Localization of Yttrium Segregation within YSZ Grain Boundary Dislocation Cores

C. Le\'on; G. S\'anchez-Santolino; J. Salafranca; M. Varela; S. J. Pennycook; S. T. Pantelides

REVIEW 2 major objections 6 minor 3 references

Localization of Yttrium Segregation within YSZ Grain Boundary Dislocation Cores

T0 review · 2 major / 6 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read Yttrium segregation at a YSZ grain boundary is not spread across the interface: it concentrates at the single atomic column under the highest expansive strain, roughly doubling the bulk concentration at that site.

desk verdict Site-specific Y segregation at a single YSZ grain-boundary column is a real advance; the load-bearing caveat is on-axis EELS channeling, which needs a control or simulation before the 'doubling' claim is taken as quantitative. read the letter →

arxiv 1908.03257 v1 pith:U322LM36 submitted 2019-08-08 cond-mat.mtrl-sci

G. Sánchez-Santolino , J. Salafranca , S. T. Pantelides , S. J. Pennycook , C. León , M. Varela This is my paper · ORCID

classification cond-mat.mtrl-sci

keywords yttriastabilizedzirconiagrainboundarysegregationyttriumdislocationcorestrainmappingelectronenergylossspectroscopyscanningtransmissionmicroscopydensityfunctionaltheory

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 aims to show where, at atomic scale, yttrium dopants sit when they segregate to a grain boundary in yttria-stabilized zirconia (YSZ), and why they sit there. Using atomic-resolution electron microscopy, the authors measure both the local strain and the chemical composition of the same atomic columns in the boundary's dislocation cores. They find that yttrium does not enrich the whole boundary uniformly: its concentration roughly doubles only at the single atomic column under the highest expansive strain, while neighboring expansive and compressive columns stay near bulk composition. Density functional theory supports this site-specific picture, identifying yttrium substitution there as the strain-relief mechanism and as a way to localize oxygen vacancies. If correct, this means grain-boundary chemistry and ionic transport in YSZ should be understood column by column, not as an averaged boundary layer.

What carries the argument

The load-bearing machinery is the pairing of an atomic-column-resolved strain map with an atomic-column-resolved composition map on the same dislocation cores. Peak-pairs analysis of a high-angle annular dark-field image produces a mean-dilatation map ($d_{xy}=\varepsilon_{xx}+\varepsilon_{yy}$) that sorts the core columns into expansive sites E1, E2, E3 and compressive site C1, with E3 carrying the largest expansion (+23.8%). EELS quantification of the Y and Zr L2,3 edges at those same columns supplies the composition per site. Density functional theory closes the argument by giving the mechanism: a Y atom substituting Zr at E3 has a segregation energy of 2.9 eV and lowers the formation energy of oxygen vacancies, so the most expansive column becomes both the Y-rich and vacancy-rich site, and the strain is released there.

What would settle it

Acquire an atomic-resolution tilt series of the same 33° [001] YSZ bicrystal and reconstruct the three-dimensional strain tensor; if the column with the largest true volumetric dilatation is not the column where Y content doubles, the strain–segregation correlation fails. A channeling-corrected EELS simulation of the same boundary could also settle the matter: if it shows the apparent Y enrichment at E3 is an artifact of electron-beam propagation, the site assignment is refuted.

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Extended reading notes

Core claim

On a 33° [001] tilt grain boundary in a 9 mol% yttria YSZ bicrystal, the boundary's dislocation cores contain three expansive atomic columns (mean dilatations +15.3%, +10.7%, +23.8%) and one compressive column (−2.7%). Site-by-site EELS quantification shows that the yttrium concentration at the most expansive column, E3, is roughly double the bulk Y content, while the other expansive columns and the compressive column stay near bulk values within error. Density functional theory calculations agree: Y substitutes Zr preferentially at E3 with a segregation energy of 2.9 eV, even though that site has the larger coordination number, and the substitution lowers the oxygen-vacancy formation energy there. The conclusion is that Y segregation at this boundary is not a uniform boundary enrichment but is localized to one atomic column, and that this localization is the strain-relaxation mechanism that also confines oxygen vacancies to the dislocation core.

Load-bearing premise

The ranking of E3 as the most expansive site comes from a two-dimensional strain map of a three-dimensional structure, and the site-to-site Y trend rests on on-axis EELS quantification despite channeling; if either gives way, the claimed correlation may not hold.

Editorial extensions

If this is right

The grain-boundary dopant excess in YSZ is concentrated at one atomic column per dislocation core, so boundary chemistry is site-specific rather than uniform.
Yttrium segregation to E3 explains how strain is relieved at the core and why oxygen vacancies are confined there, linking atomic structure to ionic-conductivity blocking.
Dopants with larger ionic radius than Zr should be expected to populate the most expansive boundary columns, giving a predictive rule for other stabilizing oxides.
The near-bulk composition at E1, E2, and C1 means that only the maximum-expansion site matters for the dominant segregation, simplifying models of boundary charge and transport.
The DFT segregation energy of 2.9 eV at E3 provides a quantitative benchmark for atomistic simulations of YSZ grain boundaries.

Reading between the lines

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

Editorial inference: the same strain–composition correlation should be testable in other fluorite and perovskite ion conductors; if it holds, the site-selection rule for oversized dopants is simply to occupy the column with the maximum expansive dilatation.
Editorial inference: a three-dimensional strain reconstruction, such as atomic-resolution electron tomography on the same boundary, would decide whether E3 remains the volume-maximizing column when out-of-plane distortions are included; if it does, the strain argument becomes quantitative rather than projected.
Editorial inference: because the Y-rich column also anchors oxygen vacancies, locally manipulating strain through epitaxy, pressure, or boundary geometry might shift the segregation column and thereby tune the grain-boundary ionic resistance.
Editorial inference: the reported site-resolved doubling suggests that continuum models using a single boundary-excess value lose information; a discrete-site segregation-energy landscape may be needed to predict transport blocking.

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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 new result here is site-specific: the same group had shown Y segregation to YSZ GB dislocation cores, but this paper identifies which atomic column takes it up — the E3 site under the largest expansive strain, where Y content reads double bulk. That is a genuine advance, and the experimental backbone is decent. The strain and EELS maps are independently measured, the composition averages come from four cores with error bars, and the DFT from their earlier paper is used as corroboration, not fitted.

I read the stress-test note as landing on the right spot. The paper is honest about its limitations, but the channeling worry is not minor. The text says the on-axis values may be 'slightly affected' and that the overall trends should be robust. Yet the E3 'doubling' is far beyond the few-percent offset they saw in their own earlier on/off-axis comparison, and that comparison was on average compositions, not on a strained dislocation core. In the core, local strain and partial column occupancy can change how the 200 keV probe propagates and how the Y/Zr L2,3 ratio is generated. A multislice EELS simulation with the DFT core model would settle it. Without that, the central quantitative claim carries a real alternative explanation.

I do not share the reader's circularity concern. Quoting one's own prior DFT segregation energy is self-citation, not a fitted circular step; the measurement does not depend on it.

Secondary cautions: one bicrystal, one boundary orientation, and a 2D strain measurement. The last point is explicitly stated in the paper, and if out-of-plane strain changes the ranking, the correlation with 'highest expansive strain' weakens but the basic segregation observation would survive. The conclusion calling this 'the main mechanism' of strain relaxation goes beyond the data; that phrasing should be tempered.

This paper deserves a serious referee. The site-specific experimental observation is likely to hold up, but the quantitative E3 value needs either an off-axis control or a channeling simulation, plus bulk error bands shown on the same graph. For a reading group on atomic-scale interfacial characterization, it is worth the time. I would cite it in a review of grain-boundary segregation in fluorite oxides.

Referee Report

2 major / 6 minor

Summary. This manuscript reports an atomic-resolution scanning transmission electron microscopy study of a 33° [001] tilt grain boundary in a 9 mol% Y2O3-doped ZrO2 bicrystal. The authors use peak pairs analysis of HAADF images to map the local mean dilatation and identify three expansive sites (E1, E2, E3) and one compressive site (C1) per dislocation core. Electron energy-loss spectrum imaging with pixel-by-pixel quantification shows that the yttrium relative concentration at the E3 site is approximately twice the bulk value, while the other core sites remain near bulk composition. Density-functional-theory calculations from the authors' earlier work (ref 16) are cited to support the assignment of E3 as the preferential Y segregation site. The paper concludes that Y segregates to the site of highest expansive strain, which is relevant to oxygen-vacancy localization and grain-boundary ionic transport in YSZ.

Significance. If the central claim holds, the paper provides a direct atomic-scale correlation between local strain and dopant segregation at a specific dislocation core, which is valuable for understanding space-charge and strain effects at YSZ grain boundaries. The experimental strengths are the site-resolved composition analysis with error bars over four dislocation cores, the simultaneous strain and composition mapping, and the explicit use of an independent DFT prediction as an interpretive anchor. The main limitations are the reliance on on-axis EELS quantification in a locally distorted region and the two-dimensional projection of the strain field; both are acknowledged in the text but are not controlled for quantitatively.

major comments (2)

[§3.3, ¶3; §3.4; Fig. 4(c)] The central quantitative claim that the Y content at E3 doubles the bulk value is based on EELS quantification performed on-axis. The manuscript states that on-axis conditions are necessary to compare specific sites and admits that 'quantitative values can be slightly affected by the not optimal conditions but the overall trends found should be robust.' However, the reported effect is a doubling (5.7% to about 11%), far beyond the few-percent offset seen in the authors' previous on/off-axis comparison (ref 16), and that comparison was for average compositions, not for a single strongly distorted dislocation core. At the core, partial column occupancy and local strain can alter channeling of the 200 keV probe and the Y/Zr L-edge ionization cross-sections, so the pixel-by-pixel relative concentrations may not reflect true compositional changes. No multislice EELS simulation using the DFT core structure is provided to rule out a channeling artifact. Given that the identification of E3 as the segregation site is the main result, this is a load-bearing gap. Please provide a multislice simulation or an equivalent control, or explicitly restrict the conclusion to a qualitative trend.
[§3.2; Fig. 2(d) and Fig. 4(c)] The strain values used to rank the expansive sites are derived from a two-dimensional projected HAADF image. The manuscript acknowledges that 'the current results reflect a two-dimensional measurement of a three-dimensional structure and the possible strain distortions along the beam direction are not considered.' The correlation between Y segregation and the 'position under higher expansive strain' therefore depends on the 2D ranking of E3. If out-of-plane strain were to reorder the expansive sequence, the claimed correlation would be weakened. Since the DFT calculations in ref 16 provide a 3D structural model, a comparison between the projected 2D strain and the 3D computed strain field at the same sites would substantiate the ranking. In the absence of such a comparison, the strain-segregation correlation is only as robust as the 2D approximation. Please add this comparison or strengthen the caveat accordingly.

minor comments (6)

[Introduction (p. 2)] The word 'demining' should be 'undermining' in the sentence 'ionic blocking processes at grain boundaries constitute one of the main obstacles demining their performance.'
[Author line] The title 'Porf. S. J. Pennycook' should be 'Prof. S. J. Pennycook.'
[§3.2] The phrase 'relative miss-orientation' should be 'relative misorientation.'
[§3.4] The phrase 'the higher expansive strain' would be more precise as 'the highest expansive strain,' since the text refers to the maximum among the three expansive sites.
[Figure 2] The text in §3.2 describes the expansive and compressive sites as marked with 'white and black dots,' while the Figure 2 caption refers to 'red and blue arrows'; please reconcile the inconsistency.
[Figure 4(c)] The caption should state explicitly that the error bars are the standard deviation over the four dislocation cores in the spectrum image region, as mentioned in the text.

Circularity Check

0 steps flagged · score 2.0 of 10

No circular derivation found; the central measurements are independent of the prior DFT calculation, which is cited transparently as context rather than as an input.

full rationale

The paper's derivation chain is experimental and self-contained at the level of its claims. The strain values (E1=+15.3%, E2=+10.7%, E3=+23.8%, C1=-2.7%) are obtained from PPA analysis of a HAADF image, and the Y, Zr, and O concentrations at each site are quantified pixel-by-pixel from EEL spectrum images. Neither of these quantities is fitted to, or defined in terms of, the DFT result. The DFT segregation energy and the identification of E3 as the preferred substitutional site are taken from the authors' earlier work (ref. 16), but the paper explicitly states that 'Such results were already reported in our previous publication, and here they are only used to discuss new experimental evidence in the context of those previous calculations.' This is a transparent, non-load-bearing self-citation: the experimental observation that Y content at E3 doubles the bulk value while E1, E2, and C1 remain near bulk does not logically depend on the prior calculation. There is no equation in which a predicted quantity is algebraically identical to an input, no fitted parameter renamed as a prediction, and no uniqueness theorem imported to forbid alternatives. The caveats about two-dimensional strain measurement and on-axis EELS channeling are experimental validity risks, not circularity: they question whether the measured signal is an accurate proxy for segregation, but they do not make the argument reduce to its own assumptions. Accordingly, the appropriate finding is low circularity, with a minor self-citation that is not load-bearing.

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

The paper introduces no free parameters or invented entities; all measured values are experimental. It relies on standard electron microscopy assumptions (HAADF contrast, PPA strain algorithm, EELS quantization), the literature rule that larger ions occupy expansive sites, and on prior DFT results from the authors' own publication (ref 16) that Y substitutes Zr at the E3 site with a segregation energy of 2.9 eV.

assumptions (4)

domain assumption The PPA algorithm reliably measures local strain from HAADF images.
Invoked in Section 3.2 as the basis for identifying expansive and compressive sites; the paper cites ref 17 for validation but provides no in-paper error analysis of the strain extraction.
domain assumption The measured 2D strain projection ranks the true 3D strain at each atomic column.
The paper explicitly states in Section 3.2 that the results reflect a two-dimensional measurement and that out-of-plane distortions are not considered; the ranking of E3 as most expansive depends on this.
domain assumption Yttrium segregation to the E3 site is confirmed by the prior DFT calculation of ref 16.
Section 3.4 quotes a 2.9 eV segregation energy and the E3 substitution result from the authors' own previous publication; that calculation is not reproduced or independently verified in this manuscript.
domain assumption EELS quantification on-axis preserves relative Y/Zr/O trends between atomic sites.
Section 3.3 concedes quantitative values can be affected by non-optimal channeling conditions but asserts trends should be robust; the central claim depends on the relative Y enhancement at E3 being real.

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Cite this review

Pith. "Pith review of Localization of Yttrium Segregation within YSZ Grain Boundary Dislocation Cores." pith.science (2026). https://pith.science/paper/U322LM36

@misc{pith2026190803257,
  author       = {Pith},
  title        = {Pith review of: Localization of Yttrium Segregation within YSZ Grain Boundary Dislocation Cores},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/U322LM36}},
  note         = {Machine review of arXiv:1908.03257}
}

read the original abstract

Ionic conductivity blocking at grain boundaries in polycrystalline electrolytes is one of the main obstacles that need to be overcome in order to improve the performance of solid state fuel cells and batteries. To this aim, harnessing the physical properties of grain boundaries in ionic conducting materials such as yttria stabilized zirconia (YSZ) down to the atomic scale arises as a greatly important task. Here we present a structural and compositional analysis of a single grain boundary in a 9 mol% yttria content YSZ bicrystal by means of aberration corrected scanning transmission electron microscopy. Our studies combine strain and compositional atomic resolution analysis with density-functional-theory calculations in order to find a preferential segregation of yttrium to the expansive atomic sites at the grain boundary dislocation cores. These results address a crucial step towards the understanding of the physical properties of grain boundaries down to atomic dimensions.

Figures

Figures reproduced from arXiv: 1908.03257 by the authors.

**Figure 1.** (a) High angle annular dark field image of a YSZ bicrystal with a 33° symmetric tilt [001] grain boundary. (b) Fast Fourier transform of the image in (a) where two symmetric reflections are marked with white arrows [PITH_FULL_IMAGE:figures/full_fig_p014_1.png] view at source ↗

**Figure 2.** (a) HAADF image of the grain boundary region. The two symmetric directions used as the principal vectors for the PPA analysis are marked with yellow arrows on the image. (b) Mean dilatation (dxy) map of the grain boundary area in %100 obtained from PPA[17] analysis. (c) dxy shown as an overlay to the HAADF image in the same color scale, where the expansive and compressive sites of the grain boundary dislocation core… view at source ↗

**Figure 4.** (a) ADF image acquired simultaneously with the EEL spectrum image in [PITH_FULL_IMAGE:figures/full_fig_p017_4.png] view at source ↗

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Works this paper leans on

3 extracted references · 3 canonical work pages

[3]

Results and Discussion 3.1. YSZ bicrystal grain boundary characterization The orientation relationship between the two single crystals and the structural quality of the sample was determined from atomic resolution STEM images. Figure 1 shows a high angle annular dark field (HAADF) image of the grain boundary region where a regular array of evenly distribu...

work page
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Varela, S.D

M. Varela, S.D. Findlay, A.R. Lupini, H.M. Christen, A.Y. Borisevich, N. Dellby, O.L. Krivanek, P.D. Nellist, M.P. Oxley, L.J. Allen, S.J. Pennycook. Phys. Rev. Lett. 2004, 92, 95502. 14. O.L. Krivanek, M.F. Chisholm, V. Nicolosi, T.J. Pennycook, G.J. Corbin, N. Dellby, M.F. Murfitt, C.S. Own, Z.S. Szilagyi, M.P. Oxley, S.T. Pantelides, S.J. Pennycook. Na...

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Van Aert, K

S. Van Aert, K. J. Batenburg, M. D. Rossell, R. Erni, G. Van Tendeloo. Nature 2011, 470, 374. 27. C.-C. Chen, C. Zhu, E. R. White, C.-Y. Chiu, M. C. Scott, B. C. Regan, L. D. Marks, Y. Huang, J. Miao. Nature 2013, 496, 74. 28. F.-R. Chen, D. Van Dyck, C. Kisielowski.Nat. Commun. 2016, 7, 10603. 29. R. Ishikawa, A. R. Lupini, Y. Hinuma, S. J. Pennycook. Ul...

work page 2011

Pith tools

Reviewed August 14, 2026 · model on record in the stance chip above.

[1] [3]

Results and Discussion 3.1. YSZ bicrystal grain boundary characterization The orientation relationship between the two single crystals and the structural quality of the sample was determined from atomic resolution STEM images. Figure 1 shows a high angle annular dark field (HAADF) image of the grain boundary region where a regular array of evenly distribu...

work page

[2] [13]

Varela, S.D

M. Varela, S.D. Findlay, A.R. Lupini, H.M. Christen, A.Y. Borisevich, N. Dellby, O.L. Krivanek, P.D. Nellist, M.P. Oxley, L.J. Allen, S.J. Pennycook. Phys. Rev. Lett. 2004, 92, 95502. 14. O.L. Krivanek, M.F. Chisholm, V. Nicolosi, T.J. Pennycook, G.J. Corbin, N. Dellby, M.F. Murfitt, C.S. Own, Z.S. Szilagyi, M.P. Oxley, S.T. Pantelides, S.J. Pennycook. Na...

work page 2004

[3] [26]

Van Aert, K

S. Van Aert, K. J. Batenburg, M. D. Rossell, R. Erni, G. Van Tendeloo. Nature 2011, 470, 374. 27. C.-C. Chen, C. Zhu, E. R. White, C.-Y. Chiu, M. C. Scott, B. C. Regan, L. D. Marks, Y. Huang, J. Miao. Nature 2013, 496, 74. 28. F.-R. Chen, D. Van Dyck, C. Kisielowski.Nat. Commun. 2016, 7, 10603. 29. R. Ishikawa, A. R. Lupini, Y. Hinuma, S. J. Pennycook. Ul...

work page 2011