REVIEW 4 major objections 6 minor 33 references
Instability of oxide perovskite surfaces induced by vacancy formation
T0 review · 4 major / 6 minor · reviewed 2026-08-16 · deepseek-v4-flash
Pith's one-line read Across nine oxide perovskites, the $\mathrm{AO}$ (001) surface is cheaper to form than the $\mathrm{BO}_2$ surface but far more prone to A-site vacancy formation, so the $\mathrm{BO}_2$ termination is the one that persists.
desk verdict Useful systematic DFT map of vacancy energetics across nine perovskites, but the BO2-prevalence claim overreaches what single-vacancy calculations can support. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing quantity is the bulk-referenced vacancy formation energy: the cost of removing an A-, B-, or O-site atom from a given layer of a $2\times2$ six-layer slab of either termination, measured relative to removing the same atom from the bulk crystal. Referencing to the bulk removes the arbitrary chemical-potential choice, so the depth profile isolates purely surface effects, and it is this quantity that produces the decisive 1–2 eV asymmetry between A-site vacancies at the $\mathrm{AO}$ surface and B-site vacancies at the $\mathrm{BO}_2$ surface. Around this core sit the surface Gibbs free energies, built with the standard excess-atom ($\Gamma$) formalism to map stability against growth conditions, and charged-vacancy transition-level calculations in $\mathrm{SrSnO}_3$, corrected for image charges, which extend the argument to the Fermi-level dependence.
What would settle it
Calculate the coupled removal energy of an A-site atom and a neighbouring oxygen at the $\mathrm{AO}$ surface in the same $2\times2$ slab geometry: if removing a whole AO unit costs far more than the isolated A-vacancy energy implies, the predicted selective degradation of the $\mathrm{AO}$ termination is weaker than claimed. Experimentally, anneal an atomically defined $\mathrm{AO}$-terminated $\mathrm{SrTiO}_3$ surface under oxygen-rich conditions and track the termination fraction, since the mechanism predicts $\mathrm{BO}_2$ coverage should rise as A-site vacancies form.
Extended reading notes
Core claim
For all nine $\mathrm{ABO}_3$ perovskites studied, density functional theory calculations on two-termination (001) slabs show that the undefected $\mathrm{AO}$ surface has the lower formation energy, and chemical-potential phase diagrams place $\mathrm{AO}$ as the stable termination across nearly all growth conditions. Introducing vacancies reverses the ranking: A-site vacancies at the $\mathrm{AO}$ surface are 1–2 eV cheaper than B-site vacancies at the $\mathrm{BO}_2$ surface, and vacancy formation energies fall systematically toward the surface relative to bulk. For charged vacancies in $\mathrm{SrSnO}_3$, the $\mathrm{AO}$ surface develops negative-formation-energy cation vacancies across most of the band gap under oxygen-rich conditions, shrinking the Fermi-level window in which the $\mathrm{AO}$ termination is stable. The paper concludes that $\mathrm{AO}$ surfaces are easy to create but prone to degrade by losing A-site cations, thereby exposing the $\mathrm{BO}_2$ layer below, so $\mathrm{BO}_2$ should be the more prevalent termination on real, aged surfaces.
Load-bearing premise
The argument assumes that single-vacancy formation energies, computed at a fixed high vacancy concentration in $2\times2$ slabs, control how the $\mathrm{AO}$ layer actually disappears, even though the paper states that removing an A atom and an O atom together cannot be represented by summing the isolated single-vacancy energies.
Editorial extensions
If this is right
- Real $\mathrm{AO}$-terminated samples should convert toward $\mathrm{BO}_2$ over time as A-site cations vacate the surface, so both terminations seen experimentally may reflect degradation rather than equilibrium formation.
- A practical synthesis route follows directly: grow or prepare the $\mathrm{AO}$ termination first, then treat the surface so its A-site depletion reveals a $\mathrm{BO}_2$ termination, the strategy the paper explicitly endorses.
- For applications requiring a chemically stable surface, such as catalysis, interfaces, and devices, the $\mathrm{BO}_2$ termination is the safer choice, whereas $\mathrm{AO}$ is only the easier starting point.
- In $\mathrm{SrSnO}_3$, the $\mathrm{BO}_2$ surface keeps a wider Fermi-level window free of compensating native vacancies, so doping and electronic applications are more reliable on the $\mathrm{BO}_2$ termination.
Reading between the lines
- The paper's own caveat, that removing A and O together cannot be handled by summing isolated single-vacancy energies, implies the cleanest numerical test of its mechanism: compute the coupled (A + O) removal cost at the $\mathrm{AO}$ surface, and a much higher coupled cost would slow the predicted AO-to-BO2 conversion considerably.
- The mechanism carries a kinetic signature the paper does not develop: on an $\mathrm{AO}$-terminated sample, $\mathrm{BO}_2$ coverage should grow with annealing time wherever A-site vacancy formation is favourable, so time- and temperature-resolved termination measurements would test the claim directly.
- Because A-site vacancy favourability traces to the ionic, weakly bonded nature of the A-site cage, the pattern that the easy termination is the vacancy-prone one is likely to extend beyond alkaline-earth perovskites to other polar oxide surfaces, where it could serve as a screening rule.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper presents a GGA-PBE DFT study of the (001) surfaces of nine oxide perovskites (ABO3 with A = Ca, Sr, Ba and B = Ti, Zr, Sn), comparing AO- versus BO2-terminated slabs, computing neutral vacancy formation energies as a function of depth, constructing chemical-potential surface phase diagrams, and performing charged-defect calculations for SrSnO3. The central claim is that the AO termination is energetically preferred on the clean surface, but A-site vacancies at the AO surface are 1-2 eV more favorable than B-site vacancies at the BO2 surface, so the AO surface degrades more readily and the BO2 termination may become more prevalent over long times. The study includes a systematic dataset across nine materials, comparison with literature titanate oxygen-vacancy energies and with Weston et al. for SrSnO3, and modern charged-defect correction schemes.
Significance. If the central conclusion were established, it would be practically relevant for perovskite surface manufacturing and for interpreting experimental observations of coexisting terminations. The paper has clear strengths: a consistent first-principles dataset across nine materials, careful treatment of charged defects with eFNV and 2D slab corrections, explicit benchmarking against known literature values, and a useful benchmark of machine-learned potentials against DFT for surface and vacancy energetics (Table SII). However, the central inference from single-vacancy energetics to full-layer degradation is not currently supported by the calculations, and the charged-defect part of the argument is demonstrated for only one of the nine materials. These issues affect the main conclusion and require substantial additional work or reframing.
major comments (4)
- [Section III D, Eq. (9), Table SVI] The conclusion that BO2 surfaces become more prevalent through degradation of AO surfaces rests on comparing the isolated A-site vacancy at the AO surface with the isolated B-site vacancy at the BO2 surface. Exposing a BO2 layer requires removing an entire AO layer, i.e., both A and O atoms, and the authors state in SI SIII G that simultaneous A- and O-site vacancy formation "cannot be considered by simply summing the relevant isolated vacancy energies." Therefore the 1-2 eV single-vacancy comparison in Table SVI does not by itself establish the proposed layer-removal pathway. I ask the authors to compute coupled A+O vacancy formation energies, at least for a subset of the nine materials, or to formulate an explicit thermodynamic model of AO-layer removal, before drawing the long-term stability conclusion.
- [Section III A and Conclusion] The clean-surface and chemical-potential results show that the AO termination is more favorable than BO2 under essentially all conditions considered (with CaTiO3 and CaSnO3 noted as exceptions in the Conclusion). The proposed AO-to-BO2 conversion would have to overcome this thermodynamic preference, yet the manuscript provides no free-energy or kinetic argument for why vacancy formation would reverse the relative stability of the two terminations. At minimum, the conclusion should be reframed as a degradation hypothesis rather than a statement that BO2 "will display better long term stability" or "will be more prevalent than initially expected."
- [Sections III C and III E] Charged-defect calculations are performed only for SrSnO3, both in bulk and in slabs, but the abstract and conclusion generalize the vacancy-stability argument to all nine perovskites, for example "Charged vacancies only drive this further under oxygen-rich conditions" and the recommendation for manufacturing BO2 surfaces of "these materials." Since the charged-vacancy stabilization is part of the degradation mechanism, the generalization should either be supported by charged-defect calculations for additional representatives (for instance one titanate and one zirconate) or explicitly flagged as an assumption.
- [Section II A and Section III D] Vacancy formation energies are reported without error bars or systematic convergence checks, at concentrations of roughly 0.021-0.042 vac/unit in 2x2 slabs, and the bulk vacancies are modeled in 2x2x2 supercells. The claim that surface defect densities are "several orders of magnitude" higher than in the bulk is quantitative and would be sensitive to these choices. A short convergence test (for example, a 3x3 surface cell or thicker slabs for one or two materials) would substantially strengthen the long-term stability discussion and is within the scope of the present study.
minor comments (6)
- [Abstract] The sentence "These results indicates that..." should read "These results indicate that..."
- [Section III C] The text uses "VMB" for the valence band maximum; it should be "VBM".
- [Section III E] There are typos in this section: "BO2-termianted" should be "BO2-terminated" and "exhbits" should be "exhibits".
- [SI SII C] The sentence "we can now simply Equation (S3)" should read "we can now simplify Equation (S3)".
- [Table SII caption] The caption lists "MPACE-MPA-0" as a model name; this should be "MACE-MPA-0" for consistency with the text.
- [Section III A] The sentence describing Fig. 2a is grammatically garbled: "here, the values are given relative to composite binary oxides (AO and BO2) and the bulk perovskite (ABO3) are presented in Fig. 2a." It should be rephrased to clearly state that the formation energies are referenced to the binary oxides.
Circularity Check
No significant circularity: all load-bearing quantities are independent DFT formation energies; the main caveat is an extrapolation limit, not a tautology.
full rationale
The paper's surface formation energies (Eq. 1) and vacancy formation energies (Eqs. 8-9) are computed from DFT total energies with stated reference phases; no parameter is fitted to the predicted outcome. The claim that A-site vacancies at AO surfaces are 1-2 eV more favourable than B-site vacancies at BO2 surfaces is a direct comparison of independently calculated formation energies, benchmarked against external literature (e.g., Weston et al. for SrSnO3 charged defects, Eglitis et al. for surface energies). The chemical-potential phase diagram framework follows Heifets et al., an external method, and the charged-defect corrections use standard published schemes (doped, qdef2d, sxdefectalign2d). Self-citations (ARTEMIS for slab construction, RAFFLE in the SI) are tool or method citations and are not load-bearing for the central energetic comparison. The SI SIII G caveat that simultaneous A- and O-site vacancy formation "cannot be considered by simply summing the relevant isolated vacancy energies" weakens the extrapolation from single-vacancy energetics to full AO-layer degradation, but this is an inference gap and a limitation statement, not a circular definition or a fitted input renamed as a prediction. No equation in the paper reduces to its own input, and no load-bearing premise rests on a self-citation chain.
Assumptions & free parameters
assumptions (4)
- domain assumption Equilibrium thermodynamics links the chemical potentials of A, B, and O through mu_ABO3 = mu_A + mu_B + 3 mu_O (Eq. 3).
- domain assumption Gibbs free energies are approximated by 0 K DFT total energies (Eq. S12 in SI SII C), neglecting vibrational, entropic, and temperature contributions.
- domain assumption GGA-PBE exchange-correlation functional gives reliable relative energies for these surfaces and vacancies.
- ad hoc to paper Charged slab defect corrections use bulk dielectric constants for SrSnO3 (epsilon_r,zz = 16.329), even though thin-film permittivity is lower.
Cite this review
Pith. "Pith review of Instability of oxide perovskite surfaces induced by vacancy formation." pith.science (2026). https://pith.science/paper/QEWOKOG4
@misc{pith2026250415857,
author = {Pith},
title = {Pith review of: Instability of oxide perovskite surfaces induced by vacancy formation},
year = {2026},
howpublished = {\url{https://pith.science/paper/QEWOKOG4}},
note = {Machine review of arXiv:2504.15857}
}
abstract
This work presents a first principles study of the (001) surface energetics of nine oxide perovskites, with a focus on the role of surface vacancies in determining termination stability. Additionally, investigation into the behaviour of vacancies as a function of depth from the surface in these perovskites, ABO$_{3}$ (A=Ca, Sr, Ba; B=Ti, Zr, Sn), is carried out, and results are compared to formation of the vacancies in bulk. Combining results from these investigations reveals a general trend for all nine perovskites - the undefected AO surface is more energetically favourable to form than the BO2 surface. This dominance of the AO over the BO2 surface is further enforced by the phase diagrams of perovskite surfaces. However, A-site vacancies at the AO surface are far more favourable (1-2 eV lower in energy) than B-site vacancies at the BO2 surface. Charged vacancies only drive this further under oxygen-rich conditions, showing a smaller range of stability for the AO than the BO2 surface. These results indicates that, whilst the AO surface is easier to form, the BO2 surface will display better long term stability, making it more suitable for use in potential applications. This study furthers the understanding of oxide perovskite (001)-terminated surface stability, which will aid in surface growth and manufacturing.
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Works this paper leans on
-
[1]
W. M. Haynes, D. R. Lide, and T. J. Bruno, eds.,CRC Handbook of Chemistry and Physics (CRC Press, 2016)
work page 2016
-
[2]
K. Momma and F. Izumi, VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data, Journal of Applied Crystallography44, 1272 (2011)
work page 2011
-
[3]
E. Heifets, J. Ho, and B. Merinov, Density functional simulation of the BaZrO3 (011) surface structure, Phys. Rev. B Condens.75, 10.1103/PHYSREVB.75.155431 (2007)
-
[4]
C. Freysoldt, J. Neugebauer, and C. G. V. de Walle, FullyAb Initio finite-sze corrections for charged-defect supercell calculations, Phys. Rev. Lett.102, 10.1103/physrevlett.102.016402 (2009)
-
[5]
Z.-J. Suo, J.-W. Luo, S.-S. Li, and L.-W. Wang, Image charge interaction correction in charged- defect calculations, Phys. Rev. B102, 10.1103/physrevb.102.174110 (2020)
-
[6]
S. R. Kavanagh, A. G. Squires, A. Nicolson, I. Mosquera-Lois, A. M. Ganose, B. Zhu, K. Brlec, A. Walsh, and D. O. Scanlon, doped: Python toolkit for robust and repeatable charged defect supercell calculations, Journal of Open Source Software9, 6433 (2024)
work page 2024
-
[7]
D. Broberg, K. Bystrom, S. Srivastava, D. Dahliah, B. A. D. Williamson, L. Weston, D. O. Scanlon, G.-M. Rignanese, S. Dwaraknath, J. Varley, K. A. Persson, M. Asta, and G. Hau- tier, High-throughput calculations of charged point defect properties with semi-local density functional theory—performance benchmarks for materials screening applications, npj Com...
-
[8]
Y. Kumagai and F. Oba, Electrostatics-based finite-size corrections for first-principles point defect calculations, Physical Review B89, 10.1103/physrevb.89.195205 (2014)
Show all 33 references
-
[9]
Freysoldt and J
C. Freysoldt and J. Neugebauer, First-principles calculations for charged defects at surfaces, interfaces, and two-dimensional materials in the presence of electric fields, Physical Review B 97, 10.1103/physrevb.97.205425 (2018)
2018 doi
-
[10]
A. M. Z. Tan, Framework for first-principles calculations of charged dopants and defects in 2d materials, https://github.com/aztan2/charged-defects-framework (2019)
2019
-
[11]
A. Jain, S. P. Ong, G. Hautier, W. Chen, W. D. Richards, S. Dacek, S. Cholia, D. Gunter, D. Skinner, G. Ceder, and K. A. Persson, Commentary: The materials project: A materials genome approach to accelerating materials innovation, APL Mater.1, 011002 (2013). S38
2013
-
[12]
Zaccone, Explaining the thickness-dependent dielectric permittivity of thin films, Physical Review B109, 10.1103/physrevb.109.115435 (2024)
A. Zaccone, Explaining the thickness-dependent dielectric permittivity of thin films, Physical Review B109, 10.1103/physrevb.109.115435 (2024)
2024 doi
-
[13]
R. B. Wexler, G. S. Gautam, E. B. Stechel, and E. A. Carter, Factors governing oxygen vacancy formation in oxide perovskites, J. Am. Chem. Soc.143, 13212 (2021)
2021
-
[14]
Weston, L
L. Weston, L. Bjaalie, K. Krishnaswamy, and C. G. V. de Walle, Origins ofn-type doping difficulties in perovskite stannates, Phys. Rev. B97, 10.1103/physrevb.97.054112 (2018)
2018 doi
-
[15]
C.Freysoldt, J.Neugebauer, A.M.Z.Tan,andR.G.Hennig,Limitationsofempiricalsupercell extrapolation for calculations of point defects in bulk, at surfaces, and in two-dimensional materials, Physical Review B105, 10.1103/physrevb.105.014103 (2022)
2022 doi
-
[16]
S. Kim, S. N. Hood, J.-S. Park, L. D. Whalley, and A. Walsh, Quick-start guide for first- principles modelling of point defects in crystalline materials, Acad. j. phys. energy2, 036001 (2020)
2020
-
[17]
H. J. Monkhorst and J. D. Pack, Special points for Brillouin-zone integrations, Physical Review B 13, 5188 (1976)
1976
-
[18]
Batatia, P
I. Batatia, P. Benner, Y. Chiang, A. M. Elena, D. P. Kovács, J. Riebesell, X. R. Advincula, M. Asta, M. Avaylon, W. J. Baldwin, F. Berger, N. Bernstein, A. Bhowmik, S. M. Blau, V. Cărare, J. P. Darby, S. De, F. Della Pia, V. L. Deringer, R. Elijošius, Z. El-Machachi, F. Falcio...
2024
-
[19]
N. T. Taylor, J. Pitfield, F. H. Davies, and S. P. Hepplestone, RAFFLE: Active learning accelerated interface structure prediction (2025)
2025
-
[20]
B. Deng, P. Zhong, K. Jun, J. Riebesell, K. Han, C. J. Bartel, and G. Ceder, Chgnet as a pretrained universal neural network potential for charge-informed atomistic modelling, Nature Machine Intelligence , 1–11 (2023). S39
2023
-
[21]
J. P. Perdew, K. Burke, and M. Ernzerhof, Generalized Gradient Approximation Made Simple, Phys. Rev. Lett.77, 3865 (1996)
1996
-
[22]
Fischer, Z
G. Fischer, Z. Wang, and S. ichiro Karato, Elasticity of CaTiO 3, SrTiO 3 and BaTiO 3 perovskites up to 3.0 GPa: the effect of crystallographic structure, Phys. Chem. 20, 10.1007/bf00207202 (1993)
1993 doi
-
[23]
Levin, T
I. Levin, T. G. Amos, S. M. Bell, L. Farber, T. A. Vanderah, R. S. Roth, and B. H. Toby, Phase equilibria, crystal structures, and dielectric anomaly in the BaZrO3–CaZrO3 system, J Solid State Chem175, 170 (2003)
2003
-
[24]
L. M. C. Honorio, M. V. B. Santos, E. C. da Silva Filho, J. A. Osajima, A. S. Maia, and I. M. G. dos Santos, Alkaline earth stannates applied in photocatalysis: prospection and review of literature, Cerâmica64, 559 (2018)
2018
-
[25]
C. Yuan, S. Ye, B. Xu, and W. Lei, Strain induced tetragonal SrTiO3 nanoparticles at room temperature, Appl. Phys. Lett.101, 071909 (2012)
2012
-
[26]
Lejaeghere, G
K. Lejaeghere, G. Bihlmayer, T. Björkman, P. Blaha, S. Blügel, V. Blum, D. Caliste, I. E. Castelli, S. J. Clark, A. Dal Corso, S. de Gironcoli, T. Deutsch, J. K. Dewhurst, I. Di Marco, C. Draxl, M. Dułak, O. Eriksson, J. A. Flores-Livas, K. F. Garrity, L. Genovese, P. Gi- anno...
2016
-
[27]
L. Lin, Q. Chen, Q. Zhang, J. He, H. Ni, J. Su, and C.-K. Duan, Trace impurity matters: Origin and modulation of near-infrared luminescence in stannates, Chemistry of Materials36, 10895–10901 (2024)
2024
-
[28]
R. I. Eglitis, Ab initio calculations of SrTiO3, BaTiO3, PbTiO3, CaTiO3 and BaZrO3 (001) and (011) surfaces, Integr. Ferroelectr.108, 11 (2009). S40
2009
-
[29]
R. I. Eglitis,Ab initio calculations of CaZrO3, BaZrO3, PbTiO3 and SrTiO3 (001), (011) and (111) surfaces as well as their (001) interfaces, Integr. Ferroelectr196, 7 (2019)
2019
-
[30]
Slassi, M
A. Slassi, M. Hammi, and O. E. Rhazouani, Surface relaxations, surface energies and electronic structures of BaSnO3 (001) surfaces: Ab Initio calculations, J. Electron. Mater. 46, 4133 (2017)
2017
-
[31]
Zhao and A
X. Zhao and A. Selloni, Structure and stability of NaTiO3(001) and KTaO3(001) surfaces, Phys. Rev. Mater.3, 10.1103/physrevmaterials.3.015801 (2019)
2019 doi
-
[32]
R. I. Eglitis and D. Vanderbilt,Ab initio calculations of BaTiO3 and PbTiO3 (001) and (011) surface terminations, Phys. Rev. B76, 10.1103/physrevb.76.155439 (2007)
2007 doi
-
[33]
Sokolović, G
I. Sokolović, G. Franceschi, Z. Wang, J. Xu, J. Pavelec, M. Riva, M. Schmid, U. Diebold, and M. Setvín, Quest for a pristine unreconstructed SrTiO 3(001) surface: An atomically resolved study via noncontact atomic force microscopy, Phys. Rev. B 103, 10.1103/phys- revb.103.l241...
2021 doi
Reviewed August 16, 2026 · model on record in the stance chip above.
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