{"id":"5eef1c32-00dd-4093-98fb-17bb7494419c","arxiv_id":"2502.10082","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":0,"one_line_summary":"The speed of sound and elastic constants of Cs3Bi2I9, MA3Bi2I9, and MA3Sb2I9 single crystals were measured by femtosecond transient reflectivity, showing hybrid compounds are softer.","lead":"Researchers measured the speed of sound in three lead-free perovskite crystals using ultrafast laser pulses. The hybrid organic-inorganic crystals turned out to be softer than the all-inorganic one, which matters for designing durable solar cells.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The measured speeds of sound are plausible, but the elastic constants in Table 1 are labeled c11 while these materials are non-cubic and the measurement is along [001]; the label should likely be c33, so the central c11 comparison is mislabeled.","rationale":"The paper's central claim is the extraction of c11 values for three lead-free halide perovskites, with the headline result that the inorganic Cs3Bi2I9 is about 30% stiffer than the two hybrid materials. Reading in good faith, the transient-reflectivity data and the extracted speeds of sound appear plausible: the Brillouin-oscillation analysis is standard, the Cs3Bi2I9 speed agrees with prior acousto-optic and transient-reflectivity measurements, and the relative ordering of stiffness is robust to the elastic-constant label. However, the step from speed of sound to c11 is not secure. The materials are almost certainly not cubic, and the measurement direction, [001], is the hexagonal c-axis, for which the relevant longitudinal elastic constant is c33. This is not a minor notational issue: it changes what physical quantity is being reported, and the abstract and conclusion explicitly name c11. The reader's weakest-assumption analysis identifies exactly this problem, and I agree. The paper can be corrected by establishing the crystal symmetry and re-labeling the constants, or by measuring along a direction that genuinely probes c11. Because the measured velocities and the softer-hybrid conclusion do not necessarily fail, the appropriate verdict remains CONDITIONAL, matching the reader's verdict; no change to that verdict is needed. Secondary weaknesses, such as the absence of error bars in Figure 4 and the reliance on only three probe wavelengths for the slopes, reinforce the need for caution but are not the most load-bearing issue.","tokens_in":7756,"tokens_out":4504,"duration_ms":44670,"concrete_test":"Perform single-crystal X-ray diffraction (or at least powder XRD) on the same crystal batches to determine the space group and confirm that the measured face is (001). If any of the materials is hexagonal or trigonal, replace Eq. (4) with V_L = sqrt(c33/rho), re-label the Table 1 entries as c33, and revise the abstract and conclusion accordingly. If the crystals prove to be cubic, the current c11 label is retained; in either case the measured speeds and the relative softness comparison are unaffected.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim rests on Eq. (4), V_L = sqrt(c11/rho), together with the statement that pump and probe are incident on the (001) plane. This identification is only correct if the longitudinal wave along the propagation direction is governed by c11, which is true for cubic symmetry with propagation along a cube axis (or for an isotropic medium). Cs3Bi2I9 is well documented to crystallize in a hexagonal structure, space group P63/mmc, and MA3Bi2I9 and MA3Sb2I9 are also commonly reported as hexagonal/trigonal layered materials. For a hexagonal crystal, propagation along [001] (the c-axis) is a pure longitudinal mode governed by c33, not c11: c11 describes in-plane basal-plane deformation. Therefore, the values in Table 1 (20.8, 15.4, and 15.0 GPa), computed as rho*V_L^2, are numerically c33, not c11, unless the crystals are cubic. The paper does not report XRD, space-group determination, or any symmetry evidence for the crystals, so the assumption underlying Eq. (4) is unverified. The speed-of-sound measurements themselves and the qualitative conclusion that the hybrid materials are softer may remain valid, but the abstract and conclusion make a quantitative claim about 'c11' that has not actually been measured.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports femtosecond transient reflectivity measurements on single crystals of the lead-free iodide perovskites Cs3Bi2I9, MA3Bi2I9, and MA3Sb2I9. Coherent acoustic phonon oscillations are observed below the excitonic resonance, and the oscillation period as a function of probe wavelength and independently measured refractive indices is used to extract the longitudinal speed of sound along the [001] direction. Combining these speeds with tabulated densities, the authors compute an elastic constant that they label c11 and conclude that the inorganic Cs3Bi2I9 is about 30% stiffer than the two hybrid compounds.","tokens_in":7954,"tokens_out":5240,"duration_ms":52247,"significance":"The speed-of-sound data for these lead-free halide perovskites are a useful addition to the literature, and the measurement protocol is largely self-contained: the speeds come from Brillouin oscillation periods and ellipsometric refractive indices, and the Cs3Bi2I9 result agrees with earlier acousto-optic and transient-reflectivity reports. The comparison between inorganic and hybrid compounds is practically relevant for optoelectronic device design. However, the central elastic-constant label is incorrect for the crystal symmetries involved, and the quantitative claim about c11 in the abstract and conclusion cannot stand as written. The measured speeds and the qualitative observation that the hybrid materials are softer remain valuable if the elastic constants are correctly identified.","major_comments":[{"comment":"Eq. (4) writes V_L = sqrt(c11/rho) for longitudinal sound propagating along the [001] direction, but this identification is valid only for cubic crystals or isotropic media. Cs3Bi2I9, MA3Bi2I9, and MA3Sb2I9 are documented in the literature as non-cubic hexagonal/trigonal materials; for such crystals, a longitudinal wave propagating along [001] (the c-axis) is governed by c33, not c11. The paper provides no XRD or space-group determination that would justify a cubic assignment. Therefore the values listed in Table 1 as c11 are, on the authors' own description of the geometry, c33 = rho*V_L^2. The labels in Table 1, Eq. (4), the abstract, and the conclusion must be corrected accordingly. The speed-of-sound measurements themselves and the qualitative conclusion of lower stiffness in the hybrids can remain, but the quantitative claim about c11 as stated is not supported. Notably, the cited agreement with Zamkov et al. for the [001] longitudinal speed actually supports the c33 interpretation.","section":"Section 4, Eq. (4), Table 1, and Abstract/Conclusion"},{"comment":"The densities used to convert V_L into elastic constants are given without citation, measurement details, or uncertainties. Since the elastic constant is computed as rho*V_L^2, any error in the density propagates linearly into the final values. The authors should state the source of each density (e.g., crystallographic data for the specific phase studied) and, if available, its precision. This is load-bearing because the numerical comparison of stiffness across the three materials depends directly on these density values.","section":"Table 1 and Section 4"}],"minor_comments":[{"comment":"The first row of Table 1 reads 'Cs3Bi2I' rather than 'Cs3Bi2I9'; this typo should be corrected.","section":"Table 1"},{"comment":"The paper states that pump and probe are incident at approximately 7 degrees from normal, while Eq. (2) is derived for normal incidence. The authors should estimate the effect of this angle or state explicitly that it is negligible at the quoted precision.","section":"Section 2, Eq. (2) and incidence angle"},{"comment":"The sentence saying that the oscillation period 'decreases with decreasing probe wavelength and with increasing light penetration depth' is unclear: Eq. (2) directly gives a decrease with decreasing wavelength, but the light penetration depth does not appear in the period formula. Please clarify whether this refers to the detection condition rather than the period itself.","section":"Section 2, text near Eq. (2)"},{"comment":"References [25] and [30] appear to denote the same paper (Science Advances 7, eabd3160). These should be consolidated to avoid duplicate citation.","section":"References"}],"recommendation":"major_revision","confidential_remarks":"The paper is not a reject: the acoustic measurements are plausible, validated on Cs3Bi2I9 against the literature, and useful for the lead-free perovskite community. The central fix is relabeling the elastic constant from c11 to c33 under the stated measurement geometry, together with a corresponding revision of the abstract and conclusion. If the authors can actually demonstrate cubic symmetry for their crystals, the original labeling could be defended, but that would contradict the known structures of these compounds, so the more likely path is a careful revision of the symmetry language."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The paper is worth reading for the data. It reports the first longitudinal sound speeds for MA3Bi2I9 and MA3Sb2I9 single crystals, measured by femtosecond transient reflectivity, and the Cs3Bi2I9 result matches previous acousto-optic and TR work. The extraction method is standard Brillouin oscillation analysis, and using three probe wavelengths to confirm the slope is a reasonable consistency check. The qualitative finding that the hybrid materials are softer than the inorganic one is likely robust.\n\nThe soft spot is the labeling of the elastic constant. The authors use Eq. (4), V_L = sqrt(c11/rho), and state the measurement is along the [001] direction. For Cs3Bi2I9, and almost certainly for the MA-based hybrids, the crystal symmetry is hexagonal or trigonal, not cubic. In that case the longitudinal wave propagating along [001] (the c-axis) is governed by c33, not c11. The paper gives no XRD or space-group data to justify the cubic assumption. So the numbers in Table 1 are probably c33 values, and the repeated claim about 'c11' in the abstract and conclusion is mislabeled. This is a correction, not a fatal flaw: the speeds of sound are what they are, and the stiffness ordering is unaffected. But anyone citing these elastic constants for the hybrid materials needs to know which constant was actually measured.\n\nMinor issues: Figure 3 is a line plot with no error bars; the slope fits use only three probe wavelengths; and the density uncertainties are not stated, even though c11 scales directly with density. The refractive index dispersion comes from ellipsometry, which is fine, but a sentence on the associated error would help.\n\nI would send this to peer review. The measurements appear clean, the paper is concise, and the new data are useful to the perovskite community. The referee should ask for the symmetry justification or a relabeling to c33, plus the missing error analysis. After that, it should be publishable as a solid experimental contribution.\n\nReading group: yes, if anyone in the group works on lead-free perovskites. I would cite the speed of sound values, but not the 'c11' label without caveat.","headline":"Solid sound-speed measurements for two new hybrid lead-free perovskites, but the elastic constant is probably mislabeled as c11 when it should be c33 for these hexagonal/trigonal crystals.","tokens_in":8554,"tokens_out":1242,"would_cite":true,"duration_ms":12684,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":["78.47.-p","62.20.Dc","63.20.-e"],"model":"deepseek-v4-flash","headline":"By timing Brillouin oscillations in reflected light, this paper measures sound speed and elastic constants in three lead-free perovskites and finds the hybrid compounds about 30% softer than Cs3Bi2I9.","keywords":["lead-free halide perovskites","femtosecond transient reflectivity","Brillouin oscillations","coherent acoustic phonons","longitudinal speed of sound","elastic constant","Cs3Bi2I9","hybrid iodide perovskites"],"falsifier":"Determine the space group of one of the measured crystals by X-ray diffraction and measure longitudinal sound velocity along two inequivalent directions, for example with Brillouin light scattering. If the velocity along [001] does not obey $v=\\sqrt{c_{11}/\\rho}$ for the true symmetry, or if it differs from the reported 1995, 2260, or 2040 m/s beyond the error bars, the elastic-constant extraction is wrong.","tokens_in":7505,"feed_emoji":"🔊","tokens_out":10773,"duration_ms":89409,"temperature":0.7,"pith_summary":"This paper aims to establish that femtosecond transient reflectivity can measure the longitudinal speed of sound in single crystals of three lead-free iodide perovskites: the inorganic Cs3Bi2I9 and the hybrid MA3Bi2I9 and MA3Sb2I9. The pump pulse launches a strain wave, and the periodic Brillouin oscillations it imprints on the reflected probe light fix the sound speed; combining that speed with known densities gives an elastic constant. The central result is that the inorganic material is stiffer, with $c_{11} = 20.8\\pm0.2$ GPa, while the two hybrid materials come in at $15.4\\pm0.2$ and $15.0\\pm0.2$ GPa, roughly 30% softer. That matters because devices made from these perovskites face repeated thermal stress, and mechanical stiffness shapes how they deform and age.","feed_headline":"Lasers clock sound speeds and find hybrid perovskites 30% softer","feed_subtitle":"Brillouin oscillations in femtosecond reflectivity yield elastic constants for stress-robust device design.","key_machinery":"The central mechanism is the coherent acoustic phonon: an ultrasonic strain pulse launched when the femtosecond pump excites electron-hole pairs and thermal stress suddenly expands the lattice. This strain field modulates the probe beam's refractive index, producing Brillouin oscillations with period $\\tau$. The relation $1/\\tau = 2nv/\\lambda$, with probe refractive index $n$ and wavelength $\\lambda$ in air, connects the period to the longitudinal sound speed $v$; plotting $1/(2n\\tau)$ against probe wavenumber gives $v$ as the slope. The elastic constant is then obtained from $v=\\sqrt{c_{11}/\\rho}$.","core_discovery":"The paper claims that coherent acoustic phonons generated by 4.5 eV pump pulses and detected through Brillouin oscillations in a white-light probe yield longitudinal sound speeds of 1995$\\pm$15 m/s for Cs3Bi2I9, 2260$\\pm$30 m/s for MA3Bi2I9, and 2040$\\pm$20 m/s for MA3Sb2I9. With tabulated densities, these correspond to $c_{11} = 20.8\\pm0.2$, $15.4\\pm0.2$, and $15.0\\pm0.2$ GPa. The authors conclude that the inorganic Cs3Bi2I9 is about 30% stiffer than the two hybrid compounds, that substituting antimony for bismuth barely changes stiffness, and that the larger transient-reflectivity oscillation amplitude in the inorganic material is consistent with its larger deformation potential.","pith_inferences":["The measured sound speeds are direct data, but the label $c_{11}$ is an interpretation: bismuth and antimony iodide perovskites often crystallize in hexagonal or trigonal space groups, in which case longitudinal propagation along [001] would be governed by $c_{33}$ rather than $c_{11}$. Relabeling would not change the speeds but would change the comparison.","A natural next step is to orient the same crystals and measure sound speed along several axes, which would map the full elastic anisotropy instead of a single constant.","The oscillation amplitude versus probe energy reported here could, with computed deformation potentials, be converted into quantitative deformation-potential values for these materials."],"forward_implications":["The reported values give device designers a direct input for estimating how lead-free perovskite films and crystals deform under thermal and mechanical stress.","The roughly 30% higher stiffness of Cs3Bi2I9 predicts that it will resist thermal cycling better than the two methylammonium compounds.","The near-equality of $c_{11}$ for MA3Bi2I9 and MA3Sb2I9 means replacing bismuth with antimony tunes optical behavior without substantially changing mechanical response.","The same optical method should transfer to other lead-free and hybrid perovskites, giving elastic data without macroscopic mechanical testing."],"supporting_citations":[{"why":"It reviews the physical mechanisms of coherent acoustic phonon generation by ultrafast laser action, grounding the strain-pulse interpretation of the oscillations.","marker":"[15]"},{"why":"It describes surface generation and detection of picosecond acoustic pulses, the basis for attributing the reflectivity oscillations to a strain field.","marker":"[16]"},{"why":"It supplies the relation $1/\\tau = 2nv/\\lambda$ that converts the measured Brillouin oscillation periods into longitudinal sound speeds.","marker":"[18]"},{"why":"It reports earlier transient-reflectivity measurements on Cs3Bi2I9 that are used to validate the speed of sound obtained here.","marker":"[24]"},{"why":"It provides the acousto-optic longitudinal speed in Cs3Bi2I9, about 1980 m/s, used as an independent check of the new value.","marker":"[32]"},{"why":"It provides the hydrothermal growth method, the ellipsometric refractive indices, and the excitonic energies used to calibrate the transient-reflectivity data.","marker":"[27]"},{"why":"It establishes the indirect-gap optical behavior that justifies treating the crystals as thick and neglecting oscillation decay.","marker":"[26]"}],"fun_headline_variants":["Laser pulses reveal hybrid perovskites are 30% softer","Inorganic lead-free perovskite stiffer than hybrid ones","Acoustic phonons clock sound speeds in lead-free perovskites","Short laser pulses measure stiffness of lead-free perovskites","Cs3Bi2I9 outperforms hybrid perovskites in rigidity"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The paper's elastic-constant comparison rests on treating a longitudinal wave along the probed direction as controlled by $c_{11}$, without establishing the crystal symmetry that would justify that identification.","fun_headline_variants_meta":{"raw":{"variants":["Laser pulses reveal hybrid perovskites are 30% softer","Inorganic lead-free perovskite stiffer than hybrid ones","Acoustic phonons clock sound speeds in lead-free perovskites","Short laser pulses measure stiffness of lead-free perovskites","Cs3Bi2I9 outperforms hybrid perovskites in rigidity"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000844,"raw_usage":{"total_tokens":3658,"prompt_tokens":911,"completion_tokens":2747,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":527,"completion_tokens_details":{"reasoning_tokens":2664}},"tokens_in":527,"tokens_out":2747,"duration_ms":17794,"temperature":1.0,"reasoning_tokens":2664,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-07T19:27:42.293635+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Determine the space group of one of the measured crystals by X-ray diffraction and measure longitudinal sound velocity along two inequivalent directions, for example with Brillouin light scattering. If the velocity along [001] does not obey $v=\\sqrt{c_{11}/\\rho}$ for the true symmetry, or if it differs from the reported 1995, 2260, or 2040 m/s beyond the error bars, the elastic-constant extraction is wrong.","supporting_citations":[],"review_version":1}