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A first-principles study on the physical properties of two-dimensional Nb3Cl8, Nb3Br8 and Nb3I8

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Pith's one-line read Spin-polarized density functional theory predicts that Nb3Cl8, Nb3Br8, and Nb3I8 monolayers are dynamically stable, easily exfoliable (0.24–0.28 J/m2), and have isotropic elastic moduli of 77–98 GPa with anisotropic tensile strengths of…

desk verdict Useful DFT add-on for a hot 2D family—new exfoliation and tensile numbers are plausible, but the phonon stability claim rests on surrogate potentials and the absolute stress scale is convention-dependent. read the letter →

arxiv 2412.03360 v1 pith:7IXAV5SD submitted 2024-12-04 cond-mat.mes-hall cond-mat.mtrl-sci

Bohayra Mortazavi , Xiaoying Zhuang , Timon Rabczuk This is my paper · ORCID
classification cond-mat.mes-hallcond-mat.mtrl-sci PACS 71.15.Mb62.20.-x68.35.Gy
keywords Nb3Cl8kagomelatticetwo-dimensionalmaterialsdensityfunctionaltheoryexfoliationenergyelasticmodulustensilestrengthphonondispersion
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

Using spin-polarized density functional theory, this paper predicts the structural, vibrational, exfoliation, and mechanical properties of three two-dimensional kagome crystals: Nb3Cl8, Nb3Br8, and Nb3I8. The central claim is that all three monolayers are dynamically stable, that their cleavage energies (0.24, 0.27, and 0.28 J/m2) are lower than graphene's 0.37 J/m2, and that they behave as isotropic-elastic but anisotropically strong materials, with elastic moduli from 98 down to 77 GPa. These results matter because a monolayer of Nb3Cl8 with topological flat bands has already been made by exfoliation, so the same route should work for the bromide and iodide cousins, and the predicted stiffness and strength offer concrete numbers for nanodevice design. The paper also finds that replacing Cl with Br and then I systematically lowers stiffness, strength, and phonon group velocities.

What carries the argument

The workhorse is a chain of first-principles and machine-learning calculations: spin-polarized DFT (generalized-gradient approximation plus dispersion correction) for geometry and stresses; moment tensor potentials, a class of machine-learned interatomic potentials, fitted to DFT data to evaluate phonon dispersions over supercells; a cleavage-energy procedure that gradually separates one layer from a six-layer slab to compute exfoliation energy; and uniaxial tensile simulations in which true stress is obtained by dividing force by the deformed real volume, using an effective monolayer thickness defined as the distance between boundary halogen atoms plus their van der Waals diameter. That thickness convention is what turns the computed in-plane forces into gigapascals, so it carries the absolute calibration of all reported moduli and strengths.

What would settle it

Indentation of a suspended Nb3Cl8 monolayer: extracting an in-plane modulus that, when converted with the paper's 6.16 Å thickness, deviates substantially from 98 GPa would falsify the quantitative prediction.

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Extended reading notes

Core claim

The paper establishes, by spin-polarized DFT with a van der Waals correction, that isolated Nb3X8 monolayers are stable in their kagome geometry, since phonon dispersions contain no imaginary frequencies. It then quantifies layer separation: exfoliation energies of 0.24, 0.27, and 0.28 J/m2 for Cl, Br, and I, all below the 0.37 J/m2 benchmark of graphene, indicating weak interlayer coupling and practical mechanical exfoliation. In the monolayer plane, the elastic moduli come out at 98, 89, and 77 GPa, independent of loading direction, while ultimate tensile strengths under uniaxial load are 10.2 (zigzag) and 8.1 (armchair) GPa for Nb3Cl8, 8.1 and 7.0 GPa for Nb3Br8, and 5.9 and 5.6 GPa for Nb3I8. The armchair direction fails more abruptly, with one Nb–X bond elongating far more than the others and with greater thickness reduction, explaining the anisotropic strength and more brittle armchair fracture.

Load-bearing premise

The load-bearing premise is that the effective monolayer thickness used to convert force to gigapascals—the distance between boundary halogen atoms plus their van der Waals diameter—is the correct thickness to use for real volume, since any other convention rescales every reported modulus and strength by the same factor.

Editorial extensions

If this is right

  • Nb3Br8 and Nb3I8 monolayers are predicted to be isolable by mechanical exfoliation, with cleavage energies within 0.03 J/m2 of the already-exfoliated Nb3Cl8.
  • In the linear regime, all three monolayers have isotropic elasticity, so measured in-plane stiffness should not depend on the loading direction.
  • Under uniaxial tension, the zigzag direction is stronger than the armchair direction in all three compounds, and armchair loading shows a sharper post-peak stress drop, indicating more brittle failure.
  • Heavier halogens systematically reduce elastic modulus, tensile strength, and phonon group velocities across the Nb3X8 family.
  • Phonon dispersions free of imaginary frequencies confirm the dynamical stability of all three free-standing monolayers.

Reading between the lines

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

  • The absolute gigapascal values inherit the paper's thickness convention; expressing the same results as in-plane force per unit length (N/m) would give convention-independent numbers and preserve the Cl→Br→I ordering, but would change how these sheets compare with graphene and other 2D materials on an absolute scale.
  • Because heavier halogens lower both stiffness and phonon group velocity, the same trend suggests lower lattice thermal conductivity in Nb3I8 than in Nb3Cl8; computing or measuring thermal transport would test this extension.
  • If the topological flat bands of Nb3Cl8 persist under strain, the predicted anisotropy suggests that uniaxial strain along the zigzag direction could tune the electronic structure over a wider range before fracture than armchair strain.
  • The cleavage-energy curves could also be used to estimate interlayer sliding barriers or the work of adhesion for heterostructures, since the same dispersion-corrected DFT setup would give the needed energy landscape.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

2 major / 5 minor

Summary. This manuscript reports spin-polarized DFT-D3 calculations of the structural parameters, phonon dispersions, exfoliation energies, and uniaxial stress-strain response of monolayer Nb3Cl8, Nb3Br8, and Nb3I8. The lattice constants are in good agreement with earlier DFT+U results. The authors find no imaginary phonon frequencies for the three monolayers, based on moment tensor potential (MTP) fitted phonon dispersions; exfoliation energies of 0.24, 0.27, and 0.28 J/m2; isotropic in-plane elasticity with elastic moduli of 98, 89, and 77 GPa; and anisotropic ultimate tensile strengths of 10.2/8.1, 8.1/7.0, and 5.9/5.6 GPa along the zigzag/armchair directions. All mechanical and phonon-frequency indicators decrease as the halogen mass increases.

Significance. The exfoliation and mechanical-response calculations are direct DFT total-energy and stress calculations, and the systematic halogen-size trends are a useful, falsifiable prediction for this recently fabricated kagome family. If the MTP-based phonon validation is strengthened, the paper would provide a compact reference dataset for Nb3X8 monolayers. The two main caveats are that the dynamic-stability claim is currently supported only by a surrogate potential with no in-paper quantitative validation, and that the GPa-scale quantities depend on a non-unique monolayer-thickness convention. Both issues are fixable and should be addressed before the quantitative abstract claims are taken as first-principles.

major comments (2)
  1. [§2, Fig. 2] The dynamical-stability conclusion is derived from phonon dispersions computed with moment tensor potentials rather than from DFT phonon calculations. The text reports no per-system validation: there are no MTP force/energy errors, no training-set statistics, no comparison of MTP dynamical matrices with DFT, and the agreement with Jiang et al. [15] is only qualitative. A missed imaginary branch in any of the three compounds would invert the headline stability claim. Please add direct DFT phonon calculations (finite-displacement or DFPT) for the three monolayers, or at a minimum a quantitative MTP-vs-DFT comparison of forces and phonon frequencies, and show an overlay of the dispersions against [15].
  2. [§3, Fig. 4] The absolute elastic moduli and tensile strengths are obtained by converting 2D in-plane stresses to GPa using an effective monolayer thickness defined as the distance between boundary halogen atoms plus their effective van der Waals diameter. This thickness is a modeling convention; alternative reasonable definitions (bulk interlayer spacing, electron-density extent, or the vdW diameter alone) would rescale all reported GPa values by tens of percent. Please report the primary 2D stiffness and strength in N/m, and present GPa values only together with an explicit statement that they are convention-dependent. This affects the numerical values quoted in the abstract and conclusions, although the halogen trends would likely survive.
minor comments (5)
  1. [§2] The phrase 'The plane wave and self-consistent loop cutoff energies were set to 300 and 10-5 eV' conflates the plane-wave energy cutoff with the electronic self-consistency criterion; please distinguish the 300 eV cutoff from the 10-5 eV convergence threshold.
  2. [Fig. 4 caption] The deformed Nb3Cl8 panels are listed as (d), (e), and (e); the last panel should be labeled (f) to match the in-text references to Fig. 4e and 4f.
  3. [§3] The compressed notation 'C11(C12) ... 105 (27) GPa' should be written explicitly, for example C11 = 105 GPa and C12 = 27 GPa for Nb3Cl8, so that the elastic constants are unambiguous.
  4. [§3] The armchair and zigzag loading directions are not defined in the text; please add a small schematic or a sentence relating these directions to the hexagonal lattice vectors.
  5. [§2] The MTP training details (training-set size, active-learning iterations, interaction order, and displacement settings used with PHONOPY) are missing; providing them, or a data-availability statement with the trained potentials, would make the phonon results reproducible.

Circularity Check

0 steps flagged · score 2.0 of 10

No significant circularity; central DFT-derived predictions are self-contained, with only minor non-load-bearing self-citations in the MTP phonon methodology.

full rationale

The paper's principal results—exfoliation energies, elastic moduli, tensile strengths, and phonon stability—are computed from spin-polarized DFT and DFT-D3 total-energy calculations with no fitting to the target quantities. Exfoliation energies are obtained by explicit layer-separation energy curves (Section 3, Fig. 3), and mechanical properties from uniaxial stress-strain simulations (Section 3, Fig. 4). The only fitted component is the moment-tensor potential used for phonon dispersions, whose accuracy is justified by self-citations [22,24]; however, the resulting dispersions are explicitly compared with independent DFT phonon results of Jiang et al. [15], so the stability claim does not reduce to the self-citation alone. The effective monolayer thickness used to convert stress to GPa is a modeling convention that scales absolute values, but it is disclosed, applied uniformly, and does not constitute a fitted parameter disguised as a prediction. No equation in the paper reduces a predicted quantity to an input by construction, and no uniqueness theorem or ansatz is imported from prior work. The absence of per-system MTP-vs-DFT force/energy metrics is a verification gap, not a circularity. The score of 2 reflects only the minor self-citations in the phonon methodology, which are not load-bearing because an external comparison is provided.

Assumptions & free parameters 1 free parameters · 3 assumptions · 0 invented entities

The central calculations are parameter-free DFT predictions, with no free parameters or invented entities beyond the modeling conventions (slab geometry, effective thickness) and the standard approximations in the exchange-correlation functional and dispersion correction.

free parameters (1)
  • Effective monolayer thickness for stress conversion = 6.16 Å (Cl), 6.51 Å (Br), 7.00 Å (I)
    Used to convert 2D stress to 3D GPa in the tensile simulations; the choice of boundary halogen distance plus vdW diameter is a modeling convention that scales all reported elastic moduli and tensile strengths.
assumptions (3)
  • domain assumption PBE+DFT-D3 accurately describes the structural, phonon, and mechanical properties of Nb3X8 monolayers.
    The paper relies on the standard GGA-PBE functional with Grimme D3 dispersion correction without benchmarking against experiments or higher-level theory for these specific materials.
  • domain assumption Moment tensor potentials trained on DFT data reproduce DFT phonon dispersions for Nb3X8.
    The paper cites prior work (references 22 and 24) for MTP accuracy but provides no in-paper validation for these specific systems, only qualitative agreement with Jiang et al.'s phonon curves.
  • domain assumption A 6-layer slab with bulk stacking pattern gives converged exfoliation energies.
    The cleavage energy is computed by separating one layer from a six-layer slab; no convergence test with respect to slab thickness or stacking variants is reported.

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Cite this review

Pith. "Pith review of A first-principles study on the physical properties of two-dimensional Nb3Cl8, Nb3Br8 and Nb3I8." pith.science (2026). https://pith.science/paper/7IXAV5SD

@misc{pith2026241203360,
  author       = {Pith},
  title        = {Pith review of: A first-principles study on the physical properties of two-dimensional Nb3Cl8, Nb3Br8 and Nb3I8},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/7IXAV5SD}},
  note         = {Machine review of arXiv:2412.03360}
}
read the original abstract

In a recent advance, Nb3Cl8 two-dimensional crystals with a kagome lattice and electronic topological flat bands has been experimentally fabricated (Nano Lett. 2022, 22, 4596). In this work motivated by the aforementioned progress, we conduct first-principles calculations to explore the structural, phonon dispersion relations, single-layer exfoliation energies and mechanical features of the Nb3X8 (X=Cl, Br, I) nanosheets. Acquired phonon dispersion relations reveal the dynamical stability of the Nb3X8 (X=Cl, Br, I) monolayers. In order to isolate single-layer crystals from bulk counterparts, we predicted exfoliation energies of 0.24, 0.27 and 0.28 J/m2, for the Nb3Cl8, Nb3Br8 and Nb3I8 monolayers, respectively, which are noticeably lower than that of the graphene. We found that the Nb3X8 monolayers are relatively strong nanosheets with isotropic elasticity and anisotropic tensile strength. It is moreover shown that by increasing the atomic weight of halogen atoms in the Nb3X8 nanosheets, mechanical characteristics decline. Presented results provide a useful vision about the key physical properties of novel 2D systems of Nb3X8 (X=Cl, Br, I).

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Works this paper leans on

27 extracted references · 13 canonical work pages

  1. [15]

    Jiang, Q

    J. Jiang, Q. Liang, R. Meng, Q. Yang, C. Tan, X. Sun, X. Chen, Exploration of new ferromagnetic, semiconducting and biocompatible Nb3X8 (X = Cl, Br or I) monolayers with considerable visible and infrared light absorption, Nanoscale. 9 (2017) 2992–3001. https://doi.org/10.1039/C6NR07231C

    work page doi:10.1039/c6nr07231c 2017

  2. [1]

    Novoselov, A.K

    K.S. Novoselov, A.K. Geim, S. V Morozov, D. Jiang, Y. Zhang, S. V Dubonos, I. V Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films., Science. 306 (2004) 666–9. https://doi.org/10.1126/science.1102896

    doi:10.1126/science.1102896 2004

  3. [2]

    Geim, K.S

    A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (2007) 183–191. https://doi.org/10.1038/nmat1849

    doi:10.1038/nmat1849 2007

  4. [3]

    Castro Neto, N.M.R

    A.H.. Castro Neto, N.M.R.. Peres, K.S.. Novoselov, A.K.. Geim, F. Guinea, The electronic properties of graphene, Rev. Mod. Phys. 81 (2009) 109–162. https://doi.org/10.1103/RevModPhys.81.109

    doi:10.1103/revmodphys.81.109 2009

  5. [4]

    Y.-L. Hong, Z. Liu, L. Wang, T. Zhou, W. Ma, C. Xu, S. Feng, L. Chen, M.-L. Chen, D.-M. Sun, X.-Q. Chen, H.-M. Cheng, W. Ren, Chemical vapor deposition of layered two-dimensional MoSi 2 N 4 materials, Science (80-. ). 369 (2020) 670–674. https://doi.org/10.1126/science.abb7023

    doi:10.1126/science.abb7023 2020

  6. [5]

    P. Li, J. Zhang, C. Zhu, W. Shen, C. Hu, W. Fu, L. Yan, L. Zhou, L. Zheng, H. Lei, Z. Liu, W. Zhao, P. Gao, P. Yu, G. Yang, Penta-PdPSe: A New 2D Pentagonal Material with Highly In-Plane Optical, Electronic, and Optoelectronic Anisotropy, Adv. Mater. 33 (2021) 2102541. https://doi.org/https://doi.org/10.1002/adma.202102541

    work page doi:10.1002/adma.202102541 2021

  7. [6]

    Y. Fang, F. Wang, R. Wang, T. Zhai, F. Huang, 2D NbOI2: A Chiral Semiconductor with Highly In-Plane Anisotropic Electrical and Optical Properties, Adv. Mater. 33 (2021) 2101505. https://doi.org/https://doi.org/10.1002/adma.202101505

    work page doi:10.1002/adma.202101505 2021

  8. [7]

    Bykov, E

    M. Bykov, E. Bykova, A. V Ponomareva, F. Tasnádi, S. Chariton, V.B. Prakapenka, K. Glazyrin, J.S. Smith, M.F. Mahmood, I.A. Abrikosov, A.F. Goncharov, Realization of an Ideal Cairo Tessellation in Nickel Diazenide NiN2: High-Pressure Route to Pentagonal 2D Materials, ACS Nano. 15 (2021) 13539–13546. https://doi.org/10.1021/acsnano.1c04325

    work page doi:10.1021/acsnano.1c04325 2021

Show all 27 references
  1. [8]

    Z. Sun, H. Zhou, C. Wang, S. Kumar, D. Geng, S. Yue, X. Han, Y. Haraguchi, K. Shimada, P. Cheng, L. Chen, Y. Shi, K. Wu, S. Meng, B. Feng, Observation of Topological Flat Bands in the Kagome Semiconductor Nb3Cl8, Nano Lett. 22 (2022) 4596–4602. https://doi.org/10.1021/acs.nano...

    2022 doi

  2. [9]

    S. Oh, K.H. Choi, S. Chae, B.J. Kim, B.J. Jeong, S.H. Lee, J. Jeon, Y. Kim, S.S. 10 Nanda, L. Shi, D.K. Yi, J.-H. Lee, H.K. Yu, J.-Y. Choi, Large-area synthesis of van der Waals two-dimensional material Nb3I8 and its infrared detection applications, J. Alloys Compd. 831 (2020)...

    2020

  3. [10]

    Conte, D

    F. Conte, D. Ninno, G. Cantele, Layer-dependent electronic and magnetic properties of ${\mathrm{Nb}}_{3}{\mathrm{I}}_{8}$, Phys. Rev. Res. 2 (2020) 33001. https://doi.org/10.1103/PhysRevResearch.2.033001

    2020 doi

  4. [11]

    Regmi, T.W

    S. Regmi, T.W. Fernando, Y. Zhao, A.P. Sakhya, G. Dhakal, I. Bin Elius, H. Vazquez, J.D. Denlinger, J. Yang, J.-H. Chu, X. Xu, T. Cao, M. Neupane, Spectroscopic evidence of flat bands in breathing kagome semiconductor Nb3I8, (2022). https://doi.org/10.48550/arxiv.2203.10547

    2022 doi arXiv

  5. [12]

    Cantele, F

    G. Cantele, F. Conte, L. Zullo, D. Ninno, Tunable electronic and magnetic properties of thin Nb$_3$I$_8$ nanofilms: interplay between strain and thickness, (2021). https://doi.org/10.48550/arxiv.2107.12836

    2021 doi arXiv

  6. [13]

    R. Peng, Y. Ma, X. Xu, Z. He, B. Huang, Y. Dai, Intrinsic anomalous valley Hall effect in single-layer $\mathrm{N}{\mathrm{b}}_{3}{\mathrm{I}}_{8}$, Phys. Rev. B. 102 (2020) 35412. https://doi.org/10.1103/PhysRevB.102.035412

    2020 doi

  7. [14]

    Kim, B.J

    B.J. Kim, B.J. Jeong, S. Oh, S. Chae, K.H. Choi, S.S. Nanda, T. Nasir, S.H. Lee, K.- W. Kim, H.K. Lim, L. Chi, I.J. Choi, M.-K. Hong, D.K. Yi, H.K. Yu, J.-H. Lee, J.-Y. Choi, Structural and Electrical Properties of Nb3I8 Layered Crystal, Phys. Status Solidi – Rapid Res. Lett. ...

    2019 doi

  8. [16]

    Kresse, J

    G. Kresse, J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B. 54 (1996) 11169–11186. https://doi.org/10.1103/PhysRevB.54.11169

    1996 doi

  9. [17]

    Perdew, K

    J.P. Perdew, K. Burke, M. Ernzerhof, Generalized Gradient Approximation Made Simple, Phys. Rev. Lett. 77 (1996) 3865–3868. https://doi.org/10.1103/PhysRevLett.77.3865

    1996 doi

  10. [18]

    Grimme, J

    S. Grimme, J. Antony, S. Ehrlich, H. Krieg, A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu, J. Chem. Phys. 132 (2010) 154104. https://doi.org/10.1063/1.3382344

    2010 doi

  11. [19]

    Kresse, J

    G. Kresse, J. Hafner, Ab initio molecular dynamics for liquid metals, Phys. Rev. B. 47 (1993) 558–561. https://doi.org/10.1103/PhysRevB.47.558

    1993 doi

  12. [20]

    Monkhorst, J

    H. Monkhorst, J. Pack, Special points for Brillouin zone integrations, Phys. Rev. B. 13 (1976) 5188–5192. https://doi.org/10.1103/PhysRevB.13.5188

    1976 doi

  13. [21]

    A. V. Shapeev, Moment tensor potentials: A class of systematically improvable interatomic potentials, Multiscale Model. Simul. 14 (2016) 1153–1173. https://doi.org/10.1137/15M1054183

    2016 doi

  14. [22]

    Mortazavi, F

    B. Mortazavi, F. Shojaei, B. Javvaji, T. Rabczuk, X. Zhuang, Outstandingly high thermal conductivity, elastic modulus, carrier mobility and piezoelectricity in two- dimensional semiconducting CrC2N4: a first-principles study, Mater. Today Energy. 22 (2021) 100839. https://doi....

    2021

  15. [23]

    A. Togo, I. Tanaka, First principles phonon calculations in materials science, Scr. Mater. 108 (2015) 1–5. https://doi.org/10.1016/j.scriptamat.2015.07.021

    2015 doi

  16. [24]

    Mortazavi, I.S

    B. Mortazavi, I.S. Novikov, E. V Podryabinkin, S. Roche, T. Rabczuk, A. V Shapeev, X. Zhuang, Exploring phononic properties of two-dimensional materials using 11 machine learning interatomic potentials, Appl. Mater. Today. 20 (2020) 100685. https://doi.org/10.1016/j.apmt.2020.100685

    2020

  17. [25]

    Henkelman, A

    G. Henkelman, A. Arnaldsson, H. Jónsson, A fast and robust algorithm for Bader decomposition of charge density, Comput. Mater. Sci. 36 (2006) 354–360. https://doi.org/10.1016/j.commatsci.2005.04.010

    2006 doi

  18. [26]

    Silvi, A

    B. Silvi, A. Savin, Classification of Chemical-Bonds Based on Topological Analysis of Electron Localization Functions, Nature. 371 (1994) 683–686. https://doi.org/10.1038/371683a0

    1994 doi

  19. [27]

    W. Wang, S. Dai, X. Li, J. Yang, D.J. Srolovitz, Q. Zheng, Measurement of the cleavage energy of graphite, Nat. Commun. (2015). https://doi.org/10.1038/ncomms8853. 12 Nb3Cl8-1L 1.00000000000000 6.7829799920112803 0.0000000000000000 0.0000000000000000 -3.3914899959582780 5.8742...

    2015 doi

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Reviewed August 11, 2026 · model on record in the stance chip above.