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arxiv: 1907.08405 · v2 · pith:PRRDNGNPnew · submitted 2019-07-19 · ❄️ cond-mat.mtrl-sci

Synthesis of MAX Phases Nb2CuC and Ti2(Al0.1Cu0.9)N by A-site Replacement Reaction in Molten Salts

Haoming Ding , Youbing Li , Jun Lu , Kan Luo , Ke Chen , Mian Li , Per O.A. Persson , Lars Hultman
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Per Eklund Shiyu Du Zhengren Huang Zhifang Chai Hongjie Wang Ping Huang Qing Huang
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Pith reviewed 2026-05-24 19:16 UTC · model grok-4.3

classification ❄️ cond-mat.mtrl-sci
keywords MAX phasesmolten salt synthesisA-site replacementNb2CuCTi2AlNMXene precursorscopper substitution
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The pith

Molten salt reactions replace Al with Cu in MAX phases to yield Nb2CuC and Ti2(Al0.1Cu0.9)N.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows that reacting Nb2AlC and Ti2AlN with copper halide salts in the molten state produces new MAX phases through A-site element exchange. Complete substitution occurs for the niobium carbide, forming Nb2CuC, while the titanium nitride reaches Ti2(Al0.1Cu0.9)N. X-ray diffraction, electron microscopy, and atomically resolved imaging confirm the structures. Density functional theory calculations establish that the new phases are stable and possess lower cleavage energies than their aluminum-based parents.

Core claim

Reacting Nb2AlC with CuCl2 or CuI molten salt produces Nb2CuC by full A-site replacement of Al by Cu, while the same process applied to Ti2AlN yields Ti2(Al0.1Cu0.9)N. Structural characterization by XRD, SEM, and STEM verifies the substitution levels. DFT calculations confirm thermodynamic stability of Nb2CuC and Ti2CuN and show that cleavage energies in the copper-containing phases are lower than in the corresponding aluminum-containing phases.

What carries the argument

A-site replacement reaction in molten copper salts, which exchanges the A-layer element while preserving the M-X layers of the MAX phase structure.

If this is right

  • Nb2CuC forms as a fully substituted new MAX phase.
  • Ti2AlN reaches near-complete copper substitution at the composition Ti2(Al0.1Cu0.9)N.
  • Copper-containing MAX phases exhibit lower cleavage energies than aluminum analogs.
  • The new phases are positioned as precursors for deriving MXene sheets.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The molten-salt exchange route may extend to other MAX phases to access additional A-site compositions not reachable by direct synthesis.
  • Reduced cleavage energy suggests these phases could exfoliate under milder conditions than traditional MAX phases.
  • Partial substitution in the titanium case offers a route to compositionally tuned properties within a single MAX family.

Load-bearing premise

XRD peak positions, SEM images, and STEM atomic columns together prove near-complete substitution without undetected secondary phases or defects, and that DFT cleavage energies correctly forecast experimental exfoliation behavior.

What would settle it

High-resolution elemental mapping or additional diffraction data revealing residual aluminum atoms or impurity phases in the final product, or failed exfoliation experiments despite the calculated lower cleavage energy.

Figures

Figures reproduced from arXiv: 1907.08405 by Haoming Ding, Hongjie Wang, Jun Lu, Kan Luo, Ke Chen, Lars Hultman, Mian Li, Per Eklund, Per O.A. Persson, Ping Huang, Qing Huang, Shiyu Du, Youbing Li, Zhengren Huang, Zhifang Chai.

Figure 1
Figure 1. Figure 1: (a) XRD patterns of the Ti2AlN and the Ti2(Al0.1Cu0.9)N obtained from the reaction of Ti2AlN and CuCl2. (b) SEM image of the Ti2(Al0.1Cu0.9)N powder and (c) corresponding EDS spectrum. (d) High-resolution (HR)-STEM images of Ti2(Al0.1Cu0.9)N showing atomic positions along [112 _ 0] direction. (e) Element mapping in STEM-EDS mode and (f) corresponding line–scan of Ti-Kα (red), Cu-Kα (green) and Al-Kα (blue)… view at source ↗
Figure 2
Figure 2. Figure 2: (a) XRD patterns of the Nb2AlC and the Nb2CuC obtained from the reaction of Nb2AlC and CuI. (b) Rietveld refinement of XRD of the Nb2CuC. The low amount of Al in Ti2(Al0.1Cu0.9)N is noteworthy. In our recent work on Ti3AlC2, only partial substitution of Cu in Ti3(Al1/3Cu2/3)C2 was achieved. Thus, the same reaction in other MAX phases should be investigated to understand the underlying mechanism [PITH_FULL… view at source ↗
Figure 3
Figure 3. Figure 3: (a) SEM image of Nb2AlC. (b) SEM image of the Nb2CuC obtained from the reaction between Nb2AlC and CuI. (c) EDS spectrum of (b). (d) - (f) EDS mapping of Nb-Lα, Cu-Kα, and C-Kα signals of (b). SEM images of Nb2AlC and Nb2CuC particles are shown in [PITH_FULL_IMAGE:figures/full_fig_p008_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: High-resolution (HR)-STEM images of Nb2CuC showing atomic positions along [112 _ 0] (a) and [11 _ 00] (b) direction, respectively. (c) STEM-EDS mapping and line– scan of Nb-Lα (red) and Cu-Kα (green) signals, respectively. In order to further determine the structure of Nb2CuC, STEM was performed [PITH_FULL_IMAGE:figures/full_fig_p009_4.png] view at source ↗
read the original abstract

New MAX phases Ti2(AlxCu1-x)N and Nb2CuC were synthesized by A-site replacement by reacting Ti2AlN and Nb2AlC, respectively, with CuCl2 or CuI molten salt. X-ray diffraction, scanning electron microscopy, and atomically-resolved scanning transmission electron microscopy showed complete A-site replacement in Nb2AlC, which lead to formation of Nb2CuC. However, the replacement of Al in Ti2AlN phase was only close to complete at Ti2(Al0.1Cu0.9)N. Density-functional theory calculations corroborated the structural stability of Nb2CuC and Ti2CuN phases. Moreover, the calculated cleavage energy in these Cu-containing MAX phases are weaker than in their Al-containing counterparts, indicating that they are precursor candidates for MXene derivation.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript reports the synthesis of Nb2CuC and Ti2(Al0.1Cu0.9)N MAX phases via A-site replacement reactions of Nb2AlC and Ti2AlN with CuCl2 or CuI in molten salts. XRD peak shifts, SEM morphology changes, and atomically-resolved STEM images are presented as evidence for complete Al-to-Cu substitution in the Nb case and near-complete substitution in the Ti case. DFT calculations are used to corroborate structural stability of the Cu-containing phases and to show that their cleavage energies are lower than those of the parent Al-containing MAX phases, positioning the new compounds as potential MXene precursors.

Significance. If the phase-purity and composition claims are substantiated, the work would add two new Cu-based MAX phases to the known set and provide a molten-salt route for A-site substitution. The experimental characterization suite (XRD/SEM/STEM) plus DFT stability and cleavage-energy results would constitute a coherent first report on these compounds, with the weaker cleavage energies offering a falsifiable prediction for future exfoliation experiments.

major comments (2)
  1. [Abstract / Results (characterization)] Abstract and characterization sections: the central claim of complete A-site replacement yielding single-phase Nb2CuC (and near-complete replacement yielding Ti2(Al0.1Cu0.9)N) rests on XRD peak positions, SEM morphology, and local STEM atomic-column images. These data are consistent with substitution but do not exclude undetected secondary phases below the XRD detection limit, local compositional inhomogeneity, or residual Al outside the imaged regions. No bulk elemental quantification (EDS maps, ICP-OES) or Rietveld-derived A-site occupancies are reported to establish average composition across the reacted powder.
  2. [DFT section] DFT cleavage-energy discussion: the statement that Cu-containing phases have weaker cleavage energies than their Al counterparts is load-bearing for the MXene-precursor claim, yet the manuscript provides neither the numerical cleavage-energy values, the computational parameters (exchange-correlation functional, slab thickness, k-point sampling), nor any comparison to experimental delamination attempts.
minor comments (2)
  1. [Abstract] The composition Ti2(Al0.1Cu0.9)N is stated without an explicit description of how the 0.1/0.9 ratio was obtained from the experimental data.
  2. [Figure captions] Figure captions and text should clarify whether the STEM images are representative of the bulk or selected regions.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the detailed and constructive comments on our manuscript. We address each of the major comments point by point below, and indicate where revisions will be made.

read point-by-point responses

  1. Referee: Abstract and characterization sections: the central claim of complete A-site replacement yielding single-phase Nb2CuC (and near-complete replacement yielding Ti2(Al0.1Cu0.9)N) rests on XRD peak positions, SEM morphology, and local STEM atomic-column images. These data are consistent with substitution but do not exclude undetected secondary phases below the XRD detection limit, local compositional inhomogeneity, or residual Al outside the imaged regions. No bulk elemental quantification (EDS maps, ICP-OES) or Rietveld-derived A-site occupancies are reported to establish average composition across the reacted powder.

    Authors: We agree with the referee that bulk compositional analysis would provide additional support for the claims of A-site substitution. The presented XRD, SEM, and STEM data offer strong evidence through peak shifts matching expected structures, morphological changes, and direct atomic imaging of Cu in A-sites. However, to strengthen the manuscript, we will include Rietveld refinement results to quantify A-site occupancies in the revised version. We note that while STEM is local, multiple regions were imaged, but acknowledge the value of bulk methods. revision: partial

  2. Referee: DFT cleavage-energy discussion: the statement that Cu-containing phases have weaker cleavage energies than their Al counterparts is load-bearing for the MXene-precursor claim, yet the manuscript provides neither the numerical cleavage-energy values, the computational parameters (exchange-correlation functional, slab thickness, k-point sampling), nor any comparison to experimental delamination attempts.

    Authors: We will revise the DFT section to include the specific numerical cleavage energy values for both Cu- and Al-containing phases, along with full computational details such as the exchange-correlation functional, slab thickness, and k-point sampling used. As the work proposes these as potential MXene precursors based on the calculated weaker cleavage energies, direct comparison to experimental delamination is not included as no such experiments were performed; this serves as a prediction for future studies. revision: yes

Circularity Check

0 steps flagged

No significant circularity; experimental synthesis and DFT are independent of inputs

full rationale

The paper reports synthesis of new MAX phases via molten-salt A-site replacement, with characterization via XRD/SEM/STEM and separate DFT calculations for stability and cleavage energies. No equations, fitted parameters, or predictions are presented that reduce by construction to the experimental inputs or to self-citations. The central claims rest on direct observation and first-principles computation rather than any of the enumerated circular patterns. Self-citations, if present, are not load-bearing for the synthesis or stability results.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The work rests on standard materials characterization assumptions and DFT applicability to MAX phases; no free parameters or invented entities are introduced in the abstract.

axioms (1)
  • domain assumption Density functional theory calculations can reliably predict structural stability and relative cleavage energies for MAX phases.
    Invoked to corroborate stability and cleavage energy comparison.

pith-pipeline@v0.9.0 · 5739 in / 1280 out tokens · 22102 ms · 2026-05-24T19:16:16.387663+00:00 · methodology

discussion (0)

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Reference graph

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