Polaron-mediated metal-insulator transition and proton conduction in hydrogenated nickelate perovskites

Hang Ma; Tianxing Ma; Ying Liang

REVIEW 2 major objections 4 minor 88 references

Polaron-mediated metal-insulator transition and proton conduction in hydrogenated nickelate perovskites

T0 review · 2 major / 4 minor · reviewed 2026-08-04 · deepseek-v4-flash

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Pith's one-line read The hydrogen-driven metal-insulator transition in nickelate perovskites is caused by electron and proton polarons that localize nickel's eg electrons into a Mott-insulating d8 state, not by simple electron filling of the nickel orbitals.

desk verdict A coherent polaron-mediated MIT mechanism in hydrogenated nickelates, with genuinely new proton-transport trends, but the central charge-transfer assignment rests on a single Hubbard U and needs a sensitivity check. read the letter →

arxiv 2608.02243 v1 pith:V54NHYAY submitted 2026-08-03 cond-mat.mtrl-sci

Hang Ma , Tianxing Ma , Ying Liang This is my paper · ORCID

classification cond-mat.mtrl-sci PACS 71.30.+h

keywords hydrogen-dopednickelatesmetal-insulatortransitionpolaronMottinsulatorligandholeprotonconductionrare-earthNdNiO3

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

This paper uses first-principles electronic-structure calculations to explain how hydrogenation turns NdNiO3 from a metal into an insulator. It argues that electrons added by hydrogen do not directly reduce nickel but instead occupy oxygen ligand-hole states in the nickel–oxygen hybridized d8L configuration. That occupation stretches the Ni–O bonds and forms electron polarons; the inserted protons act as additional polarizing centers. Together these polarons weaken nickel–oxygen hybridization, drive the nickel eg electrons into a localized d8 (t2g^6 eg^2) configuration, and open a Mott gap of about 0.81 eV. The paper also examines proton conduction, showing that hydrogenation creates fast one-dimensional proton pathways along the c-axis while suppressing three-dimensional diffusion, and that smaller rare-earth ions (Sm vs Nd) promote hydrogen uptake but impede proton mobility.

What carries the argument

The central object is the Ni–O hybridized d8L electronic configuration—a nickel d8 state carrying a hole on an oxygen ligand. The paper's mechanism is that hydrogen doping fills that ligand hole, and the resulting lattice distortion forms an electron polaron; the proton adds a second polaronic distortion. Polaron formation weakens Ni–O hybridization, which is the step that localizes the eg electrons into a d8 Mott state. For proton transport, the key distinction is between intraoctahedral transfer (proton hopping between oxygen sites within the same NiO6 octahedron) and interoctahedral transfer (proton moving through the A-site plane), whose computed energy barriers and percolation pathways

What would settle it

Resonant inelastic X-ray scattering or X-ray absorption at the Ni L-edge and O K-edge on hydrogenated NdNiO3 could directly show whether the added electrons sit in oxygen ligand holes or in Ni eg states. Alternatively, repeating the first-principles calculation with the on-site Coulomb parameter varied from 1 to 4 eV would reveal whether the d8-ligand-hole occupation and polaron localization survive; if they disappear at moderate interaction strengths, the mechanism is an artifact of the parameter choice.

Watch

Extended reading notes

Core claim

In hydrogenated NdNiO3, the electrons introduced by hydrogen fill the O 2p ligand hole of the Ni–O antibonding d8L states rather than directly converting Ni3+ to Ni2+. Filling these antibonding states elongates the Ni–O bonds, forming electron polarons, and the protons themselves act as polarizing centers. Together these polarons weaken the Ni–O σ hybridization, converting the itinerant Ni eg electron into a localized t2g^6 eg^2 configuration. The half-filled eg shell then experiences strong on-site Coulomb repulsion, opening a Mott gap of about 0.81 eV. The same calculations map proton transport: in pristine NdNiO3 the intraoctahedral proton-transfer barrier is 0.35 eV and the interoctahedr

Load-bearing premise

The entire polaron mechanism rests on the choice of the on-site Coulomb correction parameter (Ueff = 2.0 eV) and on freezing the rare-earth f electrons in the core; if the real nickel–oxygen energy balance differs, the added electrons might occupy nickel states instead of oxygen ligand holes, and the polaron-driven localization would not hold.

Editorial extensions

If this is right

The hydrogen-induced insulating phase of NdNiO3 should be classified as a polaron-driven Mott insulator, not a simple band-filling or charge-transfer insulator.
Proton conductivity in rare-earth nickelates is set by two competing effects: smaller A-site cations increase hydrogen uptake but raise migration barriers, implying an optimal cation size for electrolyte performance.
Hydrogenation creates fast one-dimensional proton channels along the c-axis while blocking three-dimensional percolation, so the material may still work well at electrolyte–electrode interfaces that exploit that direction.
The calculated 0.81 eV gap in hydrogenated NdNiO3 provides a concrete target for optical and transport measurements to confirm the insulating state.
The finding that purely electron-doped NdNiO3 has a smaller gap (0.55 eV) than hydrogenated material supports a measurable role for the proton polaron beyond electron filling.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

A similar polaron-mediated mechanism may operate in other strongly correlated oxides that spontaneously absorb hydrogen, such as cobaltites and vanadates, offering a screening criterion: materials with O 2p ligand-hole states in their metallic phase are prime candidates.
Because electron-polaron formation is driven by filling antibonding states, strain or electrostatic doping could be used instead of hydrogen to tune the transition, with hydrogen adding proton-polaron effects on top.
The suppression of three-dimensional proton diffusion under full hydrogenation suggests an intermediate hydrogen concentration may best balance polaron formation against proton mobility; measuring conductivity across H concentrations could reveal a peak.
The computed role of A-site radius implies that mixing Nd and Sm on the A-site could tune both hydrogen uptake and proton mobility, an experimentally accessible chemical substitution.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

The useful core of this paper is the mechanistic bridge it builds between ligand-hole filling and Ni eg localization. The anti-doping observation itself was already reported by Gao et al., but the authors make a stronger claim: the added electrons occupy O 2p ligand-hole states, form electron polarons, and together with proton polarons weaken Ni-O hybridization enough to drive the eg electrons into a localized d8 configuration. That is a new synthesis, and the evidence they marshal for it is more consistent than most DFT+U papers I see. The PDOS, COHP, MLWF shapes, Bader charges, structural distortions, and the electron-doped reference calculation all point the same way, and the lattice parameters match experiment well.

The proton-transport section is also a real addition. The comparison between Nd and Sm — smaller A-site cation gives easier hydrogen uptake but slower diffusion — is clear and physically plausible. The finding that hydrogenation creates a fast 1D intraoctahedral pathway while blocking 3D percolation is concrete and testable.

Where I hesitate is the Ueff = 2.0 eV sensitivity. The whole narrative depends on the added electrons going into O 2p ligand holes rather than Ni eg states. That occupation is controlled by the charge-transfer energy, and U is precisely the parameter that shifts that balance. The authors justify 2.0 eV by citing prior work and by noting that larger U gives a magnetic ground state inconsistent with experiment in the pristine phase. But the relevant quantity is not the pristine magnetic ordering — it is the energy of adding an electron to the d8L hybrid in the hydrogenated structure. They never test that. I don't think this sinks the paper, because the ligand-hole assignment is independently supported by prior XAS work and by their own electron-doped calculation, but it does mean the central causal chain has a parameter-dependence that is asserted, not demonstrated.

Two smaller issues. The electron-doped reference calculation adds eight electrons to a neutral supercell without mentioning a compensating background or finite-size corrections; for a qualitative occupation analysis this is probably minor, but it should be acknowledged. And the proton barriers and diffusion coefficients rely entirely on the CHGNet machine-learning potential with no DFT cross-check. The authors explicitly call that part qualitative, which is honest, but then the numbers in the abstract and figures (0.19 eV barrier, D values) read as more quantitative than they are.

If I were refereeing this, the main request would be: run the hydrogenated phase at a couple of other U values (and maybe one functional) and show that the occupation and the polaron picture survive. That is a few calculations, not a rewrite. The paper deserves peer review — the mechanism is plausible, the work is careful, and the proton-transport trends are useful. Send it out, with a referee asked to dig into the U sensitivity.

Referee Report

2 major / 4 minor

Summary. The paper presents first-principles DFT+U calculations on hydrogenated NdNiO3 and SmNiO3, aiming to explain the hydrogen-induced metal-insulator transition (MIT) and proton conduction. The central claim is that electrons introduced by hydrogen doping occupy O 2p ligand-hole states of the Ni-O d8L hybrid, forming electron polarons that, together with proton polarons, weaken Ni-O hybridization and drive the itinerant Ni eg electrons toward a localized d8 configuration, opening a Mott gap. The authors support this with PDOS, COHP, MLWF, Bader charge, and structural distortion analyses, and also report hydrogen insertion energies and proton migration barriers/diffusion coefficients for NdNiO3 and SmNiO3.

Significance. If the proposed mechanism is correct, it provides a coherent atomistic picture of the hydrogen-induced MIT in rare-earth nickelates, distinguishing it from the simpler electron-filling Mott picture and connecting it to polaron formation. The paper is strengthened by the use of multiple complementary electronic-structure diagnostics, the agreement of the calculated ground-state structural parameters with experiment, the absence of parameter fitting beyond a literature-inherited Ueff, and the open data statement. The proton-transport results, while more qualitative, offer comparative insight for electrolyte design. However, the central mechanistic conclusion rests on a single DFT+U parameter setting and on an under-specified electron-doped reference calculation, so the current evidence is not yet conclusive.

major comments (2)

[Computational Method, Ueff] The entire anti-doping/polaron mechanism depends on the relative energy of Ni eg vs O 2p ligand-hole states, which in this calculation is controlled by Ueff = 2.0 eV applied only to Ni 3d (with Nd 4f frozen in the core). The authors justify this value by consistency with prior PBEsol+U work and by the statement that larger U yields an incorrect magnetic ground state for pristine nickelates, but no sensitivity check is reported for the hydrogenated phase or for the electron-doped reference. The pristine magnetic ground state does not directly constrain the charge-transfer energy relevant to adding an electron to the d8L hybrid. A larger U could move the added electron from O 2p into Ni eg, collapsing the proposed mechanism. I request calculations at additional Ueff values (e.g., 1.0, 3.0, 4.0 eV) or a hybrid-functional test on HNdNiO3, reporting the PDOS/COHP/MLWF occupations, band gap, a
[Results and Discussion B] The electron-doped NdNiO3 calculation is central to isolating the electron-polaron effect from the proton-polaron effect, but the computational protocol is not specified. Adding 'eight extra electrons' to a periodic 40-atom supercell requires charge neutralization; no compensating background or other procedure is mentioned. If a uniform background was used, its effect on total energies, structural relaxations, and the resulting band gap should be discussed. It is also unclear whether the electron-doped structures were fully relaxed with the same criteria as the hydrogenated ones, and what magnetic ordering was assumed. Without this information, the 0.55 eV gap and the bond-length elongations reported for this reference cannot be properly evaluated.

minor comments (4)

[Results and Discussion B] The MLWF evidence for weakened hybridization and eg localization is presented only qualitatively ('the weight ... is reduced', 'shapes become closer to ideal'). Providing numeric MLWF occupation matrices or oxygen weights would make the central electron-localization claim more quantitative and easier to assess.
[Proton conduction C] The migration barriers and diffusion coefficients are obtained with the CHGNet machine-learning potential, not with the DFT+U approach used for the electronic-structure analysis. The manuscript does state that CHGNet is used for qualitative trends, but for a journal publication it would be helpful to benchmark at least one barrier (e.g., the intraoctahedral barrier in NdNiO3) against direct DFT+U NEB, or to state the expected MLIP error for these correlated oxides.
[General / Fig. 3] There are several presentation issues: 'electron-doped symstem' should be 'system'; the isosurface unit in Fig. 3(e) reads '0.01 e−3' and should presumably be 'e Å⁻³' or similar; the d8L notation should be defined at first use; and in the Wannier-function description it should be stated explicitly why only the spin-up channel is considered, given that spin-down states also contribute to the insulating gap.
[Table I] The column layout of Table I is hard to parse: for pristine NdNiO3, the experimental values are given alongside PBEsol+U and LDA+U+J, but for HNdNiO3 no experimental structural parameters are listed even though the text refers to agreement with experiment for the volume expansion. Please clarify which entries are experimental and cite the corresponding sources in the table caption.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the central MIT mechanism is a DFT-computed interpretation with an inherited Hubbard U and an electron-doped control; self-citations are peripheral.

full rationale

The central derivation is self-contained. The MIT mechanism is read out of PBEsol+U calculations with Ueff=2.0 eV taken from prior PBEsol+U nickelate literature (refs [35,50]) and justified by the pristine ground state, not fitted to the hydrogenated phase or to the ligand-hole occupation. The claim that doped electrons occupy O 2p ligand holes of the d8L configuration is supported by PDOS, COHP, charge-density difference, Wannier functions, and a separately computed electron-doped control (Fig. 4) that is not a re-use of the hydrogenated result. The localization of Ni eg electrons is diagnosed from MLWF weight reduction, magnetic-moment changes, and Bader charges, all direct calculated observables. No equation in the paper is equivalent by construction to an input; there is no fitted parameter renamed as a prediction, and no uniqueness theorem from the authors is invoked to force the mechanism. The self-citations [22] and [67] support proton pairing/trapping and antiferrodistortive rotations, but the same distortions and trapping are independently computed here (ISOTROPY modes, BVEL, MSD), so these citations are not load-bearing for the central claim. The absence of a Hubbard-U sensitivity sweep is a robustness/correctness limitation rather than circularity: Ueff is inherited from prior work, and changing it would change the numerical results without making the derivation tautological.

Assumptions & free parameters 1 free parameters · 4 assumptions · 0 invented entities

The central claim does not introduce new physical entities: electron and proton polarons are standard quasiparticle concepts inferred from the calculations. The main free parameter is the Hubbard U, carried over from prior literature. The axioms are the DFT/ML-potential methodological assumptions.

free parameters (1)

Ueff (Hubbard U on Ni 3d) = 2.0 eV
Applied to Ni 3d orbitals, taken from previous PBEsol+U studies [35,50] to reproduce the experimentally observed ground state, not fitted in this paper. The central MIT mechanism may be sensitive to this value.

assumptions (4)

domain assumption PBEsol+U with Ueff=2.0 eV and frozen 4f cores provides a reliable electronic structure for rare-earth nickelates.
Invoked in Computational Method; the entire MIT mechanism rests on this description.
domain assumption The ferromagnetic orthorhombic phase is the correct reference state for hydrogenated NdNiO3 at the operating temperatures of interest.
The authors choose FM over AFM based on energy ordering (Fig. A2) and prior work; if the true magnetic ordering matters for hydrogenation, the mechanism could change.
domain assumption The fully hydrogenated HNdNiO3 configuration (protons at eight apical oxygen sites) is representative of the saturated hydrogenated state.
Section II: the x=1 configuration is taken from refs [35,37]; the electronic-structure analysis is done at this composition.
domain assumption The CHGNet machine-learned potential is transferable to proton migration in these specific nickelates.
Section II: 'CHGNet is used here to explore the qualitative influence' — but quantitative barriers and D values are reported from it without direct DFT validation.

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Cite this review

Pith. "Pith review of Polaron-mediated metal-insulator transition and proton conduction in hydrogenated nickelate perovskites." pith.science (2026). https://pith.science/paper/V54NHYAY

@misc{pith2026260802243,
  author       = {Pith},
  title        = {Pith review of: Polaron-mediated metal-insulator transition and proton conduction in hydrogenated nickelate perovskites},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/V54NHYAY}},
  note         = {Machine review of arXiv:2608.02243}
}

abstract

Nickel-based perovskites, owing to their spontaneous hydrogen uptake and the dramatic increase in resistivity upon hydrogenation, have emerged as promising candidates for proton-conducting fuel cell electrolytes. However, the mechanism of the hydrogen-induced metal-insulator transition (MIT) in rare-earth nickelates remains under debate, particularly regarding whether the doped electrons occupy Ni e$_g$ states or O 2p ligand hole states. Here, we reveal a comprehensive MIT mechanism using first-principles calculations on NdNiO$_3$: the electrons introduced by hydrogen doping occupy the O 2p ligand hole states of the Ni-O hybridized d$_8$L configuration, promoting electron-polaron formation. The resulting electron polarons, together with proton polarons, weaken the Ni-O hybridization and thereby drive the originally itinerant Ni e$_g$ electrons toward localization. This generates a local d8 (t$_{2g}$$^6$e$_g$$^2$) electronic configuration, leading to a Mott transition. In addition, we also find that compared with NdNiO$_3$, SmNiO$_3$ with a smaller A-site ionic radius more readily absorbs hydrogen but exhibits weaker proton diffusion capability. Hydrogenation promotes proton permeation along the [001] direction via the intraoctahedral transfer, whereas the overall proton diffusivity is reduced. These results provide guidance for experimental screening of strongly correlated oxides as electrolyte materials and offer theoretical insights for enhancing proton conductivity in rare-earth nickelates.

Figures

Figures reproduced from arXiv: 2608.02243 by the authors.

**Figure 2.** FIG. 2. Calculated band structure of NdNiO [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗

**Figure 3.** FIG. 3. PDOS of NdNiO [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗

Figures from the paper (4 more)

**Figure 4.** Figure 4: FIG. 4. (a) Spin density distribution of NdNiO [PITH_FULL_IMAGE:figures/full_fig_p006_4.png]

**Figure 6.** Figure 6: FIG. 6. Hydrogen insertion energy as a function of hydrogen [PITH_FULL_IMAGE:figures/full_fig_p007_6.png]

**Figure 7.** Figure 7: FIG. 7. Energy barriers and schematic pathways for intraoctahedral (a) and interoctahedral (b) transfer in NdNiO [PITH_FULL_IMAGE:figures/full_fig_p008_7.png]

**Figure 8.** Figure 8: FIG. 8. Mean-square displacement (MSD) extracted from [PITH_FULL_IMAGE:figures/full_fig_p008_8.png]

Discussion (0). Continue with ORCID to comment.

Reference graph

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Wannier-interpolated band structures As shown in Fig. A1, the band structure obtained from the Wannier-interpolated tight-binding model for the spin-up channel agrees well with the DFT results, demonstrating the accuracy of the constructed Wannier functions
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In addition, we also find that compared with NdNiO3, SmNiO3 with a smaller A-site ionic radius more readily absorbs hydrogen but exhibits weaker proton diffusion capability

electronic configuration, leading to a Mott transition. In addition, we also find that compared with NdNiO3, SmNiO3 with a smaller A-site ionic radius more readily absorbs hydrogen but exhibits weaker proton diffusion capability. Hydrogenation promotes proton permeation along the [001] direction via the intraoctahedral transfer, whereas the overall proton...

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Our calculations show that orthorhombic NdNiO3 favors a FM ground state, while all AFM configurations are at least 100 meV higher in en- ergy per 40-atom supercell (Fig

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A2, based on a 40-atom orthorhombic NdNiO3 supercell, our PBEsol+U calculations yield the relative energy ordering of different magnetic configura- tions as FM<A-AFM<C-AFM<G-AFM

Magnetic ground state of NdNiO3 As shown in Fig. A2, based on a 40-atom orthorhombic NdNiO3 supercell, our PBEsol+U calculations yield the relative energy ordering of different magnetic configura- tions as FM<A-AFM<C-AFM<G-AFM. Therefore, the FM state is identified as the magnetic ground state of orthorhombic NdNiO3
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Compared to NdNiO3, the smaller A-site ionic radius in SmNiO3 leads to a larger lattice mismatch between the A- and B-site, resulting in enhanced octahedral tilting and rotation

The structural parameters of SmNiO3 TableIIpresentsthestructuralparametersofSmNiO 3. Compared to NdNiO3, the smaller A-site ionic radius in SmNiO3 leads to a larger lattice mismatch between the A- and B-site, resulting in enhanced octahedral tilting and rotation. Consequently, the Ni-O-Ni bond angles are reduced, and the amplitudes of the a−a−c0/a0a0c+ 10...
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A3, protons bound to apical O4c and equatorial O8d sites exhibit four mutually perpendicu- lar interstitial orientations

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Proton diffusion in NdNiO3 and SmNiO3 The transfer barriers for intraoctahedral and interocta- hedral transfer in NdNiO3 are 0.35 and 0.62 eV, respec- tively. The higher barrier for interoctahedral transfer arises because the proton passes through the A-O plane at the transition state, resulting in a shorter distance to the A-site cations compared to the ...
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Comparison of proton diffusion in NdNiO3 and HNdNiO3 Hydrogenation significantly reduces the intraoctahe- dral transfer barrier, while markedly increasing the inte- roctahedral barrier compared to pristine NdNiO3. This trend is consistent with the change in the distance be- tween the proton and the Nd cations at the transition state (Fig. A4(a)). For the ...

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2026 doi

Pith tools

Reviewed August 4, 2026 · model on record in the stance chip above.

[1] [1]

Wannier-interpolated band structures As shown in Fig. A1, the band structure obtained from the Wannier-interpolated tight-binding model for the spin-up channel agrees well with the DFT results, demonstrating the accuracy of the constructed Wannier functions

[2] [2]

In addition, we also find that compared with NdNiO3, SmNiO3 with a smaller A-site ionic radius more readily absorbs hydrogen but exhibits weaker proton diffusion capability

electronic configuration, leading to a Mott transition. In addition, we also find that compared with NdNiO3, SmNiO3 with a smaller A-site ionic radius more readily absorbs hydrogen but exhibits weaker proton diffusion capability. Hydrogenation promotes proton permeation along the [001] direction via the intraoctahedral transfer, whereas the overall proton...

arXiv 2026

[3] [3]

Our calculations show that orthorhombic NdNiO3 favors a FM ground state, while all AFM configurations are at least 100 meV higher in en- ergy per 40-atom supercell (Fig

only at low temper- atures (T <200K)[63, 64], we consider several conven- tional magnetic configurations for the high-temperature phase, including ferromagnetic (FM) and three types of antiferromagnetic orderings: A-type (A-AFM), C-type (C-AFM), and G-type (G-AFM), corresponding to inter- layer, intrachain, and fully staggered antiferromagnetic arrangemen...

[4] [4]

A2, based on a 40-atom orthorhombic NdNiO3 supercell, our PBEsol+U calculations yield the relative energy ordering of different magnetic configura- tions as FM<A-AFM<C-AFM<G-AFM

Magnetic ground state of NdNiO3 As shown in Fig. A2, based on a 40-atom orthorhombic NdNiO3 supercell, our PBEsol+U calculations yield the relative energy ordering of different magnetic configura- tions as FM<A-AFM<C-AFM<G-AFM. Therefore, the FM state is identified as the magnetic ground state of orthorhombic NdNiO3

[5] [5]

Compared to NdNiO3, the smaller A-site ionic radius in SmNiO3 leads to a larger lattice mismatch between the A- and B-site, resulting in enhanced octahedral tilting and rotation

The structural parameters of SmNiO3 TableIIpresentsthestructuralparametersofSmNiO 3. Compared to NdNiO3, the smaller A-site ionic radius in SmNiO3 leads to a larger lattice mismatch between the A- and B-site, resulting in enhanced octahedral tilting and rotation. Consequently, the Ni-O-Ni bond angles are reduced, and the amplitudes of the a−a−c0/a0a0c+ 10...

[6] [6]

A3, protons bound to apical O4c and equatorial O8d sites exhibit four mutually perpendicu- lar interstitial orientations

Proton configurations at oxygen sites As shown in Fig. A3, protons bound to apical O4c and equatorial O8d sites exhibit four mutually perpendicu- lar interstitial orientations. For each oxygen site, the configurational energies of these orientations depend on the distances between the proton and the two nearest A- site cations. The interstitial orientatio...

[7] [7]

Proton diffusion in NdNiO3 and SmNiO3 The transfer barriers for intraoctahedral and interocta- hedral transfer in NdNiO3 are 0.35 and 0.62 eV, respec- tively. The higher barrier for interoctahedral transfer arises because the proton passes through the A-O plane at the transition state, resulting in a shorter distance to the A-site cations compared to the ...

[8] [8]

This trend is consistent with the change in the distance be- tween the proton and the Nd cations at the transition state (Fig

Comparison of proton diffusion in NdNiO3 and HNdNiO3 Hydrogenation significantly reduces the intraoctahe- dral transfer barrier, while markedly increasing the inte- roctahedral barrier compared to pristine NdNiO3. This trend is consistent with the change in the distance be- tween the proton and the Nd cations at the transition state (Fig. A4(a)). For the ...