{"id":"002dfee8-5694-4097-be52-657a947286ce","arxiv_id":"2505.01127","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":0,"one_line_summary":"Cobalt substitution tunes EuRh2Si2 from an antiferromagnet through a sharp valence transition to a smooth crossover, with the critical endpoint between x=0.119 and x=0.166.","lead":"A team grew single crystals of a europium compound and replaced some rhodium with cobalt to squeeze its crystal lattice. They mapped where a sharp electronic transition ends, a step toward studying how materials deform near a critical point.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"CEP upper bound rests entirely on unshown heat-capacity data for a polycrystalline sample with x_EDX = 0.166; the 0.119–0.166 localization is not yet anchored by published evidence.","rationale":"The reader's weakest-assumption analysis already identifies the same load-bearing concern: the upper bound of the CEP range depends on unshown heat-capacity data for a polycrystalline x_EDX = 0.166 sample. My independent read of the manuscript confirms that this is the most consequential gap. The lower bound at x_EDX = 0.119 is supported by directly shown susceptibility data with hysteresis, and the first-order character at 0.081–0.126 is corroborated by resistivity and heat-capacity measurements. The upper bound, by contrast, has no visible data support in the paper. The authors explicitly flag the EDX spectral overlap as a source of systematic uncertainty, and the batch-averaged composition table does not include the 0.166 sample, so even the composition of the decisive sample is not fully documented. This is not an internal inconsistency in the physics, and the pressure comparison with Honda et al. provides useful external support, but the central claim's upper edge is not reproducible from the manuscript as written. The appropriate response is to require the missing data, which is exactly the CONDITIONAL verdict the reader already reached; my stress test does not change that verdict.","tokens_in":14336,"tokens_out":5437,"duration_ms":55421,"concrete_test":"Provide the heat-capacity data for the x_EDX = 0.166 polycrystalline sample, evaluated separately for heating and cooling pulses as in Fig. 6, together with the EDX analysis of the actual measured piece and, if available, magnetization data on a single crystal from the same batch. If the cooling and heating curves overlap within noise and show only a broad, non-hysteretic anomaly, the upper bound stands; if a hysteretic step appears, the CEP range must be revised upward.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim is that the critical endpoint lies at 0.119 < x_EDX < 0.166. The lower bound is supported by first-order signatures (thermal hysteresis in susceptibility and resistivity, cooling/heating split in heat capacity) for x_EDX = 0.081–0.126. The upper bound, however, rests solely on the statement in Section III.C.4 that a polycrystalline sample with x_EDX = 0.166 'enters the valence crossover regime,' which was 'observed in the heat-capacity data ... (not shown)'. No C_p(T) curve, no cooling/heating comparison, and no magnetization or resistivity data for this composition are presented, and no single crystal with this composition is described. Because the endpoint is identified by the disappearance of hysteresis, the decisive negative observation at x_EDX = 0.166 is precisely the measurement that is missing. The absence of a reported anomaly could be trivial if the polycrystal is compositionally inhomogeneous or if the heat-pulse analysis differs from that used for the single crystals; conversely, a broadened but still hysteretic anomaly would move the upper bound above 0.166. The same sample's x_EDX value is also affected by the acknowledged Co-K/Eu-Lγ1 EDX overlap (Section III.A), and Table I, which lists batch-averaged compositions, does not include x_EDX = 0.166. The CEP window is therefore not yet anchored on both sides by data that the reader can inspect.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports single-crystal growth of EuRh2Si2 and Eu(Rh1−xCox)2Si2 (x ≤ 0.23) by flux methods, and uses magnetization, resistivity, and heat-capacity measurements to map the temperature–substitution phase diagram. For EuRh2Si2 the authors identify several antiferromagnetic phases and a weak in-plane anisotropy between [100] and [110]. For substituted samples, they observe a sharp first-order valence transition with pronounced thermal hysteresis for x_EDX between 0.081 and 0.126, and locate the critical endpoint (CEP) of this transition in the range 0.119 < x_EDX < 0.166. Above this range they identify a valence-crossover regime. The central claim is that this substitution series reaches the CEP at a lower substituent concentration than Eu(Rh1−xIrx)2Si2, making it a promising system for studying critical elasticity with reduced disorder.","tokens_in":14636,"tokens_out":2126,"duration_ms":21535,"significance":"If the CEP localization is correct, the paper delivers a materially useful result: a substitution series that approaches the critical endpoint of a first-order valence transition with relatively low chemical disorder, and a consistent set of first-order signatures (hysteresis in susceptibility, resistivity, and heat capacity) on single crystals. The detailed B–T phase diagrams for EuRh2Si2, including in-plane anisotropy, also provide a solid experimental foundation for this compound. The paper's own pressure-equivalence estimate (using K = 100 GPa and Vegard's law) is clearly presented as a conversion, not as a derivation, so the central qualitative conclusion does not depend on circular reasoning.","major_comments":[{"comment":"The upper bound of the CEP window, x_EDX < 0.166, rests entirely on the statement that a polycrystalline sample with x_EDX = 0.166 'enters the valence crossover regime' based on heat-capacity data '(not shown)'. Since the CEP is identified by the disappearance of thermal hysteresis, the negative observation at this composition is load-bearing. The paper must show the C_p(T) cooling/heating data for this sample, along with its EDX-determined composition and a statement of how the crossover was assigned. As written, the reader cannot verify that the anomaly is non-hysteretic rather than absent, broadened, or obscured by the acknowledged composition uncertainties.","section":"Section III.C.4 and Fig. 7"},{"comment":"The compositions quoted in the text (x_EDX = 0.081, 0.116, 0.119, 0.126, 0.166, 0.228, 0.229) do not match the batch-averaged values in Table I (0.06, 0.08, 0.11, 0.12, 0.23). If the text values are obtained from the specific single crystal measured, that should be stated explicitly for each sample, and the x_EDX = 0.166 polycrystal should be included in the growth table. Without this connection, the phase diagram in Fig. 7 cannot be reproduced from the reported growths.","section":"Table I and Section III.C"},{"comment":"The phase diagram in Fig. 7 includes data points for x_EDX = 0.166 and x_EDX = 0.23, but the unit-cell volume data in Fig. 1(c) switch between single crystals and polycrystalline samples without a clear distinction in the main text. Since the pressure conversion pCo = ΔV/V·K depends on these volume data, the provenance of each volume point used for the pressure comparison should be stated, and the uncertainty in K (taken as a 'typical' 100 GPa from Ref. [22]) should be propagated into the reported pressure estimates.","section":"Section III.C.4 and Fig. 1(c)"}],"minor_comments":[{"comment":"The word 'substitition' should be 'substitution'.","section":"Abstract"},{"comment":"The heat-capacity hysteresis is reported as a 'large difference of ∼10 K' between cooling (96 K) and heating (106 K), but the resistivity hysteresis for the same nominal composition (x_EDX = 0.126) is given as 15 K; the discrepancy between these values and their possible origin (different sweep rates, thermal lags) deserves a brief comment.","section":"Section III.C.3"},{"comment":"The sample labels in Fig. 5(a) include both x_EDX = 0.119 and x_EDX = 0.116, and the text states that 'slight changes in the Co substitution induce large differences in TV'; given the EDX Co-K/Eu-Lγ1 overlap, the authors should indicate how robust the 0.116 vs 0.119 distinction is given the stated EDX uncertainty.","section":"Section III.C.1"},{"comment":"The sentence 'with the intermediate Eu (2+δ)+ state at high temperatures' is incomplete as written; it should describe what is observed (e.g., a broad anomaly in C_p or χ) for the x_EDX = 0.166 polycrystal.","section":"Section III.C.4"},{"comment":"The text uses both 'a−a plane' and 'a−a plane' without italicizing the crystallographic axes consistently; please standardize notation.","section":"General"}],"recommendation":"major_revision","confidential_remarks":"The paper's central claim depends on a 'not shown' heat-capacity measurement for the x_EDX = 0.166 polycrystal. This is a standard request for evidence, not a criticism of the physics. If the authors can supply the missing data (even as supplementary material), the manuscript would likely be acceptable after minor revisions. I also note that the batch-vs-single-crystal composition reporting needs tightening for reproducibility."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The paper is a solid experimental study of Co substitution in EuRh2Si2. The new content is the single-crystal growth series and the T-x phase diagram that locates the valence-transition critical endpoint somewhere between x=0.119 and 0.166. That localization is the main claim, and it is the part that needs the most careful reading.\n\nWhat the paper does well: the growth is a real achievement—single crystals up to x=0.23 with PXRD confirming Vegard-like compression. The magnetization work on pure EuRh2Si2 gives a detailed B-T phase diagram with clear basal-plane anisotropy. For the substituted crystals, hysteresis in susceptibility, resistivity, and heat capacity consistently indicate a first-order valence transition, and the heat-capacity cooling/heating split (Fig. 6) is a nice touch that is rarely seen.\n\nThe soft spot is exactly what the stress-test note flags. The upper bound of the CEP window at x=0.166 is supported only by a brief statement that a polycrystalline sample shows crossover behavior in heat capacity, with no data shown. Since the CEP is identified by the disappearance of hysteresis, the absence of the anomaly at x=0.166 is the decisive negative observation, and it is not presented. That matters. A broadened but still hysteretic anomaly would move the upper bound. The lower bound at x=0.119 is better supported by the hysteresis data in Fig. 5. The EDX overlap between Co-K and Eu-Lγ1 is acknowledged but adds uncertainty to all concentrations, especially low ones. On top of that, the concentration labels are not consistent: Table I lists x_EDX = 0.11 and 0.12, while the text and figures use 0.116, 0.119, and 0.126. It looks like the same samples, but it is hard to tell. These are addressable issues, but they need to be fixed before the CEP claim can be taken as anchored.\n\nThe comparison to hydrostatic pressure is reasonable but relies on an assumed bulk modulus of 100 GPa and Vegard's law; it is an estimate, not a precise calibration.\n\nOverall, this is honest, careful work with a clear result: the Co series reaches the valence-transition regime at low substitution, and it is a plausible platform for CEP studies with less disorder than the Ir series. The paper deserves peer review. A referee should ask for the x=0.166 heat-capacity data and a harmonization of the concentration values.","headline":"Single-crystal Co series gives a plausible CEP near x~0.14, but the upper bound of the claimed window rests on an unshown measurement.","tokens_in":15209,"tokens_out":2760,"would_cite":true,"duration_ms":25601,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"The critical endpoint of the valence transition in Eu(Rh$_{1-x}$Co$_x$)$_2$Si$_2$ lies between x = 0.119 and x = 0.166, where the first-order jump gives way to a continuous valence crossover.","keywords":["valence transition","critical endpoint","Eu(Rh1-xCox)2Si2","EuRh2Si2","chemical pressure","thermal hysteresis","valence crossover","single-crystal growth"],"falsifier":"Grow a single crystal with composition near $x=0.15$, determine the cobalt content by an independent method such as X-ray absorption or wavelength-dispersive spectroscopy, and measure temperature-dependent magnetization and heat capacity on the same crystal: a thermal hysteresis appearing for $x>0.166$, or its absence for $x<0.119$, would disprove the claimed window. Re-measuring the heat capacity and Eu valence of the $x_{\\rm EDX}=0.166$ sample would directly test the unshown data point.","tokens_in":14166,"feed_emoji":"🧲","tokens_out":7229,"duration_ms":68248,"temperature":0.7,"pith_summary":"This paper reports single-crystal growth of the series Eu(Rh$_{1-x}$Co$_x$)$_2$Si$_2$ and uses cobalt substitution as chemical pressure to sweep europium's valence instability through its end point. The central claim is that the first-order valence transition—where Eu switches from a magnetic divalent to a less magnetic intermediate-valence state—survives only up to a cobalt content between $x = 0.119$ and $x = 0.166$, and that above this window the transition smooths into a continuous valence crossover. That location matters because, compared with the iridium-substituted analogue, reaching the critical endpoint here requires far less substituent, hence less chemical disorder, making the system a cleaner platform for studying the proposed critical elasticity of the lattice near such endpoints. The paper also maps several antiferromagnetic phases and a basal-plane magnetic anisotropy in the pure compound.","feed_headline":"First-order valence jump dies between x = 0.119 and x = 0.166","feed_subtitle":"Co substitution reaches the endpoint with less disorder than iridium, enabling cleaner studies of critical behavior.","key_machinery":"The load-bearing control parameter is chemical pressure generated by substituting smaller Co for Rh in the ThCr$_2$Si$_2$ structure: the unit-cell volume shrinks with $x$, compressing the Eu site and destabilizing the large-volume Eu$^{2+}$ state. The observable that carries the argument is thermal hysteresis—the gap between the valence transition temperatures measured on cooling and heating—whose width tracks the first-order character of the transition. The paper uses the collapse of that hysteresis to locate the critical endpoint, and converts $x$ to an equivalent pressure through $\\Delta V/V \\cdot K$ with a bulk modulus $K = 100$ GPa, which permits direct comparison with hydrostatic-pressure experiments.","core_discovery":"The paper establishes that in Eu(Rh$_{1-x}$Co$_x$)$_2$Si$_2$ a sharp temperature-induced first-order valence transition is present for $0.081 \\leq x_{\\rm EDX} \\leq 0.119$, signalled by large thermal hysteresis in magnetization (about 18 K at $x=0.081$), resistivity (up to 23 K), and heat capacity (about 10 K at $x=0.126$). As $x$ increases, the transition temperature rises almost linearly from about 37 K to about 105 K and the hysteresis narrows, indicating approach to a critical endpoint. From the disappearance of hysteresis between $x_{\\rm EDX}=0.119$ and $x_{\\rm EDX}=0.166$, the paper places the critical endpoint in the window $0.119 < x_{\\rm EDX} < 0.166$; the upper bound rests on heat-capacity data from a polycrystalline sample that are not shown. Above this regime, for $x_{\\rm EDX}\\approx 0.23$, only a broad valence crossover near 220 K remains, with no first-order character. The authors conclude this is the same valence instability seen under hydrostatic pressure on EuRh$_2$Si$_2$, with the endpoint reached at an estimated chemical pressure of about 1.7 GPa.","pith_inferences":["A sharper test of the window would be to extrapolate the measured hysteresis width to zero against $x$; the paper does not perform that extrapolation, but its own data make it feasible.","If the endpoint is a genuine thermodynamic critical point, the elastic response—for instance the bulk modulus—should show anomalies in the same concentration range; the paper's motivation suggests this is the intended next measurement.","The overlap of the Co-K and Eu-L$\\gamma_1$ lines in EDX could bias $x_{\\rm EDX}$ downward, so the true endpoint composition may differ systematically from the reported window; valence-sensitive spectroscopy on the endpoint samples would settle this.","A possible fifth magnetic phase at low fields in pure EuRh$_2$Si$_2$ is only suspected from the data; field-angle-resolved magnetization could test whether a fan or spiral ground state actually lies below $B^*$."],"forward_implications":["Within the first-order regime, $T_V$ rises almost linearly from about 37 K at $x_{\\rm EDX}=0.081$ to about 105 K at $x_{\\rm EDX}=0.119$, so composition acts as a clean dial for the transition temperature.","The disappearance of hysteresis between $x=0.119$ and $0.166$ marks a boundary: below it, cooling and heating trace different states; above it, the valence change is continuous.","The equivalent hydrostatic pressure at the endpoint is approximately 1.7 GPa, close to the 2.05 GPa estimate for pure EuRh$_2$Si$_2$, so chemical and external pressure drive the same valence instability.","Because the endpoint is reached by substituting only about 12–16% cobalt, the chemical disorder at the critical point should be substantially smaller than in Eu(Rh$_{1-x}$Ir$_x$)$_2$Si$_2$, whose endpoint lies near $x \\simeq 0.5$–$0.75$."],"supporting_citations":[{"why":"Supplies the x = 0 magnetization and phase-transition baseline against which Co-substituted crystals are compared.","marker":"[14]"},{"why":"Provides the general pressure-temperature phase diagram of Eu-based ThCr2Si2 compounds and the bulk modulus used to convert Co content to chemical pressure.","marker":"[22]"},{"why":"Gives the hydrostatic-pressure study of EuRh2Si2 whose transition range and estimated critical pressure the chemical-pressure results are matched against.","marker":"[23]"},{"why":"Provides the unit-cell volume of EuCo2Si2 used to estimate the large lattice compression expected from Co substitution.","marker":"[24]"},{"why":"Provides the Ir-substituted series whose critical endpoint at higher substitution defines the higher-disorder comparison.","marker":"[25]"},{"why":"Prior hard-x-ray photoemission on polycrystalline Eu(Rh1-xCox)2Si2 showing a valence transition for low x and a crossover for higher x, motivating the single-crystal study.","marker":"[27]"},{"why":"Shows that small stoichiometry variations can shift valence transition temperatures, used to explain differences in TV among similar compositions.","marker":"[33]"}],"fun_headline_variants":["Valence jump vanishes at critical endpoint in Eu(Rh,Co)2Si2","Critical endpoint of valence transition found via Co substitution","Co doping drives first-order valence transition to its end","First-order valence transition dies near x=0.14 in Eu(Rh,Co)2Si2","Chemical pressure tunes Eu(Rh,Co)2Si2 valence transition to critical point"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The upper edge of the claimed critical-endpoint window rests on one unshown heat-capacity measurement of a polycrystalline sample with $x_{\\rm EDX}=0.166$; if that sample's cobalt concentration or its classification as a crossover is wrong, the endpoint's upper bound shifts.","fun_headline_variants_meta":{"raw":{"variants":["Valence jump vanishes at critical endpoint in Eu(Rh,Co)2Si2","Critical endpoint of valence transition found via Co substitution","Co doping drives first-order valence transition to its end","First-order valence transition dies near x=0.14 in Eu(Rh,Co)2Si2","Chemical pressure tunes Eu(Rh,Co)2Si2 valence transition to critical point"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000818,"raw_usage":{"total_tokens":3664,"prompt_tokens":1106,"completion_tokens":2558,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":722,"completion_tokens_details":{"reasoning_tokens":2458}},"tokens_in":722,"tokens_out":2558,"duration_ms":21350,"temperature":1.0,"reasoning_tokens":2458,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-16T04:25:30.870536+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Grow a single crystal with composition near $x=0.15$, determine the cobalt content by an independent method such as X-ray absorption or wavelength-dispersive spectroscopy, and measure temperature-dependent magnetization and heat capacity on the same crystal: a thermal hysteresis appearing for $x>0.166$, or its absence for $x<0.119$, would disprove the claimed window. Re-measuring the heat capacity and Eu valence of the $x_{\\rm EDX}=0.166$ sample would directly test the unshown data point.","supporting_citations":[{"cited_title":"Kakihana, D","cited_arxiv_id":null,"evidence_quote":"Supplies the x = 0 magnetization and phase-transition baseline against which Co-substituted crystals are compared."},{"cited_title":"¯Onuki, M","cited_arxiv_id":null,"evidence_quote":"Provides the general pressure-temperature phase diagram of Eu-based ThCr2Si2 compounds and the bulk modulus used to convert Co content to chemical pressure."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Gives the hydrostatic-pressure study of EuRh2Si2 whose transition range and estimated critical pressure the chemical-pressure results are matched against."},{"cited_title":"Single crystal growth and physical characterization to fine tune YbIn1-xTxCu4 (T = Au, Ag) towards the critical endpoint of the valence transition","cited_arxiv_id":"2501.17714","evidence_quote":"Provides the unit-cell volume of EuCo2Si2 used to estimate the large lattice compression expected from Co substitution."},{"cited_title":"Peters, K","cited_arxiv_id":null,"evidence_quote":"Provides the Ir-substituted series whose critical endpoint at higher substitution defines the higher-disorder comparison."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Prior hard-x-ray photoemission on polycrystalline Eu(Rh1-xCox)2Si2 showing a valence transition for low x and a crossover for higher x, motivating the single-crystal study."},{"cited_title":"Scherzberg, C","cited_arxiv_id":null,"evidence_quote":"Shows that small stoichiometry variations can shift valence transition temperatures, used to explain differences in TV among similar compositions."}],"review_version":1}