-citations appear in the provided text or abstract. All claims rest on observable data without reduction to inputs by construction, satisfying the self-contained criterion against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

This is an experimental synthesis and characterization study. The central claim rests on measured hydrogen content, neutron-determined site occupancy, and transport data rather than any theoretical model. No free parameters are fitted to produce the result; the x ~ 0.6 value comes from chemical analysis.

axioms (1)
  • domain assumption Standard assumptions of phase purity and accurate quantification in chemical analysis and neutron diffraction for hydrogenated intermetallics.
    The interpretation of a single over-hydrogenated phase depends on these common materials-science assumptions being valid.

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Reference graph

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