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1. Probing Multiscale Disorder in Pyrochlore and Related Complex Oxides in the Transmission Electron Microscope: A Review

   https://doi.org/10.3389/fchem.2021.743025  

   Wardini, Jenna L.; Vahidi, Hasti; Guo, Huiming; Bowman, William J. (November 2021, Frontiers in Chemistry)

   Transmission electron microscopy (TEM), and its counterpart, scanning TEM (STEM), are powerful materials characterization tools capable of probing crystal structure, composition, charge distribution, electronic structure, and bonding down to the atomic scale. Recent (S)TEM instrumentation developments such as electron beam aberration-correction as well as faster and more efficient signal detection systems have given rise to new and more powerful experimental methods, some of which (e.g., 4D-STEM, spectrum-imaging, in situ/operando (S)TEM)) facilitate the capture of high-dimensional datasets that contain spatially-resolved structural, spectroscopic, time- and/or stimulus-dependent information across the sub-angstrom to several micrometer length scale. Thus, through the variety of analysis methods available in the modern (S)TEM and its continual development towards high-dimensional data capture, it is well-suited to the challenge of characterizing isometric mixed-metal oxides such as pyrochlores, fluorites, and other complex oxides that reside on a continuum of chemical and spatial ordering. In this review, we present a suite of imaging and diffraction (S)TEM techniques that are uniquely suited to probe the many types, length-scales, and degrees of disorder in complex oxides, with a focus on disorder common to pyrochlores, fluorites and the expansive library of intermediate structures they may adopt. The application of these techniques to various complex oxides will be reviewed to demonstrate their capabilities and limitations in resolving the continuum of structural and chemical ordering in these systems. 

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2. Impact of thermal annealing on magnetic properties in the nanostructured compositionally complex spinel cobaltites (Mg0.2⁢Mn0.2⁢Fe0.2⁢Cu0.2⁢Zn0.2)⁢Co2⁢O4

   https://doi.org/10.1103/87mk-gdfg  

   Wang, Xin; Jorgensen, Cameron S; Kons, Corisa; Metz, Peter C; Safin, Joshua; Meier, William R; Gilbert, Dustin A; Page, Katharine; Zhang, Yuanpeng; Liu, Jue; et al (November 2025, Physical Review B)

   Exploring the controllable aspects of local atomic structure and chemical ordering and their correlations with functional properties is crucial for harnessing the potential of complex oxides in the development of advanced materials. In this work, we have investigated the sensitivity of the magnetic properties in a nanostructured metastable spinel compositionally complex oxide (CCO) composition, (Mg0.2⁢Mn0.2⁢Fe0.2⁢Cu0.2⁢Zn0.2)⁢Co2⁢O4, to local chemical segregation and phase evolution introduced through variation in post-processing heat treatment temperature. A combination of x-ray diffraction, scanning transmission electron microscopy with energy dispersive x-ray spectroscopy, first-order reversal curve (FORC) magnetometry, and neutron diffraction and total scattering analyses was employed to understand both average and local structure-property evolution. Structure analysis shows that the postannealing process triggers local and long-range cation diffusion, resulting in changes in the distribution of atoms residing on the tetrahedral and octahedral sites of the spinel structure as well as nanoscale chemical heterogeneity. FORC analysis shows that redistribution of magnetic cations induces subtle magnetic phase separation and soft to hard magnetic phase transformations, and demonstrates incipient demixing of the as-synthesized material well before detection by neutron total scattering. This work additionally highlights the necessity of a combination of advanced characterization techniques for understanding the broader crystal-chemical class of compositionally complex oxides. 

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3. Multi-Scale Microstructural Characterization of Aluminum Alloys Subject to Advanced Processing Techniques for Improving Mechanical Properties

   Wu, Yun-Hsuan (August 2025, Oregon State University)

   This work presents a multi-scale microstructural characterization of aluminum alloys processed by high-pressure torsion (HPT) and cold angular rolling process (CARP) to improve their mechanical properties. Mechanical properties such as microhardness and tensile strength were correlated with microstructural features. To understand the processing-structure-property relationships, characterization methods spanning nano- to millimeter scales were used, including X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM) EDS. TEM and STEM EDS were used to show that HPT of a Mg sheet sandwiched between Al sheets successfully produced a supersaturated solid solution (SSSS) of Mg in Al and several Al-Mg intermetallic phases, leading to significant grain refinement and increases in microhardness over pure Al. Although CARP has potential to induce the severe plastic deformation (SPD), the CARP system used in this work was not able to achieve SPD aluminum alloys. However, SEM EBSD characterization shows that CARP achieves an increase of the low-angle grain boundaries (LAGBs) and geometrically necessary dislocation (GND) density in Al-1043,which improves the mechanical properties. Moreover, a preliminary study was conducted on CAPR processed Al-6061 alloys to understand the synergistic effects precipitation and CARP-processing on the microstructure and properties. This research provides the critical insights into the capabilities and current limitations of CARP as a continuous SPD technique for aluminum alloys, and demonstrate the importance of integrated multi-scale characterization in understanding advanced materials processing. 

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4. Tuning grain boundary cation segregation with oxygen deficiency and atomic structure in a perovskite compositionally complex oxide thin film

   https://doi.org/10.1063/5.0202249  

   Guo, Huiming; Vahidi, Hasti; Kang, Hyojoo; Shah, Soham; Xu, Mingjie; Aoki, Toshihiro; Rupert, Timothy J.; Luo, Jian; Gilliard-AbdulAziz, Kandis Leslie; Bowman, William J. (April 2024, Applied Physics Letters)

   Compositionally complex oxides (CCOs) are an emerging class of materials encompassing high entropy and entropy stabilized oxides. These promising advanced materials leverage tunable chemical bond structure, lattice distortion, and chemical disorder for unprecedented properties. Grain boundary (GB) and point defect segregation to GBs are relatively understudied in CCOs even though they can govern macroscopic material properties. For example, GB segregation can govern local chemical (dis)order and point defect distribution, playing a critical role in electrochemical reaction kinetics, and charge and mass transport in solid electrolytes. However, compared with conventional oxides, GBs in multi-cation CCO systems are expected to exhibit more complex segregation phenomena and, thus, prove more difficult to tune through GB design strategies. Here, GB segregation was studied in a model perovskite CCO LaFe0.7Ni0.1Co0.1Cu0.05Pd0.05O3−x textured thin film by (sub-)atomic-resolution scanning transmission electron microscopy imaging and spectroscopy. It is found that GB segregation is correlated with cation reducibility—predicted by an Ellingham diagram—as Pd and Cu segregate to GBs rich in oxygen vacancies (VO··). Furthermore, Pd and Cu segregation is highly sensitive to the concentration and spatial distribution of VO·· along the GB plane, as well as fluctuations in atomic structure and elastic strain induced by GB local disorder, such as dislocations. This work offers a perspective of controlling segregation concentration of CCO cations to GBs by tuning reducibility of CCO cations and oxygen deficiency, which is expected to guide GB design in CCOs. 

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5. Probing Defectivity Beneath the Hydrocarbon Blanket in 2D hBN Using TEM-EELS

   https://doi.org/10.1093/mam/ozae064  

   Byrne, Dana O; Ciston, Jim; Allen, Frances I (July 2024, Microscopy and Microanalysis)

   Abstract The controlled creation and manipulation of defects in 2D materials has become increasingly popular as a means to design and tune new material functionalities. However, defect characterization by direct atomic-scale imaging is often severely limited by surface contamination due to a blanket of hydrocarbons. Thus, analysis techniques that can characterize atomic-scale defects despite the contamination layer are advantageous. In this work, we take inspiration from X-ray absorption spectroscopy and use broad-beam electron energy loss spectroscopy (EELS) to characterize defect structures in 2D hexagonal boron nitride (hBN) based on averaged fine structure in the boron K-edge. Since EELS is performed in a transmission electron microscope (TEM), imaging can be performed in-situ to assess contamination levels and other factors such as tears in the fragile 2D sheets, which can affect the spectroscopic analysis. We demonstrate the TEM-EELS technique for 2D hBN samples irradiated with different ion types and doses, finding spectral signatures indicative of boron–oxygen bonding that can be used as a measure of sample defectiveness depending on the ion beam treatment. We propose that even in cases where surface contamination has been mitigated, the averaging-based TEM-EELS technique can be useful for efficient sample surveys to support atomically resolved EELS experiments. 

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