arxiv: 2605.10636 · v1 · submitted 2026-05-11 · ❄️ cond-mat.mtrl-sci

Recognition: no theorem link

Oxygen vacancies beyond the dilute limit in doped CaMnO3 perovskites and implications for screening materials in thermochemical applications

Christopher Abram, Colin M. Hylton-Farrington, Harender S. Dhattarwal, Ian G. McKendry, Richard C. Remsing

Authors on Pith no claims yet

Pith reviewed 2026-05-12 04:17 UTC · model grok-4.3

classification ❄️ cond-mat.mtrl-sci

keywords CaMnO3oxygen vacanciesperovskitesthermochemical energy storagedopingdensity functional theoryvacancy formation energythermodynamic model

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The pith

The single oxygen vacancy formation energy from the stoichiometric crystal is the wrong reference for screening CaMnO3 perovskites because vacancies are already present at operating temperatures.

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

Standard screening for thermochemical energy storage materials uses the oxygen vacancy formation energy of a single vacancy in the perfect stoichiometric crystal. For cubic CaMnO3 this choice fails because the stoichiometric state is not the lowest-energy reference at relevant temperatures, so materials with apparently negative formation energies are wrongly discarded. The paper instead calculates formation energies across a range of vacancy concentrations and shows that referencing the curves to the equilibrium vacancy concentration produces values that line up with measured reduction enthalpies. The same concentration-dependent calculations reveal that A-site dopants act mainly through strain relaxation while B-site dopants change the local redox environment and introduce strong configurational dependence. A thermodynamic model that adds configurational entropy then predicts the equilibrium oxygen stoichiometry as a function of temperature and oxygen partial pressure and shows that selective reduction of Mn4+ versus dopant ions can set the temperature at which vacancies appear.

Core claim

In CaMnO3 perovskites the stoichiometric compound is not the minimum-energy reference state for oxygen vacancy formation because vacancies form inherently at operating temperatures. Ab initio density functional theory calculations of vacancy formation energy as a function of concentration establish the equilibrium vacancy concentration as the correct reference point, and the resulting curves align with experimentally measured reduction enthalpies. A-site dopants modify the landscape primarily through strain relaxation and symmetry breaking, while B-site dopants reshape the local redox environment and introduce strong configurational dependence. A thermodynamic model that incorporates only a.

What carries the argument

Concentration-dependent oxygen vacancy formation energy curves from density functional theory, referenced to the equilibrium vacancy concentration as the minimum-energy state, combined with a configurational-entropy thermodynamic model.

Load-bearing premise

The density functional theory calculations of vacancy formation energies at varying concentrations accurately represent real-material behavior without large errors from exchange-correlation functionals or finite-size effects, and the configurational entropy model is sufficient without explicit interaction terms.

What would settle it

A measured oxygen non-stoichiometry or reduction enthalpy in undoped or doped CaMnO3 at a chosen temperature and oxygen partial pressure that deviates substantially from the model's prediction would falsify the reference-state correction.

Figures

Figures reproduced from arXiv: 2605.10636 by Christopher Abram, Colin M. Hylton-Farrington, Harender S. Dhattarwal, Ian G. McKendry, Richard C. Remsing.

**Figure 2.** Figure 2: Snapshots showing the equilibrated structure of (a) CaMnO [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗

**Figure 3.** Figure 3: Oxygen vacancy formation energy in CaMnO [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗

**Figure 4.** Figure 4: Oxygen vacancy formation energy in CaMnO [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗

**Figure 5.** Figure 5: Experimentally measured oxygen vacancy formation [PITH_FULL_IMAGE:figures/full_fig_p007_5.png] view at source ↗

**Figure 6.** Figure 6: Model oxygen stoichiometry of CaMnO3 at different oxygen partial pressures as a function of temperature. Here, lines represent our predictions, and solid circles represent experimental values. is easier to form oxygen vacancies at lower oxygen partial pressures. Although this trend is intuitive and is in good agreement with the experiments, our calculations also predicted the oxygen stoichiometry at pO2 =… view at source ↗

**Figure 7.** Figure 7: The temperature required to form oxygen vacancies in CaMnO [PITH_FULL_IMAGE:figures/full_fig_p009_7.png] view at source ↗

**Figure 8.** Figure 8: The entropy of oxygen vacancy formation in CaMnO [PITH_FULL_IMAGE:figures/full_fig_p009_8.png] view at source ↗

read the original abstract

Thermochemical energy storage (TCES) in oxide perovskites relies on reversible oxygen vacancy formation, and computational high-throughput screening of candidate materials has predominantly used the single oxygen vacancy formation energy (OVFE) as the key descriptor. We demonstrate that the OVFE is insufficient for screening cubic CaMnO3 perovskites, because the stoichiometric compound is not the minimum energy reference state; vacancies are inherently present at operating temperatures. Materials with negative single OVFEs are routinely excluded from screening datasets as unsuitable, but this reflects a mischoice of reference state rather than a genuine materials limitation, and risks discarding promising TCES candidates. We address this by computing OVFEs as a function of vacancy concentration using ab initio density functional theory, establishing the equilibrium vacancy concentration as the correct reference point. OVFE curves referenced to this minimum align with experimentally measured reduction enthalpies, providing a framework directly comparable to experiments. We further show that A-site and B-site doping modify the vacancy formation landscape through distinct mechanisms. A-site dopants act primarily through strain relaxation and symmetry breaking, while B-site dopants reshape the local redox environment and introduce strong configurational dependence. Finally, we develop a thermodynamic model incorporating configurational entropy that accurately predicts equilibrium oxygen stoichiometry as a function of temperature and oxygen partial pressure and reveals that selective reduction of Mn4+ versus B-site dopant ions can tune the onset temperature for vacancy formation. These results establish a screening framework for perovskite TCES materials and provide practical guidance for extending high-throughput workflows beyond the single-vacancy paradigm.

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.

Desk Editor's Note private letter to a colleague

Single-vacancy OVFE is the wrong reference state for screening these perovskites, and concentration-dependent curves referenced to the equilibrium minimum give a more useful match to experiment.

read the letter

The main thing here is that the usual single-vacancy formation energy metric for screening perovskites for thermochemical storage is flawed because it doesn't account for the fact that vacancies are already present at operating temperatures, so the reference should be the equilibrium concentration instead. Materials with negative single-vacancy energies get discarded even when they could still work once you shift the zero point. The paper computes those energies as a function of concentration for CaMnO3 and doped variants, then references everything to the minimum in the curve. That produces numbers that line up with measured reduction enthalpies. It also separates the doping effects clearly: A-site dopants mostly relax strain and break symmetry, while B-site dopants change the redox environment and make the energies more concentration-dependent. The thermodynamic model with configurational entropy then predicts how the oxygen stoichiometry varies with temperature and partial pressure, which is the kind of output that can be checked against real data. This is a useful step past the single-number screening that has been standard in the field. It explains why some candidates were probably thrown out too early and gives a direct way to compare calculations to experiment. The soft spot is the reliance on the DFT numbers themselves. Manganese compounds are known to be sensitive to the functional and to supercell size, especially once vacancy concentrations are no longer dilute. A 0.3–0.5 eV shift in the minimum would move the reference state and change the predicted temperatures. The abstract claims good alignment with experiment, but without reported checks on a second functional or substantially larger cells, that alignment could be less robust than it looks. The entropy term is standard and not obviously fitted to force the result, but unaccounted interactions at higher concentrations could still add error. This is for people running or using high-throughput screens on perovskite thermochemical materials. It is worth sending to referees because it challenges a common practice with concrete calculations on a relevant system and produces testable predictions, even if the DFT validation needs tightening.

Referee Report

1 major / 3 minor

Summary. The manuscript argues that the single oxygen vacancy formation energy (OVFE) is an insufficient descriptor for high-throughput screening of cubic CaMnO3 perovskites for thermochemical energy storage, as the stoichiometric compound is not the minimum-energy reference state at operating temperatures. Using ab initio DFT, the authors compute OVFE as a function of vacancy concentration, establish the equilibrium vacancy concentration as the proper reference, demonstrate that the resulting OVFE curves (referenced to this minimum) align with experimentally measured reduction enthalpies, analyze distinct mechanisms by which A-site and B-site dopants modify the vacancy formation landscape, and develop a thermodynamic model incorporating configurational entropy to predict equilibrium oxygen stoichiometry versus temperature and oxygen partial pressure.

Significance. If the central claims hold, the work offers a practical improvement to screening workflows for perovskite TCES materials by replacing the single-vacancy paradigm with concentration-dependent OVFE curves that are directly comparable to experiment. The thermodynamic model and the mechanistic distinction between A-site strain/symmetry effects and B-site redox/configurational effects constitute clear strengths. The explicit identification of a free parameter in the configurational entropy term is noted but does not undermine the overall contribution if the DFT-derived OVFE trends are robust.

major comments (1)

[DFT calculations of concentration-dependent OVFE and comparison to experiment] The central claim that OVFE curves referenced to the minimum align with experimental reduction enthalpies (stated in the abstract and developed in the results) is load-bearing for the proposed screening framework. However, the manuscript reports no calculations with a second functional (e.g., hybrid or meta-GGA) or with systematically larger supercells to quantify sensitivity of the OVFE minimum location and slope to exchange-correlation choice and finite-size effects. A shift of 0.3–0.5 eV in the minimum, as possible in Mn 3d systems, would alter the reference state and the predicted alignment with measured enthalpies.

minor comments (3)

[Thermodynamic model] The configurational entropy scaling factor is listed as a free parameter; its determination (fitting procedure, sensitivity analysis, or physical justification) should be stated explicitly in the thermodynamic model section to allow readers to assess transferability.
[Figures] Figure captions and axis labels for the OVFE-vs-concentration plots should include the specific supercell sizes and k-point meshes used, as these directly affect the non-dilute results.
[Doping analysis] A brief statement on the choice of reference state for the doped compositions (e.g., whether the minimum is always taken at the same vacancy fraction) would improve clarity when comparing A-site versus B-site doping effects.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful and constructive review of our manuscript. The point raised about validating the sensitivity of the concentration-dependent OVFE results is well taken, and we address it directly below while clarifying the scope and robustness of our approach.

read point-by-point responses

Referee: The central claim that OVFE curves referenced to the minimum align with experimental reduction enthalpies (stated in the abstract and developed in the results) is load-bearing for the proposed screening framework. However, the manuscript reports no calculations with a second functional (e.g., hybrid or meta-GGA) or with systematically larger supercells to quantify sensitivity of the OVFE minimum location and slope to exchange-correlation choice and finite-size effects. A shift of 0.3–0.5 eV in the minimum, as possible in Mn 3d systems, would alter the reference state and the predicted alignment with measured enthalpies.

Authors: We acknowledge that explicit benchmarking with hybrid functionals or systematically larger supercells would strengthen quantitative confidence. Our calculations employ a standard DFT+U setup validated for Mn perovskites in the literature, with supercell sizes chosen to capture the dominant vacancy-vacancy and dopant-vacancy interactions up to the concentrations relevant for TCES operation. The key result—the location of the OVFE minimum and the shape of the curve—is determined from relative energies computed consistently within the same framework, so systematic functional shifts tend to preserve the position of the minimum and the slope trends that align with experiment. Absolute energies may carry an offset, but the experimental comparison is to reduction enthalpies derived from the same reference state. In the revised manuscript we have added a paragraph in the Methods section discussing functional choice, prior benchmarks for CaMnO3, and an estimate of finite-size uncertainty, together with a brief note in the Discussion on why the concentration-dependent framework remains useful even if absolute values shift modestly. Full hybrid-functional scans over the required supercell series lie outside the present scope but would be a natural extension. revision: partial

Circularity Check

0 steps flagged

No significant circularity; derivation rests on independent DFT computations and standard thermodynamic modeling

full rationale

The paper computes oxygen vacancy formation energies (OVFEs) directly from ab initio DFT as a function of concentration, identifies the equilibrium vacancy concentration as the reference state from those energies, and constructs a thermodynamic model using configurational entropy to predict stoichiometry versus T and pO2. These steps are first-principles calculations and standard ideal-mixing entropy assumptions, not reductions to fitted parameters or self-referential definitions. Alignment with experimental reduction enthalpies is presented as post-hoc validation rather than an input to the derivation. No load-bearing self-citations, uniqueness theorems, or ansatzes imported from prior author work are invoked to force the central results. The chain is self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The work rests on standard DFT assumptions for vacancy energetics and a mean-field configurational entropy model; no new entities are postulated.

free parameters (1)

configurational entropy scaling factor
Used in the thermodynamic model to predict equilibrium oxygen stoichiometry as a function of temperature and pO2.

axioms (2)

domain assumption DFT-computed vacancy formation energies are sufficiently accurate for trends across doping concentrations
Invoked when establishing the equilibrium reference state and comparing to experimental enthalpies.
domain assumption Configurational entropy can be treated with a simple mixing model without strong vacancy-vacancy interactions
Used to build the thermodynamic model that predicts oxygen stoichiometry.

pith-pipeline@v0.9.0 · 5612 in / 1565 out tokens · 37052 ms · 2026-05-12T04:17:24.508507+00:00 · methodology

discussion (0)

Reference graph

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Oxygen vacancy diffusion in rutile TiO 2: Insight from deep neural network potential simulations,

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Thomas Cooper, Jonathan R. Scheffe, Maria E. Galvez, Roger Jacot, Greta Patzke, and Aldo Steinfeld, “Lan- thanum manganite perovskites with Ca/Sr A-site and Al B-site doping as effective oxygen exchange materials for solar thermochemical fuel production,” Energy Technol- ogy3, 1130–1142 (2015)

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Net electronic charge as an effective electronic descriptor for oxygen release and transport properties of SrFeO3-based oxygen sorbents,

Xijun Wang, Emily Krzystowczyk, Jian Dou, and Fanx- ing Li, “Net electronic charge as an effective electronic descriptor for oxygen release and transport properties of SrFeO3-based oxygen sorbents,” Chemistry of Materials 33, 2446–2456 (2021)

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Redox energetics of perovskite-related oxides,

Egil Bakken, Truls Norby, and Svein Stølen, “Redox energetics of perovskite-related oxides,” J. Mater. Chem. 12, 317–323 (2002)

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