Local Atomic Environment Research Articles

Terahertz EPR spectroscopy using a 36-tesla high-homogeneity series-connected hybrid magnet

Electron Paramagnetic Resonance (EPR) is a powerful technique to study materials and biological samples on an atomic scale. High-field EPR in particular enables extracting very small g-anisotropies in organic radicals and half-filled 3d and 4f metal ions such as MnII (3d5) or GdIII (4f7), and resolving EPR signals from unpaired spins with very close g-values, both of which provide high-resolution details of the local atomic environment. Before the recent commissioning of the high-homogeneity Series Connected Hybrid magnet (SCH, superconducting + resistive) at the National High Magnetic Field Laboratory (NHMFL), the highest-field, high-resolution EPR spectrometer available was limited to 25 T using a purely resistive “Keck” magnet at the NHMFL. Herein, we report the first EPR experiments performed using the SCH magnet capable of reaching the field of 36 T, corresponding to an EPR frequency of 1 THz for g = 2. The magnet’s intrinsic homogeneity (25 ppm, that is 0.9 mT at 36 T over 1 cm diameter, 1 cm length cylinder) was previously established by NMR. We characterized the magnet’s temporal stability (5 ppm, which is 0.2 mT at 36 T over one-minute, the typical acquisition time) using 2,2-diphenyl-1-picrylhydrazyl (DPPH). This high resolution enables resolving the weak g-anisotropy of 1,3-bis(diphenylene)-2-phenylallyl (BDPA), Δg = 2.5 × 10–4 obtained from measurements at 932 GHz and 33 T. Subsequently, we recorded EPR spectra at multiple frequencies for two GdIII complexes with potential applications as spin labels. We demonstrated a significant reduction in line broadening in Gd[DTPA], attributed to second order zero field splitting, and a resolution enhancement of g-tensor anisotropy for Gd[sTPATCN]-SL.

Expandable Li Percolation Network: The Effects of Site Distortion in Cation-Disordered Rock-Salt Cathode Material

Cation-disordered rock-salt (DRX) materials receive intensive attention as a new class of cathode candidates for high-capacity lithium-ion batteries (LIBs). Unlike traditional layered cathode materials, DRX materials have a three-dimensional (3D) percolation network for Li+ transportation. The disordered structure poses a grand challenge to a thorough understanding of the percolation network due to its multiscale complexity. In this work, we introduce the large supercell modeling for DRX material Li1.16Ti0.37Ni0.37Nb0.10O2 (LTNNO) via the reverse Monte Carlo (RMC) method combined with neutron total scattering. Through a quantitative statistical analysis of the material's local atomic environment, we experimentally verified the existence of short-range ordering (SRO) and uncovered an element-dependent behavior of transition metal (TM) site distortion. A displacement from the original octahedral site for Ti4+ cations is pervasive throughout the DRX lattice. Density functional theory (DFT) calculations revealed that site distortions quantified by the centroid offsets could alter the migration barrier for Li+ diffusion through the tetrahedral channels, which can expand the previously proposed theoretical percolating network of Li. The estimated accessible Li content is highly consistent with the observed charging capacity. The newly developed characterization method here uncovers the expandable nature of the Li percolation network in DRX materials, which may provide valuable guidelines for the design of superior DRX materials.

Polygranular image guided atomistic reconstruction: A parametric model of pyrocarbon nanostructure

Atomistic modeling of disordered yet textured carbons is notoriously difficult even though it can prove extremely helpful in rationalizing structure–property relationships for this class of materials. In this work we introduce a polygranular image-guided atomistic reconstruction method, which allows building models with fine-tuned values of the in-plane (La) and out-of-plane (Lc) coherence lengths, and of the orientation angle (OA). Applying a parametric study of grain size and orientation distribution, a database of 210 models is presented with parameters spanning domains characteristic of high and medium textured pyrolytic carbons: 1.5–8 nm, 2–5.5 nm and 25–110°, for Lc, La and OA, respectively. A machine learning model based on a random forest regression shows that these three measurable properties can be accurately predicted from a limited set of microscopic information characterizing the distribution of local atomic environments in the models. Finally, the computed diffraction properties and high-resolution transmission electron microscopy images of a series of six models, that best match the properties of a set of well-characterized pyrocarbons, are extensively compared to experimental data, showing excellent agreement and drastically improving over former modeling studies on high textured pyrocarbons, in addition to providing the first atomistic model of a medium textured pyrocarbon.

Boosting the Hydroformylation Activity of a Rh/CeO2 Single-Atom Catalyst by Tuning Surface Deficiencies

Controlling the interactions between atomically dispersed metal atoms and the support plays significant roles in determining the activity and selectivity of single-atom catalysts. In this report, we tuned the local coordination environment of Rh single atoms on CeO2 via calcination to construct a highly active hydroformylation catalyst. Single-atom Rh/CeO2 calcined at a high temperature exhibits more oxygen vacancies, which lead to the formation of a large amount of low-coordination Rh active species that are more active for hydroformylation. Under the optimum conditions, the best Rh/CeO2 catalyst achieved a TOF at approximately 5000 h–1 with 100% aldehyde selectivity in propylene hydroformylation to butanal. In situ FTIR spectroscopy and in situ XPS characterizations provide strong evidence that Rh on 800 °C-calcined CeO2 is easily activated to form surface HRh(CO)2 active species, favoring propylene adsorption and CO insertion. This work highlights the significance of engineering metal–support interactions in tuning the hydroformylation performance of single-atom catalysts and contributes to mechanistic insights into single-atom Rh-catalyzed hydroformylation reactions.

Lightweight and effective tensor sensitivity for atomistic neural networks.

Atomistic machine learning focuses on the creation of models that obey fundamental symmetries of atomistic configurations, such as permutation, translation, and rotation invariances. In many of these schemes, translation and rotation invariance are achieved by building on scalar invariants, e.g., distances between atom pairs. There is growing interest in molecular representations that work internally with higher rank rotational tensors, e.g., vector displacements between atoms, and tensor products thereof. Here, we present a framework for extending the Hierarchically Interacting Particle Neural Network (HIP-NN) with Tensor Sensitivity information (HIP-NN-TS) from each local atomic environment. Crucially, the method employs a weight tying strategy that allows direct incorporation of many-body information while adding very few model parameters. We show that HIP-NN-TS is more accurate than HIP-NN, with negligible increase in parameter count, for several datasets and network sizes. As the dataset becomes more complex, tensor sensitivities provide greater improvements to model accuracy. In particular, HIP-NN-TS achieves a record mean absolute error of 0.927 kcalmol for conformational energy variation on the challenging COMP6 benchmark, which includes a broad set of organic molecules. We also compare the computational performance of HIP-NN-TS to HIP-NN and other models in the literature.

Inhibiting water-gas-shift reaction in the Fischer–Tropsch synthesis: Unraveling the roles of phase and facet over iron carbides

An effective method, regulating the phase and facet of FexCy catalysts, is proposed in this study to adjust the catalytic performance of WGSR/FTS and thus affect CO2 selectivity in the FTS reaction. The role of phase and facet over FexCy catalysts in regulating the reaction mechanisms and catalytic activity of WGSR/FTS in the FTS reaction are clearly unraveled. The results conclude that the local electronic environment of Fe atoms and the position of d-band center arisen from the phase and facet difference of FexCy play the determined role in the regulation of catalytic activity of WGSR/FTS over different FexCy catalysts. Among them, χ-Fe5C2(510) facet is screened out as the optimal catalyst to effectively inhibit CO2 formation and to present excellent FTS activity under the FTS conditions. This study gives out a theoretical guide for the design of FexCy catalysts with the specific phase and facet to regulate the activity of FTS and WGSR and therefore greatly decrease CO2 selectivity in the FTS reaction.

Graph theory-based structural analysis on density anomaly of silica glass

Analyzing the atomic structure of glassy materials is a tremendous challenge both experimentally and computationally, and the lack of direct, detailed insights into glass structure hinders our ability to navigate structure–property relationships. For instance, the structural origin of the density anomaly in silica glasses – the negative thermal expansion coefficient – is still poorly understood. Simulations based on molecular dynamics (MD) produce atomically resolved structures, but quantifying the role of disorder in the density anomaly is challenging. Here, we propose to use a graph-theoretical approach to assess topological differences between disordered structural arrangements from MD trajectories of silica glasses. A graph similarity metric quantifies the similarity between the covalent networks and can characterize the nature of the disordered solid, by comparing to reference crystalline solids, or with glasses in different thermodynamic states. This approach involves casting all-atom glass configurations as networks, and subsequently applying a graph-similarity metric (D-measure). Calculated D-measure values are then taken as the topological distances between two configurations. By measuring the topological distances of silica glass configurations across a range of temperatures, distinct topological features could be observed at temperatures higher than the fictive temperature. In addition, we compared topological distances between local atomic environments in the glass and crystalline silica phases. This approach suggests that more coesite-like and quartz-like local topologies emerge in silica glasses when the density is at a minimum during the heating process. Using graph-based analysis, we then found that the anomalous density behavior of silica glass could be attributed to the moderate increased formation of oxygen triclusters (oxygen coordinated with 3 silicon atoms), and possibly reduced number of larger sized cycles at the density minimum temperature.

Analysis of grain-boundary segregation of hydrogen in bcc-Fe polycrystals via a nano-polycrystalline grain-boundary model

Hydrogen embrittlement caused by hydrogen segregation at grain boundaries (GBs) is the most serious issue in the development of high-strength steels, but the mechanisms behind this process are still not well understood. The GB segregation behavior of hydrogen in body-centered cubic (bcc)-Fe polycrystals was comprehensively analyzed based on the interatomic potentials derived from first-principles calculations. Considering that the atomic structure of GBs is almost independent of the grain size, the GBs in polycrystals were modeled as nano-polycrystalline GBs with random orientations. The segregation energies of hydrogen for ∼17 million interstitial sites in this GB model were calculated. From these segregation energies, the effective segregation energy for the polycrystalline GB at thermal equilibrium under various temperature and hydrogen content conditions were determined, and the validity of the calculation method was verified by comparing the results with experimental data. The relationship between the segregation energy of hydrogen at each segregation site and the surrounding local atomic environment was used to identify the major hydrogen segregation sites at the atomic level, and the changes in the crystal structure near the GB that dominated segregation were clarified. The effective segregation energies at the polycrystalline GBs were in the range of −0.48 to −0.42 eV, which are in good agreement with the experimentally reported binding energy of hydrogen at GBs of bcc-Fe polycrystals (−0.52 eV). The major hydrogen segregation sites were octahedral sites with Voronoi volumes larger than 7.0 Å3, and the segregation energy was mainly due to the uniaxially distorted crystal structure in the short-axis direction of octahedral sites. Our findings and the developed calculation method contribute to the understanding of the hydrogen segregation behavior and hydrogen embrittlement mechanism in polycrystalline metallic materials.

Local structure and energy transfer of Gd3+ to Dy3+ in phosphate-based glasses

The phosphate-based glasses doped with Gd3+ and Dy3+ ions were fabricated by melt quenching at 1200 °C. The physical, structural, and luminescence properties that would suggest they are potential candidates for scintillating materials have been examined. The density and molar volume of glasses increase with increasing Gd3+ concentration. The absorption peaks indicate the Dy3+ energy transition from the 6H15/2 ground state to several excited states such as 6P7/2 (350 nm), 6P3/2 (364 nm), 6F7/2 (387 nm), 6G11/2 (426 nm), 6I15/2 (452 nm), 6F5/2 (807 nm), 6F7/2 (906 nm), 6F9/2 (1100 nm), 6H9/2 (1285 nm), and 6H11/2 (1697 nm), respectively. According to the findings, when compared to a commercially available BGO crystal, the highest value of x-ray luminescence spectrum intensity was observed at 17 mol% of Gd2O3, with 15.42% of the integral scintillation efficiency. However, the photoluminescence spectra show the maximum value at 13 mol% of Gd2O3 content with excited at 275 nm and 350 nm, while the decay time (τ) tends to decrease with increasing Gd2O3 content. Interestingly, the local structure study was conducted through XANES and EXAFS analysis to understand the local environment of Gd and Dy atoms. The XANES result indicated that the oxidation states of the Gd and Dy ions in the sample glass were +3. The EXAFS fitting parameters showed Debye-Waller factors (σ2) of 17 mol% of Gd2O3 as the highest value that represents a more asymmetric environment around Gd/Dy. Moreover, the EXAFS fitting indicates a short distance of Gd–Gd at 13 mol% Gd2O3 content is indicative of their efficient internal energy transfer between Gd–Gd.

Synchrotron X-ray absorption spectroscopy and fluorescence spectroscopy studies of collagen type I-talc intercalated nanocomposites

We investigated the properties of collagen type I-talc intercalated nanocomposites using different techniques. Understanding the conformational changes of collagen induced by clay is noticeably lacking, which is pivotal for understanding the structure of collagen-clay hybrid nanocomposites. We aim to investigate the influence of talc clay minerals on the microstructure of collagen type I molecules. Our main goal is to understand the interaction between talc and collagen molecules and to elucidate the microstructure–property relation of intercalated hybrid nanocomposites. The FT-IR spectrum of collagen type I-talc confirmed the presence of collagen amide bands and talc silicate bonds. Also, the differential scanning calorimetry (DSC) result showed that the addition of talc to the matrix could increase the denaturation temperature (Td = 40.8 °C) compared to pure collagen (Td = 19 °C). The fluorescence intensity increased in collagen type I-talc nanocomposites, indicating that the interaction of talc with collagen chains in collagen type I-talc dispersion varies the micro-environment of the tyrosine amino acids in the collagen fibrils and its fluorescence. We also analyzed X-ray absorption spectroscopy (XAS) in total fluorescence yield (TFY) and total energy yield (TEY) modes of pure collagen and collagen type I-talc nanocomposites prepared at different concentrations of talc and at different temperatures (4 °C, 8 °C, and 14 °C). We have found that the synthesis conditions have a considerable effect on the local atomic environments of nitrogen, carbon, and oxygen atoms in collagen type I-talc nanocomposites. These results are of significance for developing new applications of talc and collagen-based biomaterials in different fields, such as cellular and molecular biology, and drug delivery that enhances the optical signals are needed.

Uncertainty-aware predictions of molecular x-ray absorption spectra using neural network ensembles

As machine learning (ML) methods continue to be applied to a broad scope of problems in the physical sciences, uncertainty quantification is becoming correspondingly more important for their robust application. Uncertainty-aware machine learning methods have been used in select applications, but largely for scalar properties. In this work, we showcase an exemplary study in which neural network ensembles are used to predict the x-ray absorption spectra of small molecules, as well as their pointwise uncertainty, from local atomic environments. The performance of the resulting surrogate clearly demonstrates quantitative correlation between errors relative to ground truth and the predicted uncertainty estimates. Significantly, the model provides an upper bound on the expected error. Specifically, an important quality of this uncertainty-aware model is that it can indicate when the model is predicting on out-of-sample data. This allows for its integration with large-scale sampling of structures together with active learning or other techniques for structure refinement. Additionally, our models can be generalized to larger molecules than those used for training, and also successfully track uncertainty due to random distortions in test molecules. While we demonstrate this workflow on a specific example, ensemble learning is completely general. We believe it could have significant impact on ML-enabled forward modeling of a broad array of molecular and materials properties.

Structure and crystallization behavior of binary silver tellurite glasses and glass ceramics

Using Raman spectroscopy in conjunction with X-ray diffraction (XRD), transmission electron micrographs (TEM-EDP), and scanning electron microscopy (SEM), the local environment of Te atoms and the crystallization behavior of glasses in the system of chemical formula xAg2O. (100 − x)TeO2 (25 ≤ x ≤ 50 mol%) have been examined. Crystalline structure was evident in the XRD spectra of glasses containing 40 and 50 mol% TeO2. On the other hand, an amorphous structure has been observed in glasses with lower Ag2O concentrations (25, 30, and 35 mol%). Thermal heating could be used to crystallize the amorphous glasses’ structure. Using DSC measurements, the treatment's temperature was controlled. The species with good crystallinity include Ag2Te4O9 and Ag2Te2O5. The results of TEM and EDP, as well as both SEM and XRD, revealed that in glasses enriched with Ag2O, crystalline clustered species were formed. Raman data proved that the crystalline clustered is improved as a result of the formation of TeO3 units enriched with nonbridging oxygen bonds. In compositions containing less than 40 mol%, Ag2O plays the role of a glass modifier. At higher Ag2O concentrations, it plays the role of building crystalline clusters of the Ag2Te4O9 and Ag2Te2O5 types.

First-principles study of Xe behavior in δ-UZr2

The main problem hindering the application of uranium-zirconium (U-Zr) fuel is the swelling caused by the generation of fission gas. In this study, investigating the behavior of fission gas xenon (Xe) in δ-UZr2 using the first-principles method is the focus. The formation energies of substitutional and interstitial Xe atoms are significantly dependent on the local atomic environment. According to the present study, the Xe formation energy increases as the number of coordinated U atoms increases. δ-UZr2 exhibits a more remarkable ability to capture Xe than α-U due to the lower Xe formation energies. In addition, the interstitial Xe energetically prefers to occupy octahedral sites or form a dumbbell with a metallic atom at the 2d site along the 〈0001〉 direction. The diffusion of interstitial Xe also relies on the local environment, while Zr-rich conditions can enhance it. Compared with α-U, Xe atoms energetically preferers to form clusters in δ-UZr2

Prediction of mechanical properties of AlTiCrVNb high entropy alloys with B2 ordered structure

Chemical short-range order (CSRO) is widely reported and it's certainly related to high entropy alloys (HEAs)'s mechanical properties in many different correlative researches. B2 structure ordering is usually thought to improve alloy's structural stability and strength while sacrificing the plasticity significantly. In this work, we established a B2 ordered model by the special quasi-random structure method which considering the local atomic environment (to simulate the real situation in HEAs). Depending on this model and first principles, the elements' occupancy and the B2 structure ordering are considered to explore their influence on the mechanical properties of AlTiCrVNb HEAs. The B2-type partially ordered configuration was applied to non-iso atomic ratio alloys to investigate the different influence on alloys' mechanical properties due to the changing content of Ti or Cr and the results were verified by experiment.

Evaluating some design criteria for TiFe-based ternary hydrogen storage alloys

Elemental substitution is widely practised to fabricate TiFe-based alloys with enhanced hydrogen storage characteristics. While some hydrogen storage properties can be improved others are usually adversely affected by this method. Hence, establishing sensible alloying design criteria is important for further developing Ti-Fe-based alloys. Here, the effects of substitutional elements in Ti-Fe-based ternary alloys are assessed based on several factors such as hydrogen bond dissociation energy, lattice parameters, interstitial site size, and intermetallic phase formation. Our calculations show that all the substitutional elements studied in this work (except Ni and Co) increase the lattice parameter compared to the TiFe alloy, while they have a diverse effect on the interstitial site sizes. The thermodynamic stability of the alloys could not be adequately described based on the lattice parameter and factors such as interstitial site sizes along the local atomic environment should be also considered. Hydrogen storage capacity of the Ti-Fe-based ternary alloys also appears to be influenced by the metal-hydrogen bond dissociation energy of the substitutional elements and their capability to form intermetallic phases with Ti. TiFe-based ternary alloys were modified by V and Zr substitutions according to the above criteria. The modified alloys were mostly composed of the B2 TiFe phase, offering significantly easier activation and a higher reversible hydrogen storage capacity compared to the binary alloy.

Influence of Cr-content on the local atomic structure of Fe–Cr alloy, a study using EXAFS

High Cr ferritic/martensitic steels are considered one of the most promising structural materials for advanced nuclear power plants. Therefore, understanding properties' evolution under operation conditions is of primary importance. Neutron scattering data and more recent ab initio calculations reveal that magnetism controls the transition from the local short-range ordering of the Cr clustering inside the alloy. The transition coincides with the change in the alloy heat of formation, which shifts from a small area with negative values to a region of positive values at around 10% Cr. This work aims to investigate the effect of Cr composition on the local atomic environment of Fe–Cr alloys. This work employed EXAFS to probe the local environment of Fe–Cr alloys as a function of Cr composition. The results show the anomaly (softening) of the nearest neighbor distance around Cr atom in the 10%Cr. Furthermore, the local atomic disorder around Cr absorbing atoms is larger than Fe atoms.

Momentum-resolved electronic structure of LaTiO2N photocatalysts by resonant Soft-X-ray ARPES

Oxynitrides are promising materials for visible light-driven water splitting. However, limited information regarding their electron-momentum resolved electronic structure exists. Here, with the advantage of the enhanced probing depth and chemical state specificity of soft-X-ray ARPES, we determine the electronic structure of the photocatalyst oxynitride LaTiO2N and monitor its evolution as a consequence of the oxygen evolution reaction. After the photoelectrochemical reactions, we observe a partial loss of Ti- and La-N 2p states, distortions surrounding the local environment of titanium atoms and, unexpectedly, an indication of an electron accumulation layer at or near the surface, which may be connected with either a large density of metallic surface states or downward band bending. The distortions and defects associated with the titanium 3d states lead to the trapping of electrons and charge recombination, which is a major limitation for the oxynitride LaTiO2N. The presence of an accumulation layer and its evolution suggests complex mechanisms of the photoelectrochemical reaction, especially in cases where co-catalysts or passivation layers are used.

Local ordering in Ge/Ge–Sn semiconductor alloy core/shell nanowires revealed by extended x-ray absorption fine structure (EXAFS)

Short-range atomic order in semiconductor alloys is a relatively unexplored topic that may promote design of new materials with unexpected properties. Here, local atomic ordering is investigated in Ge–Sn alloys, a group-IV system that is attractive for its enhanced optoelectronic properties achievable via a direct gap for Sn concentrations exceeding ≈10 at. %. The substantial misfit strain imposed on Ge–Sn thin films during growth on bulk Si or Ge substrates can induce defect formation; however, misfit strain can be accommodated by growing Ge–Sn alloy films on Ge nanowires, which effectively act as elastically compliant substrates. In this work, Ge core/Ge1−xSnx (x ≈ 0.1) shell nanowires were characterized with extended x-ray absorption fine structure (EXAFS) to elucidate their local atomic environment. Simultaneous fitting of high-quality EXAFS data collected at both the Ge K-edge and the Sn K-edge reveals a large (≈ 40%) deficiency of Sn in the first coordination shell around a Sn atom relative to a random alloy, thereby providing the first direct experimental evidence of significant short-range order in this semiconductor alloy system. Comparison of path length data from the EXAFS measurements with density functional theory simulations provides alloy atomic structures consistent with this conclusion.

Nature of the Local Environment of Atoms in Ge3Sb2Te6, Ge2Sb2Te5, GeSb2Te4, and GeSb4Te7 Amorphous and Crystalline Films

Nature of the Local Environment of Atoms in Ge3Sb2Te6, Ge2Sb2Te5, GeSb2Te4, and GeSb4Te7 Amorphous and Crystalline Films

Versatile domain mapping of scanning electron nanobeam diffraction datasets utilising variational autoencoders

Characterisation of structure across the nanometre scale is key to bridging the gap between the local atomic environment and macro-scale and can be achieved by means of scanning electron nanobeam diffraction (SEND). As a technique, SEND allows for a broad range of samples, due to being relatively tolerant of specimen thickness with low electron dosage. This, coupled with the capacity for automation of data collection over wide areas, allows for statistically representative probing of the microstructure. This paper outlines a versatile, data-driven approach for producing domain maps, and a statistical approach for assessing their applicability. The workflow utilises a Variational AutoEncoder to identify the sources of variance in the diffraction signal, and this, in combination with clustering techniques, is used to produce domain maps. This approach is agnostic to domain crystallinity, requires no prior knowledge of crystal structure, and does not require simulation of a library of expected diffraction patterns.