Abstract
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.
Highlights
Crystalline materials are generally thought of as highly-ordered systems, this classification is not fully descriptive without a reference to the length-scale of ordering
The capabilities of (S)transmission electron microscopy (TEM) to serve this end are as follows: 1) The positions of atomic columns can be extracted with picometer precision in both CTEM and scanning TEM (STEM) imaging with the assistance of image analysis software, which enables the analysis of small atomic displacements and provides the basis for at least one of the many approaches to strain mapping presented here
2) The flexible illumination optics of (S)TEM systems offer unparalleled control over the volume of the electron probe on the specimen. This is especially useful for diffraction analysis, where the probe volume can be tuned across the sub-nanometer to tens of nanometers range to approach the length-scale of ordering/disordering in these systems, revealing new signatures of local-ordering not accessible with larger probes
Summary
Crystalline materials are generally thought of as highly-ordered systems, this classification is not fully descriptive without a reference to the length-scale of ordering. We present a suite of high spatial resolution electron microscopy characterization 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 isostructures of the minerals pyrochlore (A2B2O7, Fd 3 m) and fluorite (BO2, Fm 3 m) These isometric mixed-metal oxides represent two ends of an expansive library of intermediate structures that encompass varying degrees of disorder on their cationic and anionic sublattices (Mullens et al, 2021). Disorder is adopted during disordering transitions, often in lieu of long-range symmetry-lowering transformations away from their cubic parent phase This makes them ideal structures to host complex types of disorder (and/or local order) that can profoundly affect their functional properties (Subramanian et al, 1983; Maram et al, 2018), which are quite diverse. Exposure to extreme conditions can cause reconstructive phase transformations that result in a drastic change of structural symmetry elements (Talanov and Talanov, 2021), decomposition into other phases (Garg et al, 2008), or complete amorphization (Lian et al, 2002; Turner et al, 2017), all of which can occur homogeneously by modifying the long-range order, or heterogeneously by the formation of short- or medium-range domains of order/ disorder (Shamblin et al, 2016; O’Quinn et al, 2020)
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