{"id":"7d01b1dd-1c4a-48a2-979d-fb65dc46dde0","arxiv_id":"2501.09701","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":0,"one_line_summary":"Substituting small amounts of sulfur for oxygen in PbTiO3 slightly increases its tetragonality and strengthens its negative thermal expansion over a wider temperature range.","lead":"Adding a small amount of sulfur to the oxygen sites of the ferroelectric ceramic PbTiO3 makes it shrink more upon heating. This is the first demonstration that anion substitution, rather than cation substitution, can tune negative thermal expansion in this material family.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Enhanced-NTE claim may not survive a common-temperature-window comparison: PT and PTOS averages cover different ranges (763 vs 790 K) with no reported CTE uncertainties.","rationale":"Good-faith reading: this is a synthesis plus SXRD plus DFT study. The HPHT synthesis at 8 GPa and 1200 C is plausible, and the S K-edge XANES and TEM mapping support S incorporation. The small c/a increase is corroborated by Raman A1(1TO) hardening. The most important assertion for the paper's value is that sulfur substitution produces a materially stronger NTE. That assertion is quantified only by range-averaged CTE values. With no error bars and with different integration intervals (PT: 300-763 K; PTOS: 300-790 K), the 18% difference is not yet established. This is more load-bearing than the DFT supercell concentration issue, because a flawed mechanism interpretation would weaken only the explanation, whereas a flawed CTE comparison would remove the phenomenon itself. The Reader flagged the DFT concentration mapping as the weakest assumption and mentioned missing error bars in the rationale; I agree with the latter as the decisive issue, hence partial agreement. A common-window analysis with propagated uncertainties is a straightforward and decisive check. Until that is supplied, the Reader's CONDITIONAL verdict remains the correct assessment, so no verdict change is needed.","tokens_in":10460,"tokens_out":5088,"duration_ms":58546,"concrete_test":"Recompute the average volumetric CTE for PT, PTOS1, and PTOS2 over the identical range 300-763 K from the refined SXRD volumes, using standard error propagation or bootstrap from Rietveld residuals, and also report pointwise alpha_V(T). If the PTOS2 enhancement over PT is less than 3 sigma (or absent) in the common window, the headline claim is unsupported; if it persists with significance, this concern is resolved.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central experimental claim reduces to three average volumetric CTE values: -2.11 x 10^-5/K for PT over 300-763 K and -2.43 and -2.50 x 10^-5/K for PTOS1 and PTOS2 over 300-790 K. Two features make this comparison fragile. First, no uncertainty is reported for any of these averages; the statement that error bars are smaller than symbols in Fig. 3 does not guarantee that the integrated CTE difference of about 0.4 x 10^-5/K (about 18%) is statistically significant once Rietveld correlations are propagated. Second, the temperature windows differ because TC increases from about 763 K to about 790 K, and the V(T) curves are explicitly nonlinear. Since the volume contraction in PbTiO3-type ferroelectrics accelerates near TC, extending the average to 790 K may add the steepest part of the contraction to PTOS2 only. A common-window average (300-763 K) is required before claiming enhanced NTE. The DFT concentration mismatch identified by the Reader is a real limitation of the mechanism story, but it does not threaten the observation; the range and uncertainty issue threatens the title claim itself.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","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.","tokens_in":10684,"tokens_out":3768,"duration_ms":38619,"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":[{"comment":"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.","section":"Results and discussion, Fig. 3"},{"comment":"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.","section":"Results and discussion, Fig. 3 and Experimental Section"},{"comment":"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.","section":"Abstract vs. Results and discussion"},{"comment":"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.","section":"Electronic Structure Calculation, Fig. 4"}],"minor_comments":[{"comment":"In the sentence 'the hybridization between the cations and anions is crucial to the NET in PT-based ferroelectrics', 'NET' should be 'NTE'.","section":"Section 4, Electronic Structure Calculation paragraph"},{"comment":"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.","section":"Experimental Section"},{"comment":"The diffractometer manufacturer is misspelled as 'Brucker'; it should be 'Bruker'.","section":"Experimental Section"},{"comment":"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.","section":"Figure 2c and Supplementary Table 1"},{"comment":"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.","section":"Abstract and Conclusion"}],"recommendation":"major_revision","confidential_remarks":"The paper's novelty claim of being the first to study anion substitution in PbTiO3 for NTE should be checked against the literature on mixed-anion titanates; if prior work exists, the claim needs to be narrowed. The lack of error bars on the CTE values is a recurring issue in this subfield, but for a quantitative enhancement claim it is load-bearing. The common-window analysis is essential, and the DFT concentration mismatch should be disclosed clearly in the main text."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Chen — quick read on the PbTiO3 oxysulfide paper. The genuinely new thing is that they substituted S for O, not the usual A/B cation, and the high-pressure synthesis is real work: phase-pure P4mm up to x=0.02, S2- confirmed by XANES, uniform distribution, and a systematic increase in c/a with S content. That alone is a useful data point for the PT-NTE literature. The measured NTE enhancement, if it holds, is modest (roughly 18% in the average volume CTE), but in a field where a few percent matters, it is worth taking seriously.\n\nThe experimental center is believable, but the quantitative claim is shakier than the text admits. The three average CTEs are compared over different temperature windows: PT to about 763 K, PTOS to about 790 K. The V(T) curves are nonlinear and the contraction accelerates near TC, so part of the \"enhancement\" may just be the longer window. There are no numerical error bars on CTE or c/a; \"error bars smaller than symbols\" is not a substitute for propagated uncertainties. Before the title claim is accepted, the authors should give a common-window comparison (e.g., 300–763 K) and report uncertainties on the averages.\n\nThe DFT mechanism is the weakest part. One S in a 2×2×2 cell is x=0.125, more than ten times the actual x=0.01; the paper acknowledges the mismatch but then uses those charge-density plots to explain the enhancement. That is fine as a qualitative suggestion, not as evidence. The measured tetragonality trend and the SVFS values carry the argument; the hybridization story is illustrative.\n\nThere is no circularity problem: the CTE is measured directly, and the tetragonality-NTE correlation is prior art. The citation pattern looks appropriate, and the \"first anion-site substitution\" framing is accurate within their cited literature.\n\nBottom line: this deserves a serious referee. It is a solid experimental contribution with a plausible but under-quantified headline claim. I would send it to review and ask for the common-window analysis, uncertainties, and a toned-down DFT interpretation. I would bring it to reading group, mostly to discuss the temperature-window issue as a cautionary example.","headline":"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.","tokens_in":11297,"tokens_out":2130,"would_cite":true,"duration_ms":23445,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"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.","keywords":["negative thermal expansion","PbTiO3","oxysulfide","anion substitution","tetragonality","ferroelectric","hybridization","high-pressure synthesis"],"falsifier":"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.","tokens_in":10241,"feed_emoji":"📉","tokens_out":5866,"duration_ms":53038,"temperature":0.7,"pith_summary":"This paper claims that replacing a tiny amount of oxygen with sulfur in PbTiO3—the first anion-site substitution in this classic ferroelectric—increases the tetragonal distortion and strengthens the material's negative thermal expansion. For PbTiO2.98S0.02, the average volumetric coefficient of thermal expansion reaches $-2.50 \\times 10^{-5}$/K from 300 to 790 K, a deeper contraction than pristine PbTiO3's $-2.11 \\times 10^{-5}$/K measured in the same study. The authors argue that sulfur's more delocalized electrons enhance Pb–S and Ti–S hybridization, which drives the larger tetragonality and consequently a larger spontaneous volume ferroelectrostriction. This work opens a new knob—mixed-anion chemistry—for tuning NTE in ferroelectric perovskites.","feed_headline":"Sulfur makes PbTiO3 shrink more when heated","feed_subtitle":"A 2% anion swap raises tetragonality and deepens volume contraction across a broad temperature window.","key_machinery":"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.","core_discovery":"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.","pith_inferences":["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."],"forward_implications":["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."],"supporting_citations":[{"why":"Establishes the $c/a$–CTE design rule and reviews PT-based NTE materials, providing the context that increased tetragonality leads to enhanced NTE.","marker":"[3]"},{"why":"Supplies the pristine PbTiO3 CTE baseline that the enhanced values are compared against.","marker":"[36]"},{"why":"Defines spontaneous volume ferroelectrostriction ($\\omega_S$), the quantity used to link tetragonality to NTE.","marker":"[46]"},{"why":"Establishes that Pb–O/Ti–O hybridization governs ferroelectricity in PbTiO3, the premise for the anion-hybridization argument.","marker":"[47]"},{"why":"Provides experimental evidence of the hybridization in PbTiO3 that the sulfur-substitution effect is claimed to enhance.","marker":"[48]"},{"why":"Identifies the S K-edge XANES signature used to confirm S$^{2-}$ in the samples.","marker":"[42]"},{"why":"Provides the PBE functional used in the DFT calculations that support the hybridization mechanism.","marker":"[53]"}],"fun_headline_variants":["Sulfur substitution boosts negative thermal expansion in PbTiO3","Anion swap enhances PbTiO3's unusual contraction on heating","Mixed anions widen negative thermal expansion in PbTiO3","S for O improves PbTiO3's negative thermal expansion","Anion control deepens PbTiO3's shrink-on-heat effect"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"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.","fun_headline_variants_meta":{"raw":{"variants":["Sulfur substitution boosts negative thermal expansion in PbTiO3","Anion swap enhances PbTiO3's unusual contraction on heating","Mixed anions widen negative thermal expansion in PbTiO3","S for O improves PbTiO3's negative thermal expansion","Anion control deepens PbTiO3's shrink-on-heat effect"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000288,"raw_usage":{"total_tokens":1739,"prompt_tokens":1046,"completion_tokens":693,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":662,"completion_tokens_details":{"reasoning_tokens":609}},"tokens_in":662,"tokens_out":693,"duration_ms":7562,"temperature":1.0,"reasoning_tokens":609,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-10T19:44:26.891138+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"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.","supporting_citations":[],"review_version":1}