Advanced electron crystallography through model-based imaging

Ultrahigh piezoelectricity and temperature stability in piezoceramics by synergistic design

Piezoceramics with both high piezoelectric properties and broad temperature usage range are highly in demand for sensor and actuator applications. Unfortunately, the trade-off relationship between two properties poses a significant challenge that remains unresolved. Herein, through combined phase boundary engineering and process engineering, we report the simultaneous achievements of substantially enhanced piezoelectric coefficient d33 (from 784 pC/N to 855 pC/N) and piezoelectric strain d33* (from 620 pm/V to 860 pm/V), and ultrahigh temperature stability (i.e., d33 and d33* change less than 7.3% and 4.6% over 25-175 °C, respectively) in Pb0.92Ba0.08[Zr0.50+xTi0.48-x(Nb0.5Sb0.5)0.02]O3 (x = 0.4) ceramics, superior to those of other typical piezoceramics. The enhanced piezoelectricity and excellent temperature stability are attributed to three synergistic effects, namely, morphotropic phase boundary concomitant with nano-domains, reduced pores, and inhibited oxygen vacancies. Therefore, our proposed strategy provides a new paradigm to boost both piezoelectricity and its temperature stability and is beneficial to both academia and industry.

Giant enhancement and quick stabilization of capacitance in antiferroelectrics by phase transition engineering

The antiferroelectric-ferroelectric phase transition is a basic principle that holds promise for antiferroelectric ceramics in high capacitance density nonlinear capacitors. So far, the property optimization based on antiferroelectric-ferroelectric transition is solely undertaken by chemical composition tailoring. Alternately, here we propose a phase transition engineering tactic by applying pulsed electric stimulus near the critical electric field, which finally results in ~54.3% enhancement and quick stabilization of capacitance density in Pb0.97La0.02(Zr0.35Sn0.55Ti0.10)O3 antiferroelectric ceramics. Ex-situ and in-situ structural characterizations show that electric stimuli can induce the charming successive structural evolution, including domain evolution from multidomain to monodomain state, and modulation period change from 7.49 to 7.73. Structure-property correlation indicates that the antiferroelectric-ferroelectric phase transition engineering mainly stems from the unexpected irreversible recovery of the modulated structures. The present findings would deepen the understanding of the structural phase transition and provoke composition-independent post-treatment property innovation in the incommensurate antiferroelectric materials and devices.

Atom counting based on Voronoi averaged STEM intensities using a crosstalk correction scheme

If quantitative scanning transmission electron microscopy is used for very precise thickness measurements with atomic resolution, it is commonly referred to as »atom counting«. Due to scattering and the finite probe extent the signal recorded in one atomic column is dependent not only on its own height but also on the height of its neighbours. Especially for thicker specimens this crosstalk effect can have significant impact on the measured intensity. If it is not appropriately accounted for in the evaluation, it can result in a deterioration of accuracy that impedes the possibility of actual atom counting. However, as the number of possible neighbour configurations can be excessively large, a comprehensive consideration of all in the evaluation reference is neigh impossible. This work proposes a method that allows for the a-posteriori reduction of crosstalk during the evaluation by algebraic means. Based on a parametric model, which is described in detail in the article, the crosstalk is expressed by an invertible matrix. Applying the inverted matrix to the measurement yields crosstalk corrected intensity values with very little computational effort. These can subsequently be evaluated by direct comparison to simple reference data. The working principle of the method is presented on the example of crystalline gold. The crosstalk parametrisation is found by fitting a model to sets of specifically created multislice simulations. The parameters are given for both aberration corrected and uncorrected STEM. Subsequently the abilities and potential of the technique are assessed in simulative studies on multiple model systems including gold nanoparticles. Overall a significant and robust improvement of the attainable precision can be demonstrated making the proposed method a promising tool for reference-based atom counting.

Multiple Local Symmetries Result in a Common Average Polar Axis in High-Strain BiFeO_{3}-Based Ceramics

For the first time, the origin of large electrostrain in pseudocubic BiFeO_{3}-based ceramics is verified with direct structural evidence backed by appropriate simulations. We employ advanced structural and microstructural characterizations of BiFeO_{3}-based ceramics that exhibit large electrostrain (>0.4%) to reveal the existence of multiple, nanoscale local symmetries, dominantly tetragonal or orthorhombic, which have a common, averaged direction of polarization over larger, meso- or microscale regions. Phase-field simulations confirm the existence of local nanoscale symmetries, thereby providing a new vision for designing high-performance lead-free ceramics for high-strain actuators.

Giant electric field-induced strain in lead-free piezoceramics

Piezoelectric actuators are indispensable over a wide range of industries for their fast response and precise displacement. Most commercial piezoelectric actuators contain lead, posing environmental challenges. We show that a giant strain (1.05%) and a large-signal piezoelectric strain coefficient (2100 picometer/volt) are achieved in strontium (Sr)-doped (K,Na)NbO₃ lead-free piezoceramics, being synthesized by the conventional solid-state reaction method without any post treatment. The underlying mechanism responsible for the ultrahigh electrostrain is the interaction between defect dipoles and domain switching. The fatigue resistance, thermal stability, and strain value (0.25%) at 20 kilovolt/centimeter are comparable with or better than those of commercial Pb(Zr,Ti)O₃-based ceramics, showing great potential for practical applications. This material may provide a lead-free alternative with a simple composition for piezoelectric actuators and a paradigm for the design of high-performance piezoelectrics.

Recent Developments of Microscopic Study for Lanthanide and Manganese Doped Luminescent Materials

Luminescent materials are indispensable for applications in lighting, displays and photovoltaics, which can transfer, absorb, store and utilize light energy. Their performance is closely related with their size and morphologies, exact atomic arrangement, and local configuration about photofunctional centers. Advanced electron microscopy-based techniques have enabled the possibility to study nanostructures with atomic resolution. Especially, with the advanced micro-electro-mechanical systems, it is able to characterize the luminescent materials at the atomic scale under various environments, providing a deep understanding of the luminescent mechanism. Accordingly, this review summarizes the recent achievements of microscopic study to directly image the microstructure and local environment of activators in lanthanide and manganese (Ln/Mn²⁺ )-doped luminescent materials, including: 1) bulk materials, the typical systems are nitride/oxynitride phosphors; and 2) nanomaterials, such as nanocrystals (hexagonal-phase NaLnF₄ and perovskite) and 2D nanosheets (Ca₂ Ta₃ O₁₀ and MoS₂ ). Finally, the challenges and limitations are highlighted, and some possible solutions to facilitate the developments of advanced luminescent materials are provided.

Discovery of electric devil's staircase in perovskite antiferroelectric

The devil's staircase, describing step-like function for two competing frequencies, is well known over a wide range of dynamic systems including Huyghens' clocks, Josephson junction, and chemical reaction. In condensed matter physics, the devil's staircase has been observed in spatially modulated structures, such as magnetic ordering. It draws widespread attentions because it plays a crucial role in the fascinating phenomena including phase-locking behaviors, commensurate-incommensurate phase transition, and spin-valve effect. Here, we report the observation of polymorphic phase transitions consisting of several steps in PbZrO₃-based system-namely, electric devil's staircase-originated from competing ferroelectric and antiferroelectric interactions. We fully characterize a specific electric dipole configuration by decomposing this competitive interaction in terms of basic structure and modulation function. Of particular interest is that the occurrence of many degenerate electric dipole configurations in devil's staircase enables superior energy storage performance. These observations are of great significance for exploring more substantive magnetic-electric correspondence and engineering practical high-power antiferroelectric capacitors.

Decoding the Double/Multiple Hysteresis Loops in Antiferroelectric Materials

Antiferroelectric materials has become one of the most promising candidates for pulsed power capacitors. The polarization versus electric-field hysteresis loop is the key electrical property for evaluating their energy-storage performance. Here, we applied in situ biasing transmission electron microscopy to decode two representative energy-storage behaviors-namely, multiple and double hysteresis loops-in (Pb,La)(Zr,Sn,Ti)O₃ system. Simultaneous structural examination and domain/defects observation establish a direct relationship between phase transitions and hysteresis loops. Sustaining a smaller period of modulated structure to a certain applied electric field and then undergoing additional transitions among varying antiferroelectric phases with increasing modulation periods before the final antiferroelectric-ferroelectric transition leads to the favorable multiple-loop configuration that realizes a high energy-storage performance. Such phenomenon is described by a model in terms of defect-driven phase transition. The distinctive mechanisms of multiple phase transition will inspire future composition-design for high switch-fielding antiferroelectric materials.

Interface Pattern Engineering in Core-Shell Upconverting Nanocrystals: Shedding Light on Critical Parameters and Consequences for the Photoluminescence Properties

Advances in controlling energy migration pathways in core-shell lanthanide (Ln)-based hetero-nanocrystals (HNCs) have relied heavily on assumptions about how optically active centers are distributed within individual HNCs. In this article, it is demonstrated that different types of interface patterns can be formed depending on shell growth conditions. Such interface patterns are not only identified but also characterized with spatial resolution ranging from the nanometer- to the atomic-scale. In the most favorable cases, atomic-scale resolved maps of individual particles are obtained. It is also demonstrated that, for the same type of core-shell architecture, the interface pattern can be engineered with thicknesses of just 1 nm up to several tens of nanometers. Total alloying between the core and shell domains is also possible when using ultra-small particles as seeds. Finally, with different types of interface patterns (same architecture and chemical composition of the core and shell domains) it is possible to modify the output color (yellow, red, and green-yellow) or change (improvement or degradation) the absolute upconversion quantum yield. The results presented in this article introduce an important paradigm shift and pave the way toward the emergence of a new generation of core-shell Ln-based HNCs with better control over their atomic-scale organization.

Modelling ADF STEM images using elliptical Gaussian peaks and its effects on the quantification of structure parameters in the presence of sample tilt

A small sample tilt away from a main zone axis orientation results in an elongation of the atomic columns in ADF STEM images. An often posed research question is therefore whether the ADF STEM image intensities of tilted nanomaterials should be quantified using a parametric imaging model consisting of elliptical rather than the currently used symmetrical peaks. To this purpose, simulated ADF STEM images corresponding to different amounts of sample tilt are studied using a parametric imaging model that consists of superimposed 2D elliptical Gaussian peaks on the one hand and symmetrical Gaussian peaks on the other hand. We investigate the quantification of structural parameters such as atomic column positions and scattering cross sections using both parametric imaging models. In this manner, we quantitatively study what can be gained from this elliptical model for quantitative ADF STEM, despite the increased parameter space and computational effort. Although a qualitative improvement can be achieved, no significant quantitative improvement in the estimated structure parameters is achieved by the elliptical model as compared to the symmetrical model. The decrease in scattering cross sections with increasing sample tilt is even identical for both types of parametric imaging models. This impedes direct comparison with zone axis image simulations. Nonetheless, we demonstrate how reliable atom-counting can still be achieved in the presence of small sample tilt.

Comprehensive Study of Li+/Ni2+ Disorder in Ni-Rich NMCs Cathodes for Li-Ion Batteries

The layered oxides LiNixMnyCozO2 (NMCs, x + y + z = 1) with high nickel content (x ≥ 0.6, Ni-rich NMCs) are promising high-energy density-positive electrode materials for Li-ion batteries. Their electrochemical properties depend on Li+/Ni2+ cation disordering originating from the proximity of the Li+ and Ni2+ ionic radii. We synthesized a series of the LiNi0.8Mn0.1Co0.1O2 NMC811 adopting two different disordering schemes: Ni for Li substitution at the Li site in the samples finally annealed in air, and close to Ni↔Li antisite disorder in the oxygen-annealed samples. The defect formation scenario was revealed with Rietveld refinement from powder X-ray diffraction data, and then the reliability of semi-quantitative parameters, such as I003/I104 integral intensity ratio and c/(2√6a) ratio of pseudocubic subcell parameters, was verified against the refined defect concentrations. The I003/I104 ratio can serve as a quantitative measure of g(NiLi) only after explicit correction of intensities for preferred orientation. Being normalized by the total scattering power of the unit cell, the I003/I104 ratio depends linearly on g(NiLi) for each disordering scheme. The c/(2√6a) ratio appears to be not reliable and cannot be used for a quantitative estimate of g(NiLi). In turn, the volume of the R3¯m unit cell correlates linearly with g(NiLi), at least for defect concentrations not exceeding 5%. The microscopy techniques such as high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and electron diffraction tomography (EDT) allow us to study the materials locally, still, there is no proper quantitative approach for comprehensive analysis of defects. In the present work, the TEM-assisted quantitative Li+/Ni2+ disordering analysis with EDT and HAADF-STEM in six Ni-rich NMC samples with various defects content is demonstrated. Noteworthy, while PXRD and EDT methods demonstrate overall defect amounts, HAADF-STEM allows us to quantitatively distinguish regions with various disordering extents. Therefore, the combination of mentioned PXRD and TEM methods gives the full picture of Li+/Ni2+ mixing defects in Ni-rich NMCs.

Solid state chemistry for developing better metal-ion batteries

Metal-ion batteries are key enablers in today’s transition from fossil fuels to renewable energy for a better planet with ingeniously designed materials being the technology driver. A central question remains how to wisely manipulate atoms to build attractive structural frameworks of better electrodes and electrolytes for the next generation of batteries. This review explains the underlying chemical principles and discusses progresses made in the rational design of electrodes/solid electrolytes by thoroughly exploiting the interplay between composition, crystal structure and electrochemical properties. We highlight the crucial role of advanced diffraction, imaging and spectroscopic characterization techniques coupled with solid state chemistry approaches for improving functionality of battery materials opening emergent directions for further studies.

The development of high performing metal-ion batteries require guidelines to build improved electrodes and electrolytes. Here, the authors review the current state-of-the-art in the rational design of battery materials by exploiting the interplay between composition, crystal structure and electrochemical properties.

Unveiling the ferrielectric nature of PbZrO3-based antiferroelectric materials

Benefitting from the reversible phase transition between antiferroelectric and ferroelectric states, antiferroelectric materials have recently received widespread attentions for energy storage applications. Antiferroelectric configuration with specific antiparallel dipoles has been used to establish antiferroelectric theories and understand its characteristic behaviors. Here, we report that the so-called antiferroelectric (Pb,La)(Zr,Sn,Ti)O₃ system is actually ferrielectric in nature. We demonstrate different ferrielectric configurations, which consists of ferroelectric ordering segments with either magnitude or angle modulation of dipoles. The ferrielectric configurations are mainly contributed from the coupling between A-cations and O-anions, and their displacement behavior is dependent largely on the chemical doping. Of particular significance is that the width and net polarization of ferroelectric ordering segments can be tailored by composition, which is linearly related to the key electrical characteristics, including switching field, remanent polarization and dielectric constant. These findings provide opportunities for comprehending structure-property correlation, developing antiferroelectric/ferrielectric theories and designing novel ferroic materials.

The large family PbZrO₃-based solid solutions are usually considered as antiferroelectric materials with specific antiparallel polarization configuration. Here, the authors demonstrate the PbZrO₃-based material has ferrielectric dipoles ordering and configure a clear structure-property relationship.

The metamorphosis of analytical chemistry

Defining analytical chemistry as the measurement of isolated compositional features in a selected study object ignores the unique perspective that analytical chemists bring to twenty-first century science and society. In this feature article, we will discuss some of the existing preconceptions and misinterpretations of analytical chemistry that occur at present and will tackle them from the more up-to-date perspective of science in the Big Data Era. This will place their influence in context while simultaneously enlarging the scope of the discipline analytical chemistry to its well-deserved prevalent position in present-day science and technology. Graphical abstract.

Structure-Property Relationships in Lanthanide-Doped Upconverting Nanocrystals: Recent Advances in Understanding Core-Shell Structures

The production of upconverting nanostructures with tailored optical properties is of major technological interest, and rapid progress toward the realization of such production has been made in recent years. Ultimately, accurate understanding of nanostructure organization will lead to design rules for accurately tailoring optical properties. Here, the context of open questions still of general importance to the upconversion and nanocrystal communities is presented, with a particular emphasis on the structure-property relationships of core-shell upconverting nanocrystals. Although the optical properties of the latter have been thoroughly investigated, little is known regarding their atomic-scale organization. Indeed, solving the atomic-scale structure of such nanomaterials is challenging because of their intrinsic nonperiodic nature. Familiar concepts of crystallography are no longer appropriate; chemical and structural modulation waves must be introduced. To reveal the exact core-shell structures, innovative characterization techniques need to be applied and developed, as discussed herein. The continued development and application of structural characterization techniques will be vital to consolidate the currently incomplete link between atomic-scale structure and upconversion properties. This will ultimately provide a valuable contribution to the emerging detailed guidelines on how to better design upconverting nanostructures to achieve given optical properties in terms of efficiency, absorption, spectral emission, and dynamics.

Recent Advances in Transmission Electron Microscopy for Materials Science at the EMAT Lab of the University of Antwerp

The rapid progress in materials science that enables the design of materials down to the nanoscale also demands characterization techniques able to analyze the materials down to the same scale, such as transmission electron microscopy. As Belgium's foremost electron microscopy group, among the largest in the world, EMAT is continuously contributing to the development of TEM techniques, such as high-resolution imaging, diffraction, electron tomography, and spectroscopies, with an emphasis on quantification and reproducibility, as well as employing TEM methodology at the highest level to solve real-world materials science problems. The lab's recent contributions are presented here together with specific case studies in order to highlight the usefulness of TEM to the advancement of materials science.

Accurate lattice parameters from 2D-periodic images for subsequent Bravais lattice type assignments

Three different algorithms, as implemented in three different computer programs, were put to the task of extracting direct space lattice parameters from four sets of synthetic images that were per design more or less periodic in two dimensions (2D). One of the test images in each set was per design free of noise and, therefore, genuinely 2D periodic so that it adhered perfectly to the constraints of a Bravais lattice type, Laue class, and plane symmetry group. Gaussian noise with a mean of zero and standard deviations of 10 and 50% of the maximal pixel intensity was added to the individual pixels of the noise-free images individually to create two more images and thereby complete the sets. The added noise broke the strict translation and site/point symmetries of the noise-free images of the four test sets so that all symmetries that existed per design turned into pseudo-symmetries of the second kind. Moreover, motif and translation-based pseudo-symmetries of the first kind, a.k.a. genuine pseudo-symmetries, and a metric specialization were present per design in the majority of the noise-free test images already. With the extraction of the lattice parameters from the images of the synthetic test sets, we assessed the robustness of the algorithms' performances in the presence of both Gaussian noise and pre-designed pseudo-symmetries. By applying three different computer programs to the same image sets, we also tested the reliability of the programs with respect to subsequent geometric inferences such as Bravais lattice type assignments. Partly due to per design existing pseudo-symmetries of the first kind, the lattice parameters that the utilized computer programs extracted in their default settings disagreed for some of the test images even in the absence of noise, i.e., in the absence of pseudo-symmetries of the second kind, for any reasonable error estimates. For the noisy images, the disagreement of the lattice parameter extraction results from the algorithms was typically more pronounced. Non-default settings and re-interpretations/re-calculations on the basis of program outputs allowed for a reduction (but not a complete elimination) of the differences in the geometric feature extraction results of the three tested algorithms. Our lattice parameter extraction results are, thus, an illustration of Kenichi Kanatani's dictum that no extraction algorithm for geometric features from images leads to definitive results because they are all aiming at an intrinsically impossible task in all real-world applications (Kanatani in Syst Comput Jpn 35:1-9, 2004). Since 2D-Bravais lattice type assignments are the natural end result of lattice parameter extractions from more or less 2D-periodic images, there is also a section in this paper that describes the intertwined metric relations/holohedral plane and point group symmetry hierarchy of the five translation symmetry types of the Euclidean plane. Because there is no definitive lattice parameter extraction algorithm, the outputs of computer programs that implemented such algorithms are also not definitive. Definitive assignments of higher symmetric Bravais lattice types to real-world images should, therefore, not be made on the basis of the numerical values of extracted lattice parameters and their error bars. Such assignments require (at the current state of affairs) arbitrarily set thresholds and are, therefore, always subjective so that they cannot claim objective definitiveness. This is the essence of Kenichi Kanatani's comments on the vast majority of computerized attempts to extract symmetries and other hierarchical geometric features from noisy images (Kanatani in IEEE Trans Pattern Anal Mach Intell 19:246-247, 1997). All there should be instead for noisy and/or genuinely pseudo-symmetric images are rankings of the relative likelihoods of classifications into higher symmetric Bravais lattice types, Laue classes, and plane symmetry groups.

Accuracy of surface strain measurements from transmission electron microscopy images of nanoparticles

Strain analysis from high-resolution transmission electron microscopy (HRTEM) images offers a convenient tool for measuring strain in materials at the atomic scale. In this paper we present a theoretical study of the precision and accuracy of surface strain measurements directly from aberration-corrected HRTEM images. We examine the influence of defocus, crystal tilt and noise, and find that absolute errors of at least 1–2% strain should be expected. The model structures include surface relaxations determined using molecular dynamics, and we show that this is important for correctly evaluating the errors introduced by image aberrations.

Three-dimensional atomic models from a single projection using Z-contrast imaging: verification by electron tomography and opportunities

In order to fully exploit structure-property relations of nanomaterials, three-dimensional (3D) characterization at the atomic scale is often required. In recent years, the resolution of electron tomography has reached the atomic scale. However, such tomography typically requires several projection images demanding substantial electron dose. A newly developed alternative circumvents this by counting the number of atoms across a single projection. These atom counts can be used to create an initial atomic model with which an energy minimization can be applied to obtain a relaxed 3D reconstruction of the nanoparticle. Here, we compare, at the atomic scale, this single projection reconstruction approach with tomography and find an excellent agreement. This new approach allows for the characterization of beam-sensitive materials or where the acquisition of a tilt series is impossible. As an example, the utility is illustrated by the 3D atomic scale characterization of a nanodumbbell on an in situ heating holder of limited tilt range.

Challenges in the structural science of materials

Articles published recently in IUCrJ continue to exemplify the developments and challenges in the structural science of materials.