Angle Annular Dark Field Imaging Research Articles

Grain boundary junction disclinations in nanoparticles

Grain boundary junction (GBJ) disclinations are ubiquitous at the nanoscale in icosahedra and decahedra. These structural motifs consist of several tetrahedra sharing a twin grain boundary. According to the coincident site lattice (CSL) theory this twin grain boundary is referred to as Σ3<110>{111} grain boundary. In the present work, GBJ disclinations involving a low energy grain boundary Σ11<110>{113} are reported. Σ3−Σ3−Σ11 triple junction disclination is energetically preferred at the nanoscale as opposed to the ideal (without any misorientation mismatch) Σ3−Σ3−Σ9 triple junction. Molecular dynamics simulations are employed to study the disclinations in three dynamical processes key for nanoparticle synthesis − growth, coalescence, and solidification; across three metal systems Ag, Cu, and Pt which cover a range of stacking fault energies. Results indicate that Pt has a higher preference for disclinations while they are almost non-existent in Cu and to a lesser degree in Ag. Several types of disclinations have been identified and catalogued. Gaussian images (used as a proxy for simulating scanning transmission electron microscopy high angle annular dark field images) reveal that some of the disclinations induce artifacts which makes it non-trivial to identify them even when they are present in experimental particles. A key characteristic of the GBJ disclinations is the rapid change in misorientation angle around Σ11 grain boundary with distance away from the disclination. In this scenario, there is a need to reassess the methodology currently in use for identification of grain boundary type at the nanoscale.

Thermochemical interactions between Ca2Sm8(SiO4)6O2 apatite and molten calcium–magnesium-aluminosilicate (CMAS)

Thermochemical interactions between Ca2Sm8(SiO4)6O2 (CSmS) apatite and molten calcium-magnesium-aluminosilicate (CMAS) glass having CaO/SiO2 mole ratio 0.37 have been evaluated. CSmS apatite-CMAS diffusion couples were annealed at 1200, 1300, and 1400 °C for 1–50 h. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high angle annular dark field imaging, selected area electron diffraction and energy dispersive x-ray spectroscopy were used to characterize the phases formed during the reaction. Formation of a distinct cyclosilicate [Ca3Sm2(Si3O9)2] layer was observed at apatite-CMAS interface in couples heat treated at 1200 °C but not at higher temperatures. The cyclosilicate layer nucleated at the apatite-CMAS reaction front, continued to grow into the residual melt and eventually detached from the apatite substrate. Residual CMAS melt became Ca-lean when cyclosilicate was present. Formation of MgCaSi2O6 diopside and dendrites of CaSiO3 wollastonite was also observed within the residual CMAS at 1200 °C. At 1300 and 1400 °C molten CMAS, due to its much lower viscosity, infiltrated the apatite substrate through open pores and along the grain boundaries. Ca2Sm8(SiO4)6O2 apatite thermal/environmental barrier coating has the potential to provide protection against CMAS attack up to about 1200 °C but not at higher temperatures.

Atom column analysis of (Fe,Cr)2B phase in high B containing ferritic steel

This paper depicts nuances of atomistic level characterization of (Fe,Cr)2B crystal in a high B containing P91 steel using probe aberration-corrected Scanning Transmission Electron Microscope (STEM) based techniques. Substitution of Cr atoms in Fe atom positions resulted in intensity variation of atomic columns of a STEM-High Angle Annular Dark Field (HAADF) image. Image simulation has been used to understand this variation. Relative estimation of Cr atom substitution in Fe atom columns has also been quantified through the analysis of changes in peak profile intensity ratio of STEM-HAADF image in comparison with simulated images. Imaging light atoms, such as B, has recently been made possible with the advent of the Differential Phase Contrast (DPC) detectors. In this work, STEM based Integrated DPC (iDPC) has been used to obtain a very detailed image of the (Fe,Cr)2B crystal showing projection of the Fe/Cr and B atom columns along [100]. However, the observed delocalization of B atom columns was intriguing. Computational analysis using density functional theory (DFT) modelling was employed to understand the effect of Cr substitution on BB bond length with actually little or no effect. Further theoretical calculations confirmed that the energy imparted by the incident electron during imaging was responsible for the B atom delocalization.

Interactions between Ca2Gd8(SiO4)6O2 apatite and calcium-magnesium-aluminosilicate (CMAS) at elevated temperatures

High temperature corrosion behavior of Ca2Gd8(SiO4)6O2 (CGdS) apatite has been investigated in the presence of molten calcium-magnesium-aluminosilicate (CMAS) glass having the composition 21.9 CaO - 4.3 MgO - 5.4 Al2O3 - 63.0 SiO2 - 4.3 Na2O - 0.8K2O - 0.1 Fe2O3 (weight %). CGdS apatite powder was prepared by solid state synthesis from constituent oxides. Pellets of CGdS apatite + CMAS mixed powder and CGdS-CMAS diffusion couples were annealed at 1200, 1300, 1400, and 1500 °C for 1 and 20 h in ambient atmosphere. Development of phases in heat treated specimens was characterized using various analytical techniques as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high angle annular dark field imaging, selected area electron diffraction and energy dispersive X-ray spectroscopy. In both pellets and diffusion couples, monoclinic cyclosilicate Ca3Gd2(Si3O9)2 formed from reaction of apatite with CaO in the CMAS melt only in samples heat treated at 1200 °C for 1 and 20 h or at 1300 °C for 1 h. Triclinic CaSiO3 and monoclinic diopside MgCaSi2O6 were also observed in samples annealed at 1200 and 1300 °C. At 1400 and 1500 °C, because of its low viscosity, CMAS infiltrated along the pores and grain boundaries of the apatite substrates in diffusion couples. Phase compositions predicted from thermochemical computation were in good agreement with those observed experimentally. Ca2Gd8(SiO4)6O2 apatite has the potential for being an effective T/EBC in circumventing the penetration of molten CMAS up to about 1300 °C but not at higher temperatures.

Synchronization of scanning probe and pixelated sensor for image-guided diffraction microscopy

A 4-dimensional modality of a scanning transmission electron microscope (4D-STEM) acquires diffraction images formed by a coherent and focused electron beam scanning the specimen. Newly developed ultrafast detectors offer a possibility to acquire high throughput diffraction patterns at each pixel of the scan, enabling rapid tilt series acquisition for 4D-STEM tomography. Here we present a solution to the problem of synchronizing the electron probe scan with the diffraction image acquisition, and demonstrate on a fast hybrid-pixel detector camera (ARINA, DECTRIS). Image-guided tracking and autofocus corrections are handled by the freely-available microscope-control software SerialEM, in conjunction with a high angle annular dark field (HAADF) image acquired simultaneously. The open source SavvyScan system offers a versatile set of scanning patterns, operated by commercially available multi-channel acquisition and signal generator computer cards (Spectrum Instrumentation GmbH). Images are recorded only within a sub-region of the total field, so as to avoid spurious data collection during flyback and/or acceleration periods in the scan. Hence, the trigger of the fast camera follows selected pulses from the scan generator clock gated according to the chosen scan pattern. Software and protocol are provided for gating the trigger pulses via a microcontroller (ST Microelectronics ARM Cortex). We demonstrate the system on a standard replica grating and by diffraction imaging of a ferritin specimen.

Large refrigerant capacity in superparamagnetic iron nanoparticles embedded in a thin film matrix

A magnetocaloric effect (MCE) with sizable isothermal entropy change (ΔS) maintained over a broad range of temperatures above the blocking temperature is reported for a rare earth-free superparamagnetic nanoparticle system comprising of Fe–TiN heterostructure. Superparamagnetic iron (Fe) particles were embedded in a titanium nitride (TiN) thin film matrix in a TiN/Fe/TiN multilayered pattern using a pulsed laser deposition method. High angle annular dark-field images in conjunction with dispersive energy analysis, recorded using scanning transmission electron microscopy, show a clear presence of alternating layers of Fe and TiN with a distinct atomic number contrast between Fe particles and TiN. Quantitative information about the isothermal entropy change (ΔS) and the magnetocaloric effect in the multilayer Fe–TiN system has been obtained by applying Maxwell relation to the magnetization vs temperature data at various fields. With the absence of a dynamic magnetic hysteresis above the blocking temperature, the negative ΔS as high as 4.18 × 103 J/Km3 (normal or forward MCE) is obtained at 3 T at 300 K.

Effect of Ti-Hf-Zr-Cu-Ni high entropy alloy addition on laser beam welded joint of Ti2AlNb based intermetallic alloy

• In this study, LBW of Ti-22Al-27Nb was experimented through addition of an interlayer of the filler, Ti-Hf-Zr-Cu-Ni HEA, into the joint and a comparison between the LBWJs of Ti-22Al-27Nb either prepared with or without addition of the filler was established through microstructural analysis by corelating its impact on mechanical properties at room temperature. Followings are the highlights of this experimental work. • The addition of Ti-Hf-Zr-Cu-Ni HEA into FZ of LBWJ of Ti-22Al-27Nb activated heterogenous nucleation due to difference of chemical composition between the BM and the filler, resultantly, fine grained B2 phase with greater fraction of HAGBs was evolved in the FZ. Grain refinement and higher misorientation angle between GBs in the FZ contributed an increase in hardness, tensile strength and percentage elongation of HEA adulterated LBWJ of Ti-22Al-27Nb. • The average hardness of the FZ (343.5 HV) and UTS (1062 MPa) of the HEA added LBWJ was found in close agreement with that of BM, 345 HV and 1060 MPa, respectively. • The percentage elongation of the joint, prepared with an interlayer of HEA, was measured up to 11.2%, which was 83.6% of the BM. • The LBWJ of Ti-22Al-27Nb, prepared with the addition of HEA, fractured from the junction of FZ and HAZ in a ductile mode while the LBWJ prepared without addition of Ti-Hf-Zr-Cu-Ni filler was ruptured from FZ in quasi-cleavage mode. Ti 2 AlNb based intermetallic alloys, a potential competitor for next-generation super alloys, are susceptible to high-temperature embrittlement due to nucleation of a metastable single B2 phase in the fusion zone (FZ) during laser beam welding (LBW). In this study, a high entropy alloy (HEA), Ti-Hf-Zr-Cu-Ni, was self-developed and introduced as an interlayer into laser beam welded joint (LBWJ) of Ti-22Al-27Nb to analyze its impact on the evolution of microstructure in the weld zone (WZ) and subsequently on joint performance. Microstructural examination was carried out through electron probe micro analysis (EPMA), electron backscattered diffraction (EBSD) analysis, high-resolution scanning transmission electron microscopy (HRSTEM) comprising bright field (BF), selective area electron diffraction (SAED) and high angle annular dark-field (HAADF) imaging. Addition of the HEA into FZ of LBWJ triggered heterogenous nucleation during solidification, resultantly, fine-grained B2 with a greater proportion of high angle grain boundaries (HAGBs) was developed. FZ of Ti-22Al-27Nb LBWJ, prepared with an interlayer of HEA, was composed of planar, cellular, columnar and equiaxed dendritic grains; a solidification mode which was different from that observed in LBWJ prepared without adulteration of the HEA. The impact of heterogenous nucleation during epitaxial solidification on mechanical properties was established through micro vickers hardness mapping and tensile test, conducted at room temperature. The average hardness, 343.5 HV, in the FZ of LBWJ prepared with an interlayer of HEA, was compatible with that of base material (BM), 345 HV. The ultimate tensile strength (UTS), 1062 MPa, and percentage elongation, 11.2%, of the HEA tempered LBWJ were found in close approximation with that of BM, 1060 MPa and 13.4%, respectively. A ductile mode of failure was observed during tensile test of the Ti-Hf-Zr-Cu-Ni supplemented LBWJ of Ti-22Al-27Nb, while quasi-cleavage mode of fracture was apparent in the joint of Ti-22Al-27Nb welded without addition of the HEA.

Atomic-scale imaging of dopant atoms in Si-doped GaN

Determination of the concentration and distribution of trace impurity is a key factor in the manufacture of advanced semiconductor materials and is of great significance to the development of sophisticated electronics. Doping has a great influence on the performance of GaN, and a small change in the impurity content will lead to the degradation of the device performance, or even cause device failure. Determining the trace impurity and spatial distribution is therefore critical to understanding and adequately tuning functional properties. In this study, the two-dimensional distribution of Si-doped in GaN was determined by atomic resolution scanning transmission electron microscopy high angle annular dark field (STEM-HAADF) image combined with EDS mapping. Furthermore, STEM image simulation was used to estimate the distribution of trace impurity in the depth direction. This method may be useful for the determination of trace impurity concentration and distribution in semiconductor materials.

An investigation of abnormal grain growth in Zr doped CoCrFeNi HEAs through in-situ formed oxide phases

Abnormal grain growth (AGG) in nanocrystalline (CoCrFeNi)100-xZrx (x = 1 and 4 at. %) HEAs, prepared through high energy mechanical alloying, was comprehensively investigated upon annealing. Transmission electron microscopy (TEM), including high angle annular dark field imaging (HAADF) and energy dispersive spectroscopy (EDS) mapping, focused ion beam microscopy (FIB), and X-ray diffraction experiments (XRD) were utilized to investigate the microstructures as a function of added Zr content and temperature exposures. The results showed that nanocrystalline grains of the as-milled HEAs did not increase significantly upon annealing up to 700 °C as the nanocrystalline grain sizes were retained. However, grain growth was observed in (CoCrFeNi)99Zr1 after annealing at 900 °C, which turned into AGG after annealing at a higher temperature of 1100 °C, disrupting the equiaxed grain structures observed at 900 °C. Although the increased amount of Zr doping reduced the average grain size in (CoCrFeNi)96Zr4, bimodal grain structure existed in the microstructure composed of a matrix with 255 nm grain size and abnormally grown grains up to 3 μm. The observed AGG was attributed to the pinning effect of in-situ formed secondary oxide phases. The microstructural evolution as a function of Zr doping and annealing temperatures was also correlated with the microhardness test results. The AGG and bimodal grain structure reported for the Zr doped CoCrFeNi HEA may open a new avenue to produce HEAs with the enhanced strength-ductility combination due to the incorporation of larger grains and in-situ formed oxide phases in a fine-grained matrix.

Effects of Electron Microscope Parameters and Sample Thickness on High Angle Annular Dark Field Imaging.

Scanning transmission electron microscopy (STEM) developed into a very important characterization tool for atomic analysis of crystalline specimens. High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) has become one of the most powerful tools to visualize material structures at atomic resolution. However, the parameter of electron microscope and sample thickness is the important influence factors on HAADF-STEM imaging. The effect of convergence angle, spherical aberration, and defocus to HAADF imaging process has been analyzed through simulation. The applicability of two HAADF simulation software has been compared, and suggestions for their usage have been given.

Single–atom Ir1 supported on rutile TiO2 for excellent selective catalytic oxidation of ammonia

Gaseous ammonia (NH3) in the atmosphere is potentially harmful to both human health and the environment. The selective catalytic oxidation of NH3 (termed as NH3–SCO) into N2 and H2O is a promising method for decreasing NH3 emissions. A highly efficient catalyst is required for controlling NH3 emissions by this method in practice. In this study, we prepared Ir/TiO2 catalysts using different crystal structures of TiO2 (rutile, P25 or anatase) as supports by a simple impregnation method and evaluated their performance in the NH3–SCO. We found that the Ir/TiO2–R (rutile) catalyst performed better than the Ir/TiO2–P25 (mixed–phase) and Ir/TiO2–A (anatase) catalyst. High–angle annular dark–field images of the aberration–corrected scanning transmission electron microscopy revealed that the Ir species were mainly atomically dispersed on the TiO2 support in Ir/TiO2–R with 1 wt% Ir loading, whereas the Ir species agglomerated to form clusters or nanoparticles in Ir/TiO2–P25 and Ir/TiO2–A. The combined results of X–ray absorption fine structure, H2–temperature–programmed reduction, and in situ diffuse reflectance for infrared Fourier Transform spectroscopy studies suggested that atomically dispersed Ir species had stronger electronic metal–support interaction with rutile TiO2, which resulted in easier to adsorb and activate O2 at the interface and thus, better low–temperature activity of the Ir/TiO2–R catalyst.

Effects of IrO2 nanoparticle sizes on Ir/Al2O3 catalysts for the selective catalytic oxidation of ammonia

Gaseous ammonia (NH3), especially that emitted from selective catalytic reduction with NH3 (NH3–SCR) units in the atmosphere is potentially harmful to both human health and the environment. The noble metal catalytic oxidation technology placed behind the SCR unit has been considered to be a promising method for decreasing NH3 slip. In this study, we prepared Ir/Al2O3 catalysts using two different crystal structures of Al2O3 (α–Al2O3 or γ–Al2O3) as supported by an impregnation method and evaluated the performance of the selective catalytic oxidation of NH3 (NH3–SCO). We found that the Ir/γ–Al2O3 catalyst performed better than the Ir/α–Al2O3 catalysts, with 99% of NH3 converted at 260 °C. The X–ray absorption fine structure revealed that the active species on the Ir/Al2O3 catalysts are mainly oxidized IrO2. High–angle annular dark–field images of aberration–corrected scanning transmission electron microscopy revealed that IrO2 species over 1 nm in diameter are beneficial for activity. In situ diffuse reflectance for infrared Fourier transform spectroscopy studies suggested that the differences in NH3–SCO activity were caused by the activity of reactive oxygen species, and the formation of –NH2 may be the decisive step of the NH3 oxidation reaction.

Study of Lithium Disilicate Based Nano Glass Ceramics Containing P2O5

Multi component lithium disilicate based glasses containing P2O5 have been synthesized by conventional melt quenching technique. The replacement of (Li2O + SiO2) by P2O5 and its nucleating effect has been discussed. Structural features of glasses were evaluated by differential thermal analysis (DTA), Fourier transform infrared (FTIR) and Raman spectroscopy. The glass samples have been converted into glass ceramics by following three stage heat treatment schedules. X-ray diffraction analysis (XRD), field emission scanning electron microscopy (FESEM), high angle annular dark field imaging (HAADF), energy dispersive X-ray analysis (EDX) and high resolution transmission electron microscopy (HR-TEM) has been carried out for the glass ceramics. Vickers microhardness (Hv) and Vickers indentation fracture toughness (KIC) of all the glass ceramics have been measured. UV–Visible spectroscopy study has been carried out for glass ceramics to investigate the optical properties. The glass ceramic with 1.5 mol % P2O5 has the highest transmittance. Glass ceramics with P2O5 ≥ 1 mol % having Li2Si2O5 (LS2) as main phase exhibit high Hv about 6.71–6.82 GPa which is suitable for dental and armor applications.

Insight on precipitate evolution during additive manufacturing of stainless steels via in-situ heating-cooling experiments in a transmission electron microscope

During additive manufacturing of alloys, just after local heat-matter interactions, a molten material undergoes rapid solidification. Then, for the rest of the building time, it is subjected to cooling/heating cycles in the solid-state i.e., solid-state thermal cycling. The thermo-mechanical forces generated during solid-state thermal cycling can trigger a plethora of micro-mechanisms that can bring about significant microstructural changes that determine the eventual mechanical properties of as-built parts. In this work, the aim is to gain insight on solid-state thermal cycling-driven evolution of submicron-sized precipitates in an austenitic stainless steel using transmission electron microscopy. To that end, thin-film lamellae are extracted from a pre-built sample and subjected to different in-situ solid-state thermal cycles inside a transmission electron microscope. The solid-state thermal cycles are designed to understand the role of temperature amplitude and rate, number and type of thermal cycles, and post-process annealing on precipitate evolution. High angle annular dark field imaging and energy dispersive X-ray spectroscopy before and after each thermal cycle provide a deep insight on the contribution of different thermal cycling factors on the evolution of precipitate composition, size and morphology. Common trends include diffusion of Mn and Si from Mn-Si-rich oxides into the surrounding matrix, formation of Cr rings around oxide precipitates and S redistribution in non-oxide precipitates. Similar Cr rings and S distributions were also found in precipitates in as-built samples studied in (Upadhyay et al. [30]), which strongly supports the representativeness of these results with respect to what occurs during additive manufacturing.

Electron tomography of microelectronic device interconnects

Abstract As the dimensions of microelectronic devices continue to decrease, single transmission electron microscopy images are not able to properly represent the 3D structures when the structure’s curvature is comparable to the sample thickness. Electron tomography was used to study cylindrical vias coated with Ta-barrier layers and Cu-seed layers in 3D. Tomography reconstructions from both bright field images and high angle annular dark field images are presented. Fidelity of the reconstruction from single-axis and dual-axis tilt series is compared. Strategies for improving the fidelity of the reconstruction and making electron tomography practically applicable for device failure analysis of microelectronic industry are discussed.

Xenon bubbles formed by ion implantation in zirconium alloy films

The xenon content of a Zircaloy-4 thin film was quantified in a spatially resolved way using high angle annular dark field (HAADF) images and DualEELS, a type of electron energy loss spectroscopy that takes spectra from the high- and low-loss regions in quick succession. The xenon in the films was implanted using a tandem accelerator. The HAADF images show that the xenon had coalesced into bubbles. A semi-empirical standard was used created for the quantification using pre-existing xenon data and experimental data scaled using a Hartree Slater cross-section. This standard was used to calculate the number of atoms in the xenon bubbles and their densities and pressures were then calculated. In total, 244 were bubbles were analysed. The mean diameter, density and pressure across all the bubbles were 21.2 Å, 2355 kg/m3 and 5.27 GPa respectively. Most of the bubbles were gaseous xenon. The separation of the bubbles was also analysed. This work is a good demonstration of a characterisation technique for end-of-life structural materials and the technique can be easily applied to small gas bubbles in other materials.

A study on strengthening and toughening mechanism of laser beam welded joint prepared between Ti–22Al–27Nb and Ti–6Al–4V with an interlayer of Nb

In this study, laser beam welding (LBW) between Ti–22Al–27Nb and Ti–6Al–4V was experimented with and without an interlayer of Nb to analyze and compare the effect of Nb on evolution of microstructure in fusion zone (FZ) and its impact on weld joint performance at room as well as high temperature (650 °C). The nucleation and evolution of phases and crystals were examined through X-ray diffraction (XRD), electron probe microanalysis (EPMA), electron back scattered diffraction (EBSD), high resolution transmission electron microscopy (HRTEM) and high angle annular dark field (HAADF) imaging, respectively. The weld joint performance was evaluated through tensile and hardness tests. The adulteration of LBW joint between Ti–22Al–27Nb and Ti–6Al–4V through pure Nb contributed an increase of 28.70% in yield strength (YS), 10.22% in ultimate tensile strength (UTS) and 12.81% in percentage elongation. The evident reason for such an improvement in joint performance was attributed to the suppression of α′ martensite and nucleation of single phase B2 with fine grained structure, which provided greater fraction of high angle grain boundaries (HAGBs) in FZ. The LBW joint between Ti–22Al–27Nb and Ti–6Al–4V with an interlayer of Nb was ruptured from the base material (BM) Ti–22Al–27Nb during tensile straining at room temperature. All the LBW joints between Ti–22Al–27Nb and Ti–6Al–4V, either prepared with or without an interlayer of Nb, were fractured from the BM Ti–6Al–4V during tensile test at 650 °C.

Catalytic performance and mechanistic evaluation of sulfated CeO2 cubes for selective catalytic reduction of NOx with ammonia

Sulfated CeO2 cubes were prepared by the impregnation of CeO2 cubes by ammonium sulfates, and further evaluated in selective catalytic reduction of NOx with ammonia (NH3-SCR). Catalytic activity tests indicated that NOx reduction conversions and N2 selectivity of sulfated CeO2 cubes could be significantly improved compared to pure CeO2 cubes. The synthesized sulfated CeO2 cubes were further characterized by atom-resolved high angle annular dark-field (HAADF) imaging, Fourier-transform infrared spectroscopy (FTIR) by pyridine adsorption, and temperature-programmed reduction by H2 (H2-TPR). The characterization results showed that sulfates were primarily dispersed through the corners, edges, and surfaces of CeO2 cubes, and did not significantly affect the crystal structures of CeO2 cubes. Sulfation treatment could create and strengthen Brønsted acid sites originated from the protons on surface sulfates, further facilitating ammonia adsorption and activation. The kinetic data indicated that the apparent reaction order of NO, O2, and NH3 was 0.95 to 1.01, −0.01 to 0.00, and −0.18 to −0.15, respectively. It could speculate that gaseous phase NO involving in NO catalytic oxidation was the rate-determining step over sulfated CeO2 cubes for NH3-SCR reaction. The presence of NH3 slightly inhibited the SCR reaction rate due to the competitive adsorption blocking NO oxidation sites.

A novel approach for 3D morphological characterization of silica nanoparticle population through HAADF-STEM

The morphology of amorphous silica NPs in three dimensions (3D) of space is analysed using a single technique: the transmission electron microscope in Scanning Transmission Electron Microscopy with High Angle Annular Dark Field (HAADF-STEM) imaging mode. For this purpose, a method consisting in adjusting the HAADF-STEM signal by a function describing the thickness of material in a sphere has been developed to determine the height of the nano-object and tested on particles having quasi-spherical shape. This approach is suitable only in the case of a nanomaterial chemically homogeneous. A reference present in the image is required for calibrating the signal strength. This reference can be either a particle of known shape or a particle whose height was previously measured with another technique (i.e. Atomic Force Microscopy). Thus, the study of the small silica particles by HAADF-STEM highlighted their spheroidal shape but also their preferential orientation on the substrate.

Extended B-Site Vacancy Content Range and Cation Ordering in Twinned Hexagonal Perovskites Ba8Cr4-xTa4+0.6xO24.

The new 8-layer twinned hexagonal solid solution Ba8Cr4-xTa4+0.6xO24 (x = 0.0-3.0) was isolated through the aliovalent substitution of Ta5+ for Cr3+ in Ba2CrTaO6, showing the widest B-site vacancy content range among the 8-layer twinned hexagonal perovskites. Ba8Cr4-xTa4+0.6xO24 forms a simple 8-layer hexagonal perovskite structure within 0.0 ≤ x < 2.4 and a tripled 8-layer hexagonal perovskite superstructure within 2.4 ≤ x ≤ 3.0. The latter shows expanded a and b axes by times in comparison to the simple 8-layer hexagonal perovskite structure owing to the partial face-sharing octahedral (FSO) B cation ordering along the ab plane. The B-cation and vacancy distributions in the tripled superstructure were characterized by neutron and X-ray powder diffraction and further confirmed by a scanning transmission electron microscopy-high angle annular dark field imaging and intensity profile analysis. The formation of 8-layer twinned hexagonal perovskites Ba8Cr4-xTa4+0.6xO24 in an extended solid solution range can be attributed to the presence of both covalent B-B and B-O-B bonding and B-site vacancies in the FSO sites. This work provides an effective way of combining covalent B-B and B-O-B bonding and vacancy creation as well as the cationic ordering in the FSO sites to reduce electrostatic repulsion, which could further enable the stabilization of new hexagonal perovskite compounds.