REVIEW 4 major objections 5 minor 8 references
Mixed anion control of enhanced negative thermal expansion in the oxysulfide of PbTiO3
T0 review · 4 major / 5 minor · reviewed 2026-08-10 · deepseek-v4-flash
Pith's one-line read Substituting a small amount of sulfur for oxygen in PbTiO3 deepens its negative thermal expansion, pushing the average volumetric contraction to $-2.50 \times 10^{-5}$/K over 300–790 K.
desk verdict A credible but quantitatively under-supported claim of anion-tuned NTE in PbTiO3; the experimental trend is plausible, but the headline comparison needs common temperature windows and error bars. 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 central mechanism is the spontaneous volume ferroelectrostriction ($\omega_S$), which quantifies how the ferroelectric order contributes to the anomalous volume contraction; the paper also uses DFT charge-density maps and electron localization functions to show that sulfur doping enhances Pb/Ti–O/S hybridization. The $\omega_S$ values extracted from the temperature-dependent volume data are 3.36% ($x = 0.01$) and 3.64% ($x = 0.02$), both larger than pristine PT's 3.1%, tying the enhanced NTE to a stronger ferroelectrovolume effect.
What would settle it
Measure $c/a$ and volumetric CTE on PbTiO3-xSx samples with sulfur content verified independently (e.g., by ion chromatography or atom-probe tomography); if the enhancement over pristine PbTiO3 does not track sulfur content, the claim fails—or, at the theory level, repeat the DFT in a supercell large enough to represent $x = 0.02$ and check whether the hybridization enhancement persists.
Extended reading notes
Core claim
On its own terms: substituting S$^{2-}$ for O$^{2-}$ at the anion site of PbTiO3 further increases the already large $c/a$ ratio (from 1.064 to ~1.065 for $x = 0.01$ and 0.02) and pushes the average volumetric CTE from $-2.11 \times 10^{-5}$/K (pristine PT, RT–763 K) to $-2.43 \times 10^{-5}$/K ($x = 0.01$) and $-2.50 \times 10^{-5}$/K ($x = 0.02$), extending the NTE window to 790 K. The enhanced NTE is attributed to enhanced hybridization between Pb/Ti and the O/S anions, particularly along the polar [001] axis, as supported by DFT charge-density and electron-localization-function calculations on a $2 \times 2 \times 2$ supercell. The paper interprets this as evidence that anion substitution, not just A/B-site cation substitution, can control the ferroelectric-volume effect that drives NTE in PbTiO3-based materials.
Load-bearing premise
The calculations that explain the mechanism put one or two sulfur atoms in a small 40-atom cell, which is roughly 12 times more sulfur than the real samples contain; if that exaggerated concentration is what actually changes the bonding, the explanation may not apply to the real material.
Editorial extensions
If this is right
- Small amounts of sulfur ($\le 2$%) can push PbTiO3's NTE beyond that of many cation-doped variants, with performance comparable to framework NTE materials like ZrW2O8.
- The NTE operation window widens from 763 K to 790 K, which matters for thermal-expansion compensation at higher temperatures.
- Because the enhanced NTE is tied to increased tetragonality, anion substitution provides a complementary route to the well-known $c/a$–CTE design rule for PT-based ferroelectrics.
- Other mixed-anion PbTiO3 compounds (e.g., oxynitrides, oxyfluorides) can plausibly be screened using the same high-pressure synthesis and the same $c/a$–$\omega_S$ logic.
Reading between the lines
- If the hybridization mechanism is robust, even larger NTE may be achievable with more polarizable anions (Se$^{2-}$, Te$^{2-}$) or by placing sulfur on both anion sites, though solubility limits and synthesis pressure will constrain this.
- The DFT supercell overestimates sulfur concentration by ~12-fold (one S in a 40-atom cell is $x = 0.125$ vs. the nominal 0.01); at the true dilute limit the enhanced hybridization might be weaker or qualitatively different, so the mechanism should be re-examined with larger supercells or embedded-cluster methods.
- The paper's own CTE for pristine PT ($-2.11 \times 10^{-5}$/K) differs from the literature value ($-1.99 \times 10^{-5}$/K); the claimed enhancement (from $-2.11$ to $-2.50$) is ~18%, so a careful interlaboratory comparison of the same PTOS2 sample would help confirm the effect size.
- The same anion-substitution strategy could be tested on other ferroelectric NTE perovskites (e.g., BiFeO3-PbTiO3 solid solutions) to see whether the $c/a$–NTE enhancement generalizes beyond PbTiO3.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper reports the synthesis of oxysulfide PbTiO3-xSx (x = 0.01 and 0.02) by high-pressure high-temperature methods, with synchrotron XRD, Raman, XANES, and DFT characterization. The central claim is that sulfur substitution increases the tetragonality (c/a) and enhances negative thermal expansion, giving an average volumetric CTE of -2.50 x 10^-5/K from 300 to 790 K for x = 0.02, compared with -2.11 x 10^-5/K for pristine PbTiO3 measured from 300 to 763 K. The enhanced NTE is attributed, on the basis of DFT charge density and electron localization function calculations, to enhanced Pb/Ti-O/S hybridization in sulfur-doped supercells.
Significance. If confirmed, the work introduces anion-site substitution as a new tuning knob for negative thermal expansion in PbTiO3-based ferroelectrics, complementing the extensive prior work on A- and B-site cation substitution. The experimental observation is based on direct diffraction measurements, and the paper provides an independent DFT mechanism study, which are strengths. However, the headline quantitative claim rests on a comparison of average CTE values that (i) cover different temperature windows and (ii) are reported without uncertainties. The DFT mechanism additionally uses sulfur concentrations about 12.5 times larger than the experimental ones, so the mechanistic attribution is qualitative rather than quantitative. The paper is potentially publishable after the experimental comparison is placed on a statistically sound common-window footing and the mechanism claim is appropriately framed.
major comments (4)
- [Results and discussion, Fig. 3] The central claim of enhanced NTE rests on comparing average volumetric CTEs over different temperature ranges: pristine PT is averaged from RT to 763 K, while PTOS1 and PTOS2 are averaged from RT to 790 K. Since the V(T) curves are explicitly nonlinear and the volume contraction in PbTiO3-type ferroelectrics accelerates near TC, extending the averaging window to 790 K for PTOS only adds the steepest part of the contraction for those samples. A common-window comparison (e.g., 300-763 K for all three compositions) is required, along with pointwise CTE curves, before the title claim can be considered established.
- [Results and discussion, Fig. 3 and Experimental Section] No numerical uncertainties are reported for the average volumetric CTE values. The statement that error bars are smaller than symbols in Fig. 3 does not quantify the uncertainty in the integrated CTE, which involves propagation of Rietveld parameter correlations across many temperatures. Please report standard deviations or confidence intervals for the average CTEs and for the c/a values, and state whether the PTOS1/PTOS2 differences from PT are statistically significant.
- [Abstract vs. Results and discussion] The abstract reports an average volumetric CTE of -1.99 x 10^-5/K for pristine PbTiO3, while the main text reports -2.11 x 10^-5/K for the same quantity measured in this study. This discrepancy changes the claimed enhancement from about 26% to about 18%. Please reconcile the two numbers and clearly identify which baseline is used for the comparison with PTOS2.
- [Electronic Structure Calculation, Fig. 4] The DFT mechanism uses one or two sulfur substitutions in a 40-atom 2x2x2 supercell, corresponding to x = 0.125 and 0.25, roughly 12.5 times larger than the experimental x = 0.01 and 0.02. The paper acknowledges this approximation, but the mechanistic conclusion that the observed NTE enhancement is caused by enhanced Pb/Ti-O/S hybridization relies on this concentration mapping. Please either explicitly reframe the DFT result as a qualitative trend that may not be quantitatively transferable to the measured compositions, or provide additional evidence (e.g., concentration-dependent hybridization indicators such as Bader charges or crystal orbital Hamilton populations) to support the extrapolation.
minor comments (5)
- [Section 4, Electronic Structure Calculation paragraph] In the sentence 'the hybridization between the cations and anions is crucial to the NET in PT-based ferroelectrics', 'NET' should be 'NTE'.
- [Experimental Section] Reference 50 (Shirane et al., Acta Crystallogr. 1956) is cited for the FullProf software, but that reference is not the FullProf manual or a FullProf-related paper. Please correct the citation.
- [Experimental Section] The diffractometer manufacturer is misspelled as 'Brucker'; it should be 'Bruker'.
- [Figure 2c and Supplementary Table 1] Please provide actual numeric uncertainties for the lattice parameters and c/a ratio, rather than only the statement that error bars are smaller than symbols.
- [Abstract and Conclusion] The abstract and conclusion state that the enhanced NTE is attributed to enhanced hybridization between 'Pb and O/S', while the main text refers to 'Pb/Ti and O/S'. Please make the wording consistent.
Circularity Check
No significant circularity: the central NTE enhancement is a direct diffraction measurement; DFT and SVFS are explanatory or descriptive, not fitted predictions.
full rationale
The paper's headline result—enhanced negative thermal expansion in S-substituted PbTiO3—is an experimental measurement, not a model output. The average volumetric CTE values for PTOS1 and PTOS2 (-2.43 and -2.50 x 10^-5/K) and pristine PT (-2.11 x 10^-5/K in the body text, -1.99 x 10^-5/K in the abstract) are obtained by Rietveld refinement of temperature-dependent synchrotron X-ray diffraction data; no parameter fitted to the volume data is renamed as a prediction. The DFT charge-density and electron-localization-function analysis uses experimental lattice parameters with relaxed internal coordinates to rationalize enhanced Pb/Ti-O/S hybridization, but it does not compute or predict the CTE, so the mechanism discussion is not used to force the measured outcome. The SVFS values (3.36% and 3.64%) are defined from the same measured Vexp against an extrapolated nominal volume and are descriptive restatements of the volume data rather than independent predictions; this is a presentational redundancy, not a circular derivation of the central claim. Cited prior work (e.g., refs. 3, 40, 44, and 46) supplies interpretive correlations and the SVFS baseline, but the central comparison between PT and PTOS is measured in this study, and those citations are not used to generate the reported CTE values. The acknowledged DFT concentration mismatch (x = 0.125 and 0.25 in the supercell versus x = 0.01 and 0.02 in the samples) is a modeling limitation, not circularity. The differing averaging windows (300-763 K versus 300-790 K) and the absence of reported uncertainties are statistical and comparison-robustness concerns, not circularity. No load-bearing step in the derivation chain reduces by construction to its own input.
Assumptions & free parameters
assumptions (4)
- ad hoc to paper The 40-atom supercell with one or two sulfur substitutions faithfully represents PbTiO3-xSx with x = 0.01 and x = 0.02.
- domain assumption The enhanced tetragonality-to-NTE correlation established in cation-substituted PbTiO3 transfers to anion-substituted PbTiO3.
- domain assumption PBE DFT charge-density and electron localization function maps reliably indicate the hybridization changes responsible for NTE.
- domain assumption The A1(1TO) Raman mode frequency tracks the ferroelectric order parameter and tetragonality in these compounds.
Cite this review
Pith. "Pith review of Mixed anion control of enhanced negative thermal expansion in the oxysulfide of PbTiO3." pith.science (2026). https://pith.science/paper/GXJQT3VX
@misc{pith2026250109701,
author = {Pith},
title = {Pith review of: Mixed anion control of enhanced negative thermal expansion in the oxysulfide of PbTiO3},
year = {2026},
howpublished = {\url{https://pith.science/paper/GXJQT3VX}},
note = {Machine review of arXiv:2501.09701}
}
abstract
The rare physical property of negative thermal expansion (NTE) is intriguing because materials with large NTE over a wide temperature range can serve as high-performance thermal expansion compensators. However, applications of NTE are hindered by the fact that most of the available NTE materials show small magnitudes of NTE, and/or NTE occurs only in a narrow temperature range. Herein, for the first time, we investigated the effect of anion substitution instead of general Pb/Ti-site substitutions on the thermal expansion properties of a typical ferroelectric NTE material, PbTiO3. Intriguingly, the substitution of S for O in PbTiO3 further increases the tetragonality of PbTiO3. Consequently, an unusually enhanced NTE with an average volumetric coefficient of thermal expansion $\bar{\alpha}_V$ = -2.50 $\times$ 10$^{-5}$/K was achieved over a wide temperature range (300 -- 790 K), which is contrasted to that of pristine PbTiO3 ($\bar{\alpha}_V$ = -1.99 $\times$ 10$^{-5}$/K RT -- 763 K). The intensified NTE is attributed to the enhanced hybridization between Pb/Ti and O/S atoms by the substitution of S, as evidenced by our theoretical investigations. We therefore demonstrate a new technique for introducing mixed anions to achieve large NTE over a wide temperature range in PbTiO3-based ferroelectrics.
Reference graph
Works this paper leans on
-
[1]
1 K. Takenaka, Sci. Technol. Adv. Mater., 2012, 13, 013001. 2 P. J. Attfield, Front. Chem., 2018, 6,
work page 2012
-
[347]
11 I. Yamada, K. Tsuchida, K. Ohgushi, N. Hayashi, J. Kim, N. Tsuji, R. Takahashi, M. Matsushita, N. Nishiyama, T. Inoue, T. Irifune, K. Kato, M. Takata and M. Takano, Angew. Chem., 2011, 50, 6579-6582. 12 H. Yamamoto, T. Imai, Y . Sakai and M. Azuma, Angew. Chem. Int. Ed., 2018, 57, 8170-8173. 13 M. Sato, V . Warne-Lang, Y . Kadowaki, N. Katayama, Y . Ok...
work page 2011
-
[371]
3 J. Chen, L. Hu, J. X. Deng and X. R. Xing, Chem. Soc. Rev., 2015, 44, 3522-3567. 4 C. Linda, Materials, 2012, 5, 1125-1154. 5 Q. Li, K. Lin, Z. Liu, L. Hu, Y . Cao, J. Chen and X. R. Xing, Chem. Rev., 2022, 122, 8438-
work page 2015
-
[2458]
47 R. E. Cohen, Nature, 1992, 358, 136-138. 48 Y . Kuroiwa, S. Aoyagi, A. Sawada, J. Harada, E. Nishibori, M. Takata and M. Sakata. Phys. Rev. Lett., 2001, 87, 217601. 49 S. Kawaguchi, M. Takemoto, K. Osaka, E. Nishibori, C. Moriyoshi, Y . Kubota, Y . Kuroiwa and K. Sugimoto, Rev. Sci. Instrum., 2017, 88, 085111. 50 G. Shirane, R. Pepinsky and B. C. Fraze...
work page 1992
-
[4690]
23 C. Wang, L. Chu, Q. Yao, Y . Sun, M. Wu, L. Ding, J. Yan, Y . Na, W. Tang, G. Li, Q. Huang and J. W. Lynn, Phys. Rev. B, 2012, 85, 220103(R). 24 B. K. Greve, K. L. Martin, P . L. Lee, P. J. Chupas, K. W. Chapman and A. Wilkinson, J. Am. Chem. Soc., 2010, 132, 15496-15498. 25 L. Hu, J. Chen, L. Fan, Y . Ren, Y . Rong, Z. Pan, J. Deng, R. Yu and X. R. Xi...
work page 2012
-
[5593]
9 Y . W. Long, N. Hayashi, T. Saito, M. Azuma, S. Muranaka and Y . Shimakawa, Nature, 2009, 458, 60-63. 10 M. Azuma, W. T. Chen, H. Seki, M. Czapski, S. Olga, K. Oka, M. Mizumaki, T. Watanuki, N. Ishimatsu, N. Kawamura, S. Ishiwata, M. G. Trucker, Y . Shimakawa and P . J. Attfield, Nat. Commun., 2011, 2,
work page 2009
-
[7674]
35 S. C. Abrahams, S. K. Kurtz and P. B. Jamieson, Phys. Rev., 1968, 172, 551-553. 36 J. Chen, X. Xing, R. Yu and G. Liu, J. Am. Ceram. Soc., 2005, 88, 1356-1358. 37 J. Chen, X. Xing, R. Yu and G. Liu, Appl. Phys. Lett., 2005, 87, 231915. 38 T. Yang, Y . Wang, L. Fan, N. Wang, K. Lin, J. Chen and X. R. Xing, J. Phys. Chem. C, 2020, 124, 20445-20449. 39 J....
work page 1968
-
[8486]
6 T. A. Mary, J. S. O. Evans, T. V ogt and A. W. Sleight, Science, 1996, 272, 90-92. 7 X. R. Xing, J. Deng, J. Chen and G. Liu, Rare Metal, 2003, 22, 294-297. 8 X. X. Jiang, M. S. Molokeev, L. Dong, Z. Dong, N. Wang, L. Kang, X. Li, Y . Li, C. Tian, S. Peng, W. Li and Z. Lin, Nat. Commun., 2020, 11,
work page 1996
Reviewed August 10, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.