Atomic-resolution Energy Dispersive X-ray Research Articles

Advanced rare earth tantalate RETaO4 (RE=Dy, Gd and Sm) with excellent oxygen/thermal barrier performance

Thermal barrier coatings (TBCs) materials with lowered thermal and oxygen ion conductivity can provide thermal and oxidative protection for high temperature hot-end components in aeronautical engines and gas turbines. The rare-earth tantalate RETaO4 (RE = Dy, Gd and Sm) ceramics with monoclinic (m) phase were successfully synthesized via spark plasma sintering. Oxygen vacancies responsible for the thermal and oxygen ion conductivities of RETaO4 were demonstrated by atomic-resolution energy dispersive X-ray and X-ray photoelectron spectroscopy. Among the three samples, DyTaO4 has excellent oxygen/thermal barrier performance. Compared to the current service thermal barrier coating material ZrO2-8 wt% Y2O3 (8 YSZ), DyTaO4 has an ultra-low oxygen ion conductivity benefiting from low oxygen vacancy concentration and strong stretching force constants. The intrinsic thermal conductivity of DyTaO4 is 68.2% less than that of 8 YSZ. Additionally, the thermal expansion rate curves indicate that the phase transformation does not happen from room temperature to 1200 °C. The above results demonstrate that high-growth rate thermally grown oxide can be retarded by creating dense DyTaO4 coating with lowered thermal and oxygen ion conductivity.

Unravelling Structure and Formation Mechanisms of Ruddlesden-Popper-Phase-like Nanodomains in Inorganic Lead Halide Perovskites.

Ultrastable CsPbBr3 nanoplates against electron beam irradiations are fabricated and nanodomains with anomalous high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) contrasts are observed within CsPbBr3 nanoplates. Atomic resolution energy dispersive X-ray spectroscopy (EDS) mapping, which requires even higher beam currents and may cause significant damages on electron beam sensitive materials, are obtained without any detectable damages or decomposition. Combining HAADF-STEM images, atomic resolution EDS mapping, and image simulations has revealed detailed structure and chemistry of the nanodomains to be induced by Ruddlesden-Popper faults (RP faults) rather than any chemical intermixing or formation of new phases. A formation mechanism is also proposed on the basis of the atomic structure of the nanodomains. This result promotes an atomic-level understanding of inorganic lead halide perovskites and may help to reveal their structure-property relationship.

Age-hardening structure and mechanism of Cu–3at%Ni–1.5 at%Si Corson alloy

The age-hardening structure and mechanism of a Cu–3at%Ni–1.5 at%Si Corson alloy were elucidated using high-resolution scanning transmission electron microscopy. The alloy samples were solution-treated at 850 °C and reached peak strength after 6 h of aging at 450 °C. The hardening structure is composed of δ′-Ni2Si precipitates, which are slightly distorted from δ-Ni2Si (oP12, Pnma, C23, Prototype: Co2Si) by the Cu matrix to increase coherence. After identifying the structure using atomic-resolution energy dispersive X-ray spectroscopy, the electron diffraction pattern could be completely indexed. The δ′-Ni2Si precipitates have stacking faults on the (004)δ'-Ni2Si planes, which result in dark bands in the transmission electron microscopy image of the precipitates and streaks in the electron diffraction pattern. No interface-like crystal habit plane or rigid boundary was observed between the δ′-Ni2Si precipitates and the Cu matrix, indicating a diffuse boundary. The δ′-Ni2Si lattice had corresponding planes that were almost parallel and approximately equal to all the planes in a particular slip system: {111}Cu<110>Cu. From their orientation relationships, the precipitates were to be coherent with the matrix. Under the conditions of this study, the precipitates grew via Ostwald ripening and supported the peak strength via precipitation hardening with the cutting mechanism.

Atomic-resolution energy dispersive X-ray spectroscopy mapping of η precipitates in an Al-Mg-Zn-Cu alloy

Energy-dispersive X-ray spectroscopy (EDS) mapping has been performed to determine the distributions of the component atoms of η-Mg(Zn,Cu)2 precipitates in an Al-Mg-Zn-Cu aluminium alloy. High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) was concomitantly employed to recognize the configurations of atomic columns. Mg and Zn maps showed their alternating atomic columns. The Cu-map revealed that the Al matrix adjacent to the interfaces of (0001)η is segregated by Cu, giving an evidence of the effect of Cu additions on the precipitation and growth of η-Mg(Zn,Cu)2 precipitates. Furthermore, the elongated-hexagonal lattice defects located inside the zig-zag stacking configurations of (21¯1¯0)η are related to the Zn-depleted zones, which are presumably occupied by Mg atoms.

Flux Growth and Superconducting Properties of (Ce,Pr)OBiS2 Single Crystals.

Ce1−xPrxOBiS2 (0. 1 ≤ x ≤ 0.9) single crystals were grown using a CsCl flux method. Their structural and physical properties were examined by X-ray diffraction, X-ray absorption, transmission electron microscopy, and electrical resistivity. All of the Ce1−xPrxOBiS2 single crystals with 0.1 ≤ x ≤ 0.9 exhibited tetragonal phase. With increasing Pr content, the a-axis and c-axis lattice parameters decreased and increased, respectively. Transmission electron microscope analysis of Ce0.1Pr0.9OBiS2 (x = 0.9) single crystal showed no stacking faults. Atomic-resolution energy dispersive X-ray spectrometry mapping revealed that Bi, Ce/Pr, O, and S occupied different crystallographic sites, while Ce and Pr randomly occupied the same sites. X-ray absorption spectra showed that an increase of the Pr ratio increased the ratio of Ce4+/Ce3+. All of the Ce1−xPrxOBiS2 crystals showed superconducting transition, with a maximum transition temperature of ~4 K at x = 0.9.

Shear deformation determined by short-range configuration of atoms in topologically close-packed crystal

Dislocation behaviors, which determine mechanical properties of materials, are generally believed to be controlled by long-range lattice translational order. For intermetallic compounds, however, their structures are often closely related to atomic environments, with the short-range configurations possibly deviating from the long-range translational order. Thus, how dislocations move in these crystals is obscure. Here, we resolve the shear deformation, dominated by atomic-environment polyhedra, in a representative topologically close-packed phase with (Cr,Ni,Al)2Nb stoichiometry, using aberration-corrected scanning transmission electron microscopy combining with atomic-resolution energy dispersive X-ray spectroscopy. Dislocations there have Burgers vectors deviating from slip planes and surprisingly move by switching between two different slip planes. Long-range diffusion and local composition variation assist the dislocation motion, as demonstrated by high temperature quasi-static and room temperature impact deformations. These discoveries demonstrate the defining role of short-range configuration in the deformation of complex structured intermetallics and shed light on the novel behavior of dislocations.

Transition-Metal Distribution in Brownmillerite Ca2FeCoO5.

Ca2Fe2-xCoxO5 (0 ≤ x ≤ 1) with higher Co content, which crystallizes in a brownmillerite-type structure, is currently one of the best oxygen-evolution-reaction (OER) catalysts. Identifying the Fe/Co occupancies at the octahedral (Oh) and tetrahedral (Td) sites in the structure is the foundation for the understanding of the role of cobalt in each site and the exploration of further improvement in the OER activity. Here, we investigate the Fe/Co distribution in Ca2FeCoO5 by means of atomic-resolution energy dispersive X-ray spectroscopy in scanning transmission electron microscopy and dynamical image simulations combined with systematic density functional theory calculations. Our careful microscopic study reveals the absence of long-range Fe/Co order within the transition-metal (TM) layers, but cobalt is slightly enriched at the Td and Oh sites in the as-synthesized (1100 °C) and 800 °C annealed for a month samples, respectively. The observed Co site preferences are interpretable from the viewpoints of TM ionic size effect and ligand field effect, which are competitive around a crossover point at a certain temperature between 800 and 1100 °C. We also elucidate that the as-synthesized sample with Co enrichment at the Td site shows the better OER activity, and the optimum annealing temperature for more OER active Ca2FeCoO5 should be higher than the crossover temperature.

Direct atomic identification of cation migration induced gradual cubic-to-hexagonal phase transition in Ge2Sb2Te5

GeTe-Sb2Te3 pseudobinary system, especially Ge2Sb2Te5 alloy, is the most desirable material to be commercialized in phase change random access memory. Directly resolving the local atomic arrangement of Ge2Sb2Te5 during intermediate steps is an effective method to understand its transition mechanism from face-centered-cubic to hexagonal phases. In this study, we provide insights into the atomic arrangement variation during face-centered-cubic to hexagonal transition process in Ge2Sb2Te5 alloy by using advanced atomic resolution energy dispersive X-ray spectroscopy. Induced by thermal annealing, randomly distributed germanium and antimony atoms would migrate to the specific (111) layer in different behaviors, and antimony atoms migrate earlier than germanium atoms during the phase transition process, gradually forming intermediate structures similar to hexagonal lattice. With the migration completed, the obtained stable hexagonal structure has a partially ordered stacking sequence described as below: -Te-Sbx/Gey-Te-Gex/Sby-Te-Gex/Sby-Te-Sbx/Gey-Te- (x > y), which is directly related to the migration process. The current visual fragments suggest a gradual transition mechanism, and guide the performance optimization of Ge2Sb2Te5 alloy.

On the quantitativeness of grain boundary chemistry using STEM EDS: A ZrO2 Σ9 model grain boundary case study

Atomic-resolution energy dispersive X-ray spectroscopy (EDS) in scanning transmission electron microscopy (STEM) has recently been shown to be a powerful approach to investigate local chemistry of nanoscale structures quantitatively. While most of the studies have been focused on the quantification of the chemical composition in bulk crystals, few were discussed on interfaces. In this study, we theoretically explored the applicability of STEM EDS for the quantification of local chemistry in grain boundaries (GBs), where the electron channeling can be dramatically changed compared with the bulk due to non-periodic atomic arrangement. We find that: (1) line scan analysis across the GBs or mapping analysis, which have been widely used for interface analysis, sometimes leads to misinterpretation of true interface chemistry. (2) Tilting the specimen, which is effective to reduce the effects of scattering, is not always useful for the quantification of GBs. (3) EDS analysis covering the whole GB structure unit, such as using a box scan, can provide true chemical information. Our study provides useful insights into characterization of interface chemistry using STEM EDS.

Probing the effect of electron channelling on atomic resolution energy dispersive X-ray quantification

Advances in microscope stability, aberration correction and detector design now make it readily possible to achieve atomic resolution energy dispersive X-ray mapping for dose resilient samples. These maps show impressive atomic-scale qualitative detail as to where the elements reside within a given sample. Unfortunately, while electron channelling is exploited to provide atomic resolution data, this very process makes the images rather more complex to interpret quantitatively than if no electron channelling occurred. Here we propose small sample tilt as a means for suppressing channelling and improving quantification of composition, whilst maintaining atomic-scale resolution. Only by knowing composition and thickness of the sample is it possible to determine the atomic configuration within each column. The effects of neighbouring atomic columns with differing composition and of residual channelling on our ability to extract exact column-by-column composition are also discussed.

Elemental and lattice-parameter mapping of binary oxide superlattices of (LaNiO3)4/(LaMnO3)2 at atomic resolution

We construct the elemental distribution and lattice strain maps from the measured atomic column positions in a (LaNiO3)4/(LaMnO3)2 superlattice over a large field of view. The correlation between the distribution of B-cations and the lattice parameter in the form of Vegard’s law is validated using atomic resolution energy dispersive x-ray spectroscopy (EDS). The maps show negligible Mn intermixing in the LaNiO3 layer, while Ni intermixing in the LaMnO3 layer improves away from the substrate interface to 9.5 atomic% from the 8th period onwards, indicating that the superlattice interfacial sharpness is established as the distance from the substrate increases. The maps allow an observation of the compositional defects of the B-sites, which is not possible by Z-contrast alone. Thus, this study demonstrates a promising approach for atomic scale correlative study of lattice strain and composition, and a method for the calibration of atomic resolution EDS maps.

Realisation of magnetically and atomically abrupt half-metal/semiconductor interface: Co2FeSi0.5Al0.5/Ge(111).

Halfmetal-semiconductor interfaces are crucial for hybrid spintronic devices. Atomically sharp interfaces with high spin polarisation are required for efficient spin injection. In this work we show that thin film of half-metallic full Heusler alloy Co2FeSi0.5Al0.5 with uniform thickness and B2 ordering can form structurally abrupt interface with Ge(111). Atomic resolution energy dispersive X-ray spectroscopy reveals that there is a small outdiffusion of Ge into specific atomic planes of the Co2FeSi0.5Al0.5 film, limited to a very narrow 1 nm interface region. First-principles calculations show that this selective outdiffusion along the Fe-Si/Al atomic planes does not change the magnetic moment of the film up to the very interface. Polarized neutron reflectivity, x-ray reflectivity and aberration-corrected electron microscopy confirm that this interface is both magnetically and structurally abrupt. Finally, using first-principles calculations we show that this experimentally realised interface structure, terminated by Co-Ge bonds, preserves the high spin polarization at the Co2FeSi0.5Al0.5/Ge interface, hence can be used as a model to study spin injection from half-metals into semiconductors.

Composition measurement in substitutionally disordered materials by atomic resolution energy dispersive X-ray spectroscopy in scanning transmission electron microscopy

The increasing use of energy dispersive X-ray spectroscopy in atomic resolution scanning transmission electron microscopy invites the question of whether its success in precision composition determination at lower magnifications can be replicated in the atomic resolution regime. In this paper, we explore, through simulation, the prospects for composition measurement via the model system of AlxGa1−xAs, discussing the approximations used in the modelling, the variability in the signal due to changes in configuration at constant composition, and the ability to distinguish between different compositions. Results are presented in such a way that the number of X-ray counts, and thus the expected variation due to counting statistics, can be gauged for a range of operating conditions.

Influence of experimental conditions on atom column visibility in energy dispersive X-ray spectroscopy

Here we report the influence of key experimental parameters on atomically resolved energy dispersive X-ray spectroscopy (EDX). In particular, we examine the role of the probe forming convergence semi-angle, sample thickness, lattice spacing, and dwell/collection time. We show that an optimum specimen-dependent probe forming convergence angle exists to maximize the signal-to-noise ratio of the atomically resolved signal in EDX mapping. Furthermore, we highlight that it can be important to select an appropriate dwell time to efficiently process the X-ray signal. These practical considerations provide insight for experimental parameters in atomic resolution energy dispersive X-ray analysis.

Atom site preference and γ′/γ mismatch strain in Ni[sbnd]Al[sbnd]Co[sbnd]Ti superalloys

The structure and chemistry of NiCoAlTi quaternary superalloys are investigated at the microstructural and atomic scale. Atom probe tomograghy (APT) is used to quantify phase compositions and elemental partitioning behavior. Using aberration corrected electron microscopy, the site occupancy of the atoms within the A3 B structure (L12–type) γ′ phase is determined via atomic resolution energy dispersive X-ray spectroscopy (EDX). Ni is observed to preferentially occupy the A sub-lattice positions for all alloys, while Al and Ti occupy the B sub-lattice. In contrast, Co's site preference changes from a random distribution to the A sub-lattice as the alloy composition changes, in agreement with theoretical predictions from literature. Finally, the lattice strain across the γ′/γ interfaces is measured as a function of alloy composition.

The study of Al3(Mn, Pd) crystal and Al–Mn–Pd decagonal quasicrystal by spherical aberration-corrected scanning transmission microscopy and atomic-resolution energy dispersive X-ray spectroscopy

An Al3Mn-type Al3(Mn, Pd) crystal and an Al–Mn–Pd decagonal quasicrystal (DQC) in an Al70Mn20Pd10 alloy are studied using a spherical aberration (Cs)-corrected scanning transmission electron microscope (STEM) with high-angle annular dark-field (HAADF) and annular bright-field (ABF) techniques, together with atomic-resolution energy dispersive X-ray spectroscopy (EDS). Mn and Pd atomic positions in the Al3(Mn, Pd) structure projected along the b-axis (pseudo-tenfold rotational axis) are represented by separate bright dots in observed HAADF-STEM images. Besides, Al as well as Mn and Pd atomic positions are represented as dark dots in ABF-STEM images. Most Mn and Pd atomic positions in the Al3(Mn, Pd) structure can be observed on atomic-resolution EDS maps. On the basis of the good correlation between the STEM images and the EDS maps, and also considering the structure of the Al3(Mn, Pd) crystal, which was determined by X-ray diffraction using a single crystal, observed HAADF and ABF-STEM images of the Al–Mn–Pd DQC have been interpreted. Pd and Mn atomic positions in the Al–Mn–Pd DQC can be detected on the observed EDS maps. It can be seen that Pd is enriched around the centre of the columnar clusters, having a decagonal section with 2 nm in diameter. It can therefore be concluded that Pd plays an important role in the stabilization of the decagonal clusters, which form the Al–Mn–Pd DQC structure.

Chemical ordering of Co and Ni in a W-(AlCoNi) crystalline approximant related to Al–Co–Ni decagonal quasicrystals studied by atomic resolution energy-dispersive X-ray spectroscopy

A W-(AlCoNi) crystalline approximant, which is closely related to Al–Co–Ni decagonal quasicrystals, in an Al72.5Co20Ni7.5 alloy has been studied by atomic resolution energy-dispersive X-ray spectroscopy (EDS), in an instrument attached to a spherical aberration (Cs)-corrected scanning transmission electron microscope. On high-resolution EDS maps of Co and Ni elements, obtained by integrating many sets of EDS data taken from undamaged areas, chemical ordering of Co and Ni is clearly detected. In the structure of the W-(AlCoNi) phase, consisting of arrangements of transition-metal (TM) atoms located at vertices of pentagonal tilings and pentagonal arrangements of mixed sites (MSs) of TM and Al atoms, Co atoms occupy the TM atom positions with the pentagonal tiling and Ni is enriched in part of the pentagonal arrangements of MSs.

Structure of a crystalline approximant related to Al–Co–Ni decagonal quasicrystals studied by spherical aberration (Cs)-corrected scanning transmission electron microscopy and atomic-resolution energy dispersive X-ray spectroscopy

Six types of decagonal quasicrystals (DQCs) found in Al–Co–Ni alloys with a wide compositional ratio of Co/Ni are considered to be stabilized by chemical ordering of Co and Ni, but the elucidation of this ordering has never been performed on alloys containing neighbouring elements Co and Ni. In order to examine the chemical ordering, an Al–Co–Ni crystalline phase, the PD3c phase, which is an important approximant for understanding the structures of ordered Al–Co–Ni DQCs, in an Al71.5Co16Ni12.5 alloy has been studied by spherical aberration (Cs)-corrected scanning transmission electron microscopy (STEM) and atomic-resolution energy dispersive X-ray spectroscopy (EDXS). From the combination of observed high-angle annular dark-field and annular bright-field STEM observations, an atomic arrangement of the crystalline approximant can be directly derived. The chemical ordering of Co and Ni in the arrangement of transition-metal (TM) atoms and mixed sites (MSs) of TM and Al atoms can be clearly detected on atomic-resolution EDXS maps obtained with the special technique. Co atoms are located at the TM atom positions arranged in pentagonal tiling with a bond length of 0.76 nm, whereas Ni is enriched in the MSs located in pentagonal tiles of Co atoms. It can be concluded that the change of an area ratio of Co-rich clusters to Ni-rich MSs produces various types of DQCs with different compositional Ni/Co ratios.

Pb5Fe3TiO11Cl: A rare example of Ti(IV) in a square pyramidal oxygen coordination

A new oxychloride Pb5Fe3TiO11Cl has been synthesized using the solid state method. Its crystal and magnetic structure was investigated in the 1.5–550K temperature range using electron diffraction, high angle annular dark field scanning transmission electron microscopy, atomic resolution energy dispersive X-ray spectroscopy, neutron and X-ray powder diffraction. At room temperature Pb5Fe3TiO11Cl crystallizes in the P4/mmm space group with the unit cell parameters a=3.91803(3)Å and c=19.3345(2)Å. Pb5Fe3TiO11Cl is a new n=4 member of the oxychloride perovskite-based homologous series An+1BnO3n−1Cl. The structure is built of truncated Pb3Fe3TiO11 quadruple perovskite blocks separated by CsCl-type Pb2Cl slabs. The perovskite blocks consist of two layers of (Fe,Ti)O6 octahedra sandwiched between two layers of (Fe,Ti)O5 square pyramids. The Ti4+ cations are preferentially located in the octahedral layers, however, the presence of a noticeable amount of Ti4+ in a five-fold coordination environment has been undoubtedly proven using neutron powder diffraction and atomic resolution compositional mapping. Pb5Fe3TiO11Cl is antiferromagnetically ordered below 450(10)K. The ordered Fe magnetic moments at 1.5K are 4.06(4)μB and 3.86(5)μB on the octahedral and square-pyramidal sites, respectively.