Structural disorder plays a key role in tuning the properties of functional materials such as thermoelectrics and ionic conductors, yet its detailed characterization has long faced significant challenges. Traditional X-ray and neutron diffraction techniques rely heavily on high-quality, large-size single crystals or high-purity powders, making them difficult to apply directly to micro/nano-scale or multiphase mixed materials. The rise of three-dimensional electron diffraction (3D ED) has broken this limitation. Owing to its strong scattering power for submicron single crystals, 3D ED enables high-precision determination of the average structure of tiny crystals, opening a new vista for microstructural analysis.

Although 3D ED has achieved breakthroughs in average structure determination, accurately visualizing local disorder, trace interstitial atoms, and continuous ion migration channels still faces severe bottlenecks. Traditional Fourier reconstruction methods are limited by reciprocal-space truncation effects and initial model bias, which can easily obscure weak electron density features and introduce artifacts. Overcoming the dual limitations of dynamical scattering and Fourier truncation to establish a model-independent reconstruction framework suitable for electron diffraction has become a core challenge for achieving high-fidelity imaging and dynamic evolution observation of disordered structures at the submicron scale.

To address the above issues, a team led by the Shanghai Institute of Ceramics, Chinese Academy of Sciences, among others, conducted systematic research using ZEPTOOLS' FIB cryo-stage and in-situ high-tilt heating chip holder. The team developed a maximum entropy method reconstruction framework specifically designed for 3D electron diffraction (3D ED-MEM), which overcomes the truncation effects and model bias of conventional Fourier reconstruction. For the first time, it achieves high-fidelity, model-independent real-space imaging of trace interstitial ions and continuous ion migration channels in submicron crystals.

Title: Visualizing Structural Disorder in Submicron Crystals by 3D Electron Diffraction-Maximum Entropy Method
Journal: Journal of the American Chemical Society
URL: https://doi.org/10.1021/jacs.6c02708

To tackle the challenges of Fourier truncation artifacts and initial model bias in reconstructing local disordered structures via 3D ED, the research team constructed a maximum entropy method electron density reconstruction framework specifically oriented to 3D electron diffraction (3D ED-MEM). In this method, dynamical refinement is first performed to account for multiple scattering effects, extracting high-precision electron diffraction structure factors. These are then converted, using the Mott formula, into structure factors compatible with real-space electron density, followed by iterative maximum entropy algorithm calculations to produce model-unbiased electron density maps. Using polycrystalline silicon (Si) as a standard reference sample, the team systematically evaluated the effects of sample thickness and resolution limits on reconstruction quality. The results demonstrate that, under controlled sample thickness, the real-space electron density distributions obtained by 3D ED-MEM are in excellent agreement with those from synchrotron X-ray diffraction reconstructions, validating the high fidelity and reliability of this computational framework for real-space electron density analysis in submicron crystals.

In addressing the challenge of characterizing static local disorder in complex thermoelectric materials, the team applied 3D ED-MEM to the typical thermoelectric compound Zn₄Sb₃. Due to trace interstitials and local disorder of Zn ions in Zn₄Sb₃, conventional crystallographic average structure models and difference Fourier maps struggle to identify sparse interstitial atoms against strong background noise, leading to long-standing mismatches between crystal density and stoichiometry. By applying the 3D ED-MEM reconstruction framework, the team directly visualized three distinct, trace-occupied Zn²⁺ interstitial ions located in lattice gaps within submicron single-crystal grains, without needing to preset atomic positions or specific disorder models. This enabled high-precision real-space capture of static defect structures that are invisible to conventional diffraction methods, providing direct structural evidence for understanding how disorder suppresses lattice thermal conductivity.

In exploring the dynamic disorder evolution mechanism of the liquid-like flow characteristics in the superionic conductor Ag₂Se, micro/nano sample preparation and in-situ variable-temperature testing played a crucial role. To prevent cation migration or anomalous precipitation during focused ion beam processing caused by ion beam bombardment or local heating, the team used ZEPTOOLS' FIB cryo-stage to maintain the sample at -190 °C during delicate thinning, successfully preparing ultra-thin micro/nano single crystals of Ag₂Se while preserving phase purity. The sample was then loaded onto ZEPTOOLS' in-situ high-tilt heating chip holder, and high-quality in-situ 3D ED data were stably collected during heating from room temperature to 473 K. Thanks to the nondestructive cryo-preservation of the microscale phase state by the ZEPTOOLS FIB cryo-stage and the excellent temperature stability of the in-situ heating holder, the team obtained high-quality variable-temperature diffraction data before and after the phase transition. Through 3D ED-MEM, they successfully reconstructed real-space electron density distribution maps of silver ions at different temperatures, visually displaying the continuous migration channels and percolation paths formed by silver ions crossing the grid in the high-temperature phase of Ag₂Se, achieving high-resolution in-situ observation of dynamic ion diffusion behavior in the superionic state.

This research fundamentally expands the application scope of 3D electron diffraction, transitioning it from mere average crystal structure determination to high-precision real-space electron density and quantitative disorder analysis. The 3D ED-MEM method overcomes the strong dependence of traditional diffraction methods on large single crystals, providing a general means for structural disorder analysis in complex functional materials for which large single crystals cannot be grown or high-purity powders are difficult to obtain. The data processing workflow and in-situ TEM experimental protocol established in this paper define a scientific paradigm for quantitatively mapping local defects and dynamic migration channels at the submicron scale, bearing significant academic value for investigating the structure-property relationships between disorder and macroscopic performance in solid-state materials.

ZEPTOOLS, as a Chinese high-end precision instrument company, is a first-class supplier of in-situ electron microscopy characterization solutions. Its PicoFemto series of in-situ transmission electron microscopy characterization solutions have successively provided technical support for significant research results by domestic and international users. The following figures show the ZEPTOOLS in-situ products used in this study: