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arxiv: 2606.12150 · v1 · pith:CVHXNB67new · submitted 2026-06-10 · ❄️ cond-mat.mtrl-sci

Tracking atomic-scale interdiffusion in immiscible bimetallic nanoparticles via four-dimensional electron tomography

Jisheng Xie , Dijin Jiang , Zhen Sun , Yiheng Dai , Zezhou Li , Yao Zhang , Jihan Zhou This is my paper

Pith reviewed 2026-06-27 09:11 UTC · model grok-4.3

classification ❄️ cond-mat.mtrl-sci
keywords interdiffusionimmiscible bimetallic nanoparticlesPdIrfour-dimensional electron tomographyatomic-scale mixingsurface reconstructionin situ STEMnanoscale materials
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The pith

Immiscible PdIr nanoparticles intermix at the atomic level starting at 200°C through distinct surface and diffusion steps.

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

Researchers used four-dimensional electron tomography to watch how palladium and iridium atoms, which do not mix in bulk, begin to combine in nanoparticles at surprisingly low temperatures. They observed atom hopping that rebuilds the particle surface at 200°C, followed by flattening at 300°C, and then iridium atoms diffusing inward at 400°C via temporary intermediate positions. This process continues until the particles reach a mixed state near 900°C. The results demonstrate that size effects create new kinetic routes for mixing that are blocked in larger materials.

Core claim

Ex situ four-dimensional atomic resolution electron tomography combined with in situ scanning transmission electron microscopy reveals the atomic scale miscible transition driven by interdiffusion in immiscible PdIr nanoparticles at temperatures far below the melting point. The pathway involves surface reconstruction atom hopping at 200°C and surface flattening at 300°C, followed by a critical transition at 400°C where Ir interfacial diffusion and discrete Ir intermediates drive miscible intermixing. Upon reaching the nanoscale melting point 900°C, collective inward Ir diffusion yields the thermodynamically stable IrPd configuration.

What carries the argument

Four-dimensional atomic resolution electron tomography combined with in situ scanning transmission electron microscopy that tracks the temperature-dependent sequence of surface atom hopping, flattening, and Ir interfacial diffusion through discrete intermediates.

Load-bearing premise

The observed structural changes result from thermally driven interdiffusion rather than electron-beam artifacts, sample preparation damage, or uncontrolled temperature gradients.

What would settle it

If PdIr nanoparticles heated to the same temperatures without any electron beam exposure show no atomic intermixing when subsequently imaged, the claim that the changes are caused by interdiffusion would be falsified.

Figures

Figures reproduced from arXiv: 2606.12150 by Dijin Jiang, Jihan Zhou, Jisheng Xie, Yao Zhang, Yiheng Dai, Zezhou Li, Zhen Sun.

Figure 1
Figure 1. Figure 1: Overview of the miscibility features in Pd@Ir. (A) In situ ADF-STEM images of Pd-Ir NPs at R.T. and 900°C, and three schematic pathways in the miscible transition between 200 and 400oC revealed by 4D-AET. The Pd@Ir NPs undergo “atom hopping, surface flattening, and discrete diffusion” and ultimately transitions into the thermodynamically stable Ir@Pd configuration at Tm. At the nanoscale, Tc/Tm is reduced … view at source ↗
Figure 2
Figure 2. Figure 2: Quantification of 4D atomic evolution and chemical order. (A) Distribution of distances to the surface for migrated atoms in Particles 1-3 at different annealing stages. (B to D) Surface reconstruction pathways within island-like regions at 200°C (atom hopping) (B), 300°C (surface flattening) (C) and 400°C (surface flattening) (D). Green, yellow and gray represent Pd, Ir and other atoms in particle, respec… view at source ↗
read the original abstract

The interdiffusion of immiscible elements is generally considered both thermodynamically unfavorable and kinetically hindered. At the nanoscale, however, the mixing behavior of multielements materials often diverges from bulk equilibrium, yet a quantitative, atomically resolved description of this transformation has remained challenging. Using ex situ four dimensional atomic resolution electron tomography combined with in situ scanning transmission electron microscopy, here we reveal the atomic scale miscible transition driven by interdiffusion in immiscible PdIr nanoparticles at temperatures far below the melting point. The pathway involves surface reconstruction atom hopping at 200oC and surface flattening at 300oC, followed by a critical transition at 400oC where Ir interfacial diffusion and discrete Ir intermediates drive miscible intermixing. Upon reaching the nanoscale melting point 900oC, collective inward Ir diffusion yields the thermodynamically stable IrPd configuration. Our findings provide quantitative atomic scale insights into how metastable nanostructures evolve through distinct intermediates, offering a design framework for advanced multielement materials.

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 use of ex situ four-dimensional atomic-resolution electron tomography combined with in situ scanning transmission electron microscopy to track atomic-scale interdiffusion in immiscible PdIr nanoparticles. It describes a temperature-dependent pathway involving surface reconstruction and atom hopping at 200°C, surface flattening at 300°C, a critical transition at 400°C driven by Ir interfacial diffusion and discrete intermediates leading to miscible intermixing, and collective inward Ir diffusion at 900°C to the thermodynamically stable configuration.

Significance. If the structural changes are confirmed to arise from thermally driven interdiffusion, the work would deliver quantitative atomic-scale observations of how metastable nanostructures evolve through distinct intermediates in immiscible bimetallic systems below the bulk melting point. The combination of in situ STEM heating with ex situ 4D tomography is a methodological strength that enables direct visualization of atom trajectories and could inform design principles for multielement nanomaterials.

major comments (2)
  1. [Abstract] Abstract: the central claim that distinct transitions at 200°C, 300°C, and especially the critical 400°C transition are 'driven by interdiffusion' is load-bearing, yet the described experimental workflow provides no beam-off controls, dose-series comparisons, or beam-heating estimates to exclude electron-beam-induced artifacts (local heating, knock-on displacement, or radiolysis) during in situ STEM or ex situ tomography.
  2. [Experimental workflow] Experimental methods (as summarized): without quantitative reporting of electron dose, temperature calibration under beam illumination, or control experiments separating thermal from beam effects, attribution of the observed Ir diffusion and intermixing specifically to thermal activation at 400°C cannot be rigorously distinguished from beam-assisted processes.
minor comments (2)
  1. [Abstract] Abstract: temperatures are inconsistently formatted as '200oC' rather than the conventional '200 °C'.
  2. [Abstract] Abstract: 'four dimensional' should be hyphenated as 'four-dimensional' for standard technical usage.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive feedback on potential electron-beam artifacts. We address each major comment below and will revise the manuscript to strengthen the distinction between thermal and beam-induced effects where possible.

read point-by-point responses

  1. Referee: [Abstract] Abstract: the central claim that distinct transitions at 200°C, 300°C, and especially the critical 400°C transition are 'driven by interdiffusion' is load-bearing, yet the described experimental workflow provides no beam-off controls, dose-series comparisons, or beam-heating estimates to exclude electron-beam-induced artifacts (local heating, knock-on displacement, or radiolysis) during in situ STEM or ex situ tomography.

    Authors: We agree that explicit controls are important for rigorous attribution. The ex situ 4D tomography workflow heats particles without the electron beam present during the temperature ramp, with atomic-resolution imaging performed afterward at ambient temperature; the in situ STEM component images changes only after the target temperature is reached. In the revised manuscript we will add quantitative electron-dose values for both modalities, literature-based estimates of beam-induced local heating in nanoparticles, and a discussion of why the observed temperature-specific stepwise transitions (surface hopping at 200 °C, flattening at 300 °C, interfacial diffusion at 400 °C) are inconsistent with constant beam-driven processes. Dedicated beam-off heating controls and full dose-series comparisons were not performed in the original study; we will note this limitation explicitly while arguing that the multi-particle reproducibility and thermodynamic consistency support the thermal interpretation. revision: yes

  2. Referee: [Experimental workflow] Experimental methods (as summarized): without quantitative reporting of electron dose, temperature calibration under beam illumination, or control experiments separating thermal from beam effects, attribution of the observed Ir diffusion and intermixing specifically to thermal activation at 400°C cannot be rigorously distinguished from beam-assisted processes.

    Authors: We accept that the current methods section lacks the requested quantitative details. The revised manuscript will report the electron doses used during in situ STEM heating and ex situ tomography, clarify the heating-stage temperature calibration procedure, and include an estimate of beam-heating contributions using established models for metallic nanoparticles. While separate beam-off control experiments were not included in the original workflow (the ex situ design intentionally minimizes beam exposure during heating), the distinct, temperature-thresholded structural pathways observed across multiple particles provide internal evidence favoring thermal activation. We will expand the methods and discussion sections accordingly to address this point directly. revision: yes

Circularity Check

0 steps flagged

No circularity: purely observational experimental report with no derivations or fitted predictions

full rationale

The paper reports experimental observations of atomic-scale structural changes in PdIr nanoparticles using in-situ STEM and ex-situ 4D tomography. The abstract and described pathway detail temperature-dependent transitions (surface reconstruction at 200°C, flattening at 300°C, intermixing at 400°C) without any equations, parameters, predictions, or first-principles derivations. No self-citations, ansatzes, or uniqueness theorems are invoked in a load-bearing mathematical sense. The central claim is an attribution of observed dynamics to thermal interdiffusion, which is an interpretive step but does not reduce to a self-definitional or fitted-input circularity by construction. This matches the default expectation for non-circular experimental papers.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Experimental microscopy study; no mathematical model, free parameters, or new physical entities are introduced.

pith-pipeline@v0.9.1-grok · 5731 in / 1073 out tokens · 29067 ms · 2026-06-27T09:11:46.567838+00:00 · methodology

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    Chemicals Sodium tetrachloropalladate(II) (Na2PdCl4, ≥99.99%) was purchased from Sigma -Aldrich; iridium (III) acetylacetonate (Ir(acac)3) was purchased from Bide Pharmatech, Shanghai, China; ethanol, cyclohexane and acetone were purchased from TGchem, Beijing, China; potassium bromide (KBr, 99.7%) was purchased from MREDA, Beijing, China; potassium chlor...

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    Synthesis of Pd NPs PVP (315 mg), KBr (15 mg), KCl (610.5 mg) and L-ascorbic acid (180 mg) were dissolved in 24 mL of deionized water , and the solution was heated to 80° C. Na2PdCl4 (30 mg) was dissolved in 9 mL deionized water, sonicated for 10 min, and then combined with the preheated solution. The mixture was stirred at 80° C for 4 h, and allowed to c...

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    Synthesis of Pd@Ir NPs The Pd seeds dispersion obtained above (4.7 mL) was first mixed with 6 mL of benzyl alcohol and stirred at 200° C in an open vessel for 5 min to complete phase transfer of the seeds from ethanol to benzyl alcohol. The resulting solution was then cooled to room temperature. An Ir precursor solution was prepared by dissolving 30.6 mg ...

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    Specimen preparation 18 A single-slot silicon TEM window with a SiN x membrane (purchased from NORCADA , Canada) was used for the specimen preparation. To further enhance the conductivity of the membrane, a 4 -nm-thick carbon layer was deposited onto the TEM window using a sputter coater (pulse mode , Leica EM ACE600). The grid was then heated at 350 ℃ fo...

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    4D Tomography experiment Three Pd@Ir NPs were randomly selected for the 4D tomography experiment. To obtain NPs in different diffusion states, the three NPs were annealed under an argon flow at 200, 300, and 400° C, respectively. For each particle, four tomographic tilt series were collected at cumulative annealing times of 0, 30, 60, and 120 min (figs. S...

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    Operation parameters used in acquisitions of tomographic tilt series All tomographic tilt series were acquired in ADF-STEM mode on a n aberration-corrected FEI Themis Z microscope . The operation parameters were: accelerating voltage 300 kV, convergence semi-angle 25 mrad, HAADF detector inner semi -angle 40.6 mrad and HAADF detector outer semi-angle 200 ...

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    missing wedge

    Element classification Initial element classification was conducted using the K-means clustering algorithm (50). For each atomic coordinate, the integrated intensity was calculated by summing the intensity signal within a 7× 7× 7 box centered on the atom . These integrated intensit ies were then grouped into two classes: Pd (lower intensity) and Ir (highe...

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    Identification of common positions and chemically consistent atoms First, an ideal FCC lattice was fitted to each dataset by a least-squares fitting approach (20), matching all atomic positions well. Next, we compared the two atomic models obtained from the same NP at different annealing times. Atomic pairs were identified by requiring that the two atoms ...

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    Quantification of chemical short-range-order parameter (CSROP) To evaluate short -range order in each NP, a pairwise multicomponent short -range-order parameter was applied (54), which is denoted as: 𝛼𝑖𝑗 𝑚 = (𝑝𝑖𝑗 𝑚 − 𝐶𝑗) / (𝛿𝑖𝑗 − 𝐶𝑗) Here, m indicates the m-th nearest-neighbor shell around a central atom i, 𝑝𝑖𝑗 𝑚 represents the averaged probability of det...

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    Calculation of interfacial diffusion coefficients of Ir atoms The radial concentration profiles of Ir at different depths were quantified based on the “onion peeling” method described in the previous section (f ig. S32). To eliminate interference from particle reshaping during surface reconstruction, we selected the change s in Ir concentration s with the...

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