Local Atomic Arrangement Research Articles

Probing Active Sites on Pd/Pt Alloy Nanoparticles by CO Adsorption.

We studied the adsorption of CO on Pd/Pt nanoparticles (NPs) with varying compositions using polarization-dependent Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS) and theoretical calculations (DFT). We prepared PtPd alloy NPs via physical vapor codeposition on α-Al2O3(0001) supports. Our morphological and structural characterization by scanning electron microscopy and grazing incidence X-ray diffraction revealed well-defined, epitaxial NPs. We used CO as a probe molecule to identify the particles' surface active sites. Polarization-dependent FT-IRRAS enabled us to distinguish CO adsorption on top and side facets of the NPs. The role of the Pd/Pt alloy ratio on CO adsorption was investigated by comparing the experimental CO stretching band frequency for different alloy arrangements to the results for pure Pd and Pt NPs. Moreover, we studied the influence of hydrogen adsorption on the NP surface composition. We determined the dependence of the IR bands on the local atomic arrangement via DFT calculations, revealing that both bulk alloy composition and neighboring atoms influence the wavenumber of the bands.

Structural evolution of liquid silicates under conditions in Super-Earth interiors

Molten silicates at depth are crucial for planetary evolution, yet their local structure and physical properties under extreme conditions remain elusive due to experimental challenges. In this study, we utilize in situ X-ray diffraction (XRD) at the Matter in Extreme Conditions (MEC) end-station of the Linear Coherent Linac Source (LCLS) at SLAC National Accelerator Laboratory to investigate liquid silicates. Using an ultrabright X-ray source and a high-power optical laser, we probed the local atomic arrangement of shock-compressed liquid (Mg,Fe)SiO3 with varying Fe content, at pressures from 81(9) to 385(40) GPa. We compared these findings to ab initio molecular dynamics simulations under similar conditions. Results indicate continuous densification of the O-O and Mg-Si networks beyond Earth’s interior pressure range, potentially altering melt properties at extreme conditions. This could have significant implications for early planetary evolution, leading to notable differences in differentiation processes between smaller rocky planets, such as Earth and Venus, and super-Earths, which are exoplanets with masses nearly three times that of Earth.

A Fundamental Correlative Spectroscopic Study on Li1‐xNiO2 and NaNiO2

AbstractThe intricate relationship between local atomic arrangements and electronic states significantly influences the electrochemical properties of Li‐ion battery cathode materials. Despite decades of investigation, a consensus regarding the local atomic and electronic structure of LiNiO2 remains elusive. This ambiguity stems from the potential distortion of Ni sites, either via Jahn‐Teller (JT) distortion or bond disproportionation (BD), complicating the understanding of the charge compensation mechanism involving Ni and O. This study compares the structures of LiNiO2 and NaNiO2, a JT system, using an innovative approach that integrates bulk spectroscopy techniques on standardized interoperable samples for enhanced reliability. While X‐r and theoretical calculations fail to differentiate between the proposed scenarios, Raman spectroscopy highlights local structural distinctions between monoclinic NaNiO2 and rhombohedral LiNiO2. HAXPES confirms various formal oxidation states for Ni, supported by RIXS data indicating 3d8 states, emphasizing negative charge transfer from Ni and some bond disproportionation in LiNiO2. Regarding charge compensation, XRS and RIXS suggest oxygen hole involvement in redox activity, whereas Raman spectroscopy does not detect molecular oxygen. This comprehensive spectroscopic analysis highlights the importance of correlative characterization workflows in elucidating complex structural‐electrochemical relationships.

New ion radii for oxides and oxysalts, fluorides, chlorides and nitrides.

Ion radii are derived here from the characteristic (grand mean) bond lengths for (i) 135 ions bonded to oxygen in 459 configurations (on the basis of coordination number) using 177 143 bond lengths extracted from 30 805 ordered coordination polyhedra from 9210 crystal structures; and (ii) 76 ions bonded to nitrogen in 137 configurations using 4048 bond lengths extracted from 875 ordered coordination polyhedra from 434 crystal structures. There are two broad categories of use for ion radii: (1) those methods which use the relative sizes of cation and anion radii to predict local atomic arrangements; (2) those methods which compare the radii of different cations (or the radii of different anions) to predict local atomic arrangements. There is much uncertainty with regard to the relative sizes of cations and anions, giving rise to the common failure of type (1) methods, e.g. Pauling's first rule which purports to relate the coordination adopted by cations to the radius ratio of the constituent cation and anion. Conversely, type (2) methods, which involve comparing the sizes of different cations with each other (or different anions with each other), can give very accurate predictions of site occupancies, physical properties etc. Methods belonging to type (2) can equally well use the characteristic bond lengths themselves (from which the radii are derived) in place of radii to develop correlations and predict crystal properties. Extensive quantum-mechanical calculations of electron density in crystals in the literature indicate that the radii of both cations and anions are quite variable with local arrangement, suggesting significant problems with any use of ion radii. However, the dichotomy between the experimentally derived ion radii and the quantum-mechanical calculations of electron density in crystals is removed by the recognition that ion radii are proxy variables for characteristic bond lengths in type (2) relations.

Thermodynamic, structural, surface and transport properties of Au-Cu melt

Thermodynamic, structural, surface and transport properties of Au-Cu liquid alloy were calculated on the basis of different theoretical modeling equations. The thermodynamic properties, such as excess Gibbs free energy of mixing, enthapy of mixing and activity were estimated and compared with the available experimental and literature data on the basis of simple theory of mixing (STM). The best fit values of model parameters were estimated using the experimental values of excess Gibbs free energy of mixing and enthalpy of mixing of the system at 1550 K. Using the same model parameters and frame, the concentration fluctuation in long wave-length limit and Warren-Cowley short range order parameter were calculated and analysed to understand the local arrangement of atoms in the liquid mixture. The surface tension and extent of surface segregation of atoms in the initial melt were computed using Butler’s model. In transport properties, the ratio of mutual to intrinsic diffusion coefficients was calculated using STM and viscosity was calculated using Kaptay equation. The system showed complete ordering tendency at its melting temperature.

Tailoring Local Chemical Ordering via Elemental Tuning in High-Entropy Alloys.

Due to the large multi-elemental space desired for property screening and optimization, high-entropy alloys (HEAs) hold greater potential over conventional alloys for a range of applications, such as structural materials, energy conversion, and catalysis. However, the relationship between the HEA composition and its local structural/elemental configuration is not well understood, particularly in noble-metal-based HEA nanomaterials, hindering the design and development of nano-HEAs in energy conversion and catalysis applications. Herein, we determined precise atomic-level structural and elemental arrangements in model HEAs composed of RhPtPdFeCo and RuPtPdFeCo to unveil their local characteristics. Notably, by changing just one constituent element in the HEA (Rh to Ru), we found dramatic changes in the elemental arrangement from complete random mixing to a local single elemental ordering feature. Additionally, we demonstrate that the local ordering in RuPtPdFeCo can be further controlled by varying the Ru concentration, allowing us to toggle between local Ru clustering and distinct heterostructures in multicomponent systems. Overall, our study presents a practical approach for manipulating local atomic structures and elemental arrangements in noble-metal-based HEA systems, which could provide in-depth knowledge to mechanistically understand the functionality of noble-metal-based HEA nanomaterials in practical applications.

Symmetry and order of the kinetic heterogeneity in Pd-Si amorphous alloys

In amorphous alloys, the atomic arrangement exhibits short-range order while lacking long-range order. Despite this absence of long-range order, the local atomic arrangements and interactions can still significantly influence the movement of atoms.The microstructural features and structural evolution mechanisms of amorphous materials are key areas of research, and the dynamics of amorphous alloys can provide insights into their formation processes and structural evolution. The cage effect refers to the phenomenon where atoms are trapped by their surrounding atoms, making it difficult for them to migrate or diffuse freely. This leads to slower diffusion rates and higher viscosities in these materials. Atomic concentration is one of the crucial factors that influence the structure and properties of amorphous materials. Changes in concentration can significantly alter the material’s structure. Adjusting the atomic concentration can lead to differences in the diffusion rates between elements in the amorphous alloys, resulting in a heterogeneous distribution of elements in different regions, which in turn affects the deformation characteristics of amorphous materials.This study aims to investigate the effect of Pd atomic concentration on the diffusion hindrance of Si atoms, as well as its impact on the local symmetry and order of the system. To achieve this objective, molecular dynamics simulations are employed to explore the relaxation process of atoms in Pd-Si amorphous alloys at different Pd atomic concentrations, and parameters related to atomic diffusion, displacement distribution, system symmetry, and order are analyzed. The results show that increasing the concentration of Pd atoms leads to a more pronounced hindrance in the diffusion of Si atoms, manifested by an increase in the abnormal peak values of the non-Gaussian parameters and a decrease in the standard deviation of the displacements. This indicates that a higher Pd atom concentration enhances the cage effect of Si atoms, thus restricting their diffusion. Additionally, the increase in Pd concentration promotes the transition from unsaturated to saturated bond types in the Pd-Si amorphous alloys, and also leads to a decrease in the system's configurational entropy. This consequently enhances the local symmetry and order of the Pd-Si amorphous alloys, resulting in Si atoms locate at the center of more closed, higher-symmetry, and more compact cluster structures, which strengthens their cage effect and local symmetry. This study investigates the impact of Pd atom concentration on the diffusion behavior and local environment of Si atoms, providing a new insight into the structural evolution of amorphous alloys.

Local mechanism for magnetoelastic coupling and magnetic frustration in high-pressure synthetized NixTi2S4 intercalated sulfide

The local atomic arrangement correlates to the short-range magnetic interactions mediated by electron hopping between Ni eg states, suggesting the occurrence of magnetoelastic coupling and magnetic frustration in Ni0.8Ti2S4.

Multilevel atomic structural model for interstratified opal materials

The structure of opal has long fascinated scientists. It occurs in a number of structural states, ranging from amorphous to exhibiting features of stacking disorder. Opal-CT, where C and T signify cristobalite- and tridymite-like interstratification, represents an important link in the length scales between amorphous and crystalline states. However, details about local atomic (dis)order and arrangements extending to long-range stacking faults in opal polymorphs remain incompletely understood. Here, a multilevel modeling approach is reported that considers stacking states in correlation with the abundance of C and T segments as a high-level structural parameter (i.e. not each atom). Optimization accounting for inter-tetrahedral bond lengths and angles and the regularity of the silicate tetrahedra is included as lower levels of structural parameters. Together, a set of parameters with both coarse-grained and atomistic features for different levels of structural details is refined. Structural disorder at the ∼10–100 Å distance scale is evaluated using experimental pair distribution function and diffraction datasets, comparing peak intensities, widths and asymmetry. This work presents a complete multilevel structural description of natural opal-CT and explains many of the unusual features observed in X-ray powder diffraction patterns. This modeling approach can be adopted generally for analyzing layered materials and their assembly into 3D structures.

Photoelectron holographic evidence for the incorporation site of Se and suppressed atomic displacement of the conducting layer of La(O,F)BiSSe

La(O,F)BiS2-x Se x is a layered material that is considered to be a candidate exotic superconductor as well as a promising thermoelectrical material. We performed soft X-ray photoelectron holography to study the Se incorporation site and the local atomic arrangement of the conducting layer. A comparison of the experimental holograms with the simulated holograms indicates that Se atoms preferentially occupy the S sites in the conducting Bi–S plane of La(O,F)BiS2. A comparison between the state-of-the-art holographic reconstructions of La(O,F)BiSSe and La(O,F)BiS2 suggests that Se substitution suppresses the displacement of S atoms in La(O,F)BiS2. These results provide photoelectron holographic evidence for the Se incorporation site and the Se-induced suppression of in-plane disorder.

Atomic-scale probing of short-range order and its impact on electrochemical properties in cation-disordered oxide cathodes

Chemical short-range-order has been widely noticed to dictate the electrochemical properties of Li-excess cation-disordered rocksalt oxides, a class of cathode based on earth abundant elements for next-generation high-energy-density batteries. Existence of short-range-order is normally evidenced by a diffused intensity pattern in reciprocal space, however, derivation of local atomic arrangements of short-range-order in real space is hardly possible. Here, by a combination of aberration-corrected scanning transmission electron microscopy, electron diffraction, and cluster-expansion Monte Carlo simulations, we reveal the short-range-order is a convolution of three basic types: tetrahedron, octahedron, and cube. We discover that short-range-order directly correlates with Li percolation channels, which correspondingly affects Li transport behavior. We further demonstrate that short-range-order can be effectively manipulated by anion doping or post-synthesis thermal treatment, creating new avenues for tailoring the electrochemical properties. Our results provide fundamental insights for decoding the complex relationship between local chemical ordering and properties of crystalline compounds.

Origin of spin glass magnetic behavior and phase stability of flux-grown RE(Al, Cr, Fe)B4 (RE = Ho and Er) single crystals

The YCrB4 type (α-phase, orthorhombic with space group Pbam) α-HoAlB4 and α-ErAlB4 single crystals which were self-flux grown exhibit the spin glass magnetic behavior, which is anomalous compared to the other reported rare earth system phases of α-REAlB4. Local atomic arrangements with the atomic-resolution for α-HoAlB4 and α-ErAlB4 compounds have been investigated by spherical aberration (Cs)-corrected scanning transmission electron microscopy (STEM) with high-angle annular detector dark-field (HAADF) and annular bright-field (ABF), and STEM-energy-dispersive X-ray spectroscopy (EDS) techniques to elucidate the origin of the exotic magnetic behavior. Observed HAADF- and ABF-STEM images and STEM-EDS chemical maps taken with the incident beam parallel to the [001] direction clearly indicate the existence of substitutions for Al atom sites. The phase stability of RAlB4 (R = Ho ∼ Lu) compounds was discussed from the viewpoint of crystallographic features with lattice parameters and compositions. Based on the presented results, it could be concluded that the origin of the spin glass behavior for self-flux grown α-HoAlB4 and α-ErAlB4 single crystals is the unexpected substitution of Cr/Fe atoms for Al ones. Simultaneously, the substitution by Cr/Fe atoms for Al atoms is indicated to play a significant role to stabilize the YCrB4-type α-HoAlB4 and α-ErAlB4 compounds.

Structural analysis of non-stoichiometric lithium cuprates, Li2+2xCu1-2xO2-x. Effects of lithium content and thermal treatments

Lithium cuprate (Li2CuO2) is being used for a wide range of applications due to its high lithium diffusion through the layer structure. Moreover, Li2+2xCu1-xO2-x non-stoichiometric material shows enhanced physicochemical properties. Therefore, lithium location understanding is highly important for lithium cuprate applications. This paper reports the structural coherence analysis, local and long atomic arrangement of Li2+2xCu1-xO2-x using X-ray diffraction (XRD), pair distribution function (PDF) and solid-state nuclear magnetic resonance (NMR) techniques. Li2CuO2, containing different excess quantities of lithium (from 0 to 60 at%), were synthesized by solid-state reaction. The synthesized ceramics presented nonstoichiometric structures, with Li2CuO2 type-structure. Two structural models were proposed to explain the high enhancement physicochemical properties of these ceramics; (i) the extra lithium atoms substitute copper sites, and (ii) lithium species occupy interstitial sites in the crystalline structure. Additionally, further thermal treatments rearrange the non-stoichiometric crystalline structures into the stable Li2CuO2 phase.

TimeSOAP: Tracking high-dimensional fluctuations in complex molecular systems via time variations of SOAP spectra.

Many molecular systems and physical phenomena are controlled by local fluctuations and microscopic dynamical rearrangements of the constitutive interacting units that are often difficult to detect. This is the case, for example, of phase transitions, phase equilibria, nucleation events, and defect propagation, to mention a few. A detailed comprehension of local atomic environments and of their dynamic rearrangements is essential to understand such phenomena and also to draw structure-property relationships useful to unveil how to control complex molecular systems. Considerable progress in the development of advanced structural descriptors [e.g., Smooth Overlap of Atomic Position (SOAP), etc.] has certainly enhanced the representation of atomic-scale simulations data. However, despite such efforts, local dynamic environment rearrangements still remain difficult to elucidate. Here, exploiting the structurally rich description of atomic environments of SOAP and building on the concept of time-dependent local variations, we developed a SOAP-based descriptor, TimeSOAP (τSOAP), which essentially tracks time variations in local SOAP environments surrounding each molecule (i.e., each SOAP center) along ensemble trajectories. We demonstrate how analysis of the time-series τSOAP data and of their time derivatives allows us to detect dynamic domains and track instantaneous changes of local atomic arrangements (i.e., local fluctuations) in a variety of molecular systems. The approach is simple and general, and we expect that it will help shed light on a variety of complex dynamical phenomena.

Mechanical properties and thermal stability of thin film metallic glass compared to bulk metallic glass from ambient to elevated temperatures

Metallic glasses can exhibit a wide range of local atomic arrangements which are determined by their processing history, and which control their mechanical properties. While recent studies show that thermally stable thin film metallic glasses with high strength and plasticity can be synthesized using physical vapor deposition, they often differ in terms of their structure and mechanical performance from bulk metallic glasses. In that aspect, two different fabrication methods – physical vapor deposition and arc melting – were utilized to fabricate compositionally similar Zr-based glasses in thin-film and bulk form. The as-fabricated samples were characterized in terms of oxygen concentration, element composition, and atomic structure. The mechanical response of the thin film metallic glass (TFMG) and the bulk metallic glass (BMG) was examined using in situ micropillar compression in a scanning electron microscope over a temperature range of room temperature to 500 °C. While the two glasses show overall similar failure characteristics, the TFMG exhibits higher strength together than the BMG without sacrificing plasticity at all testing temperatures. At elevated temperatures, the deformation mechanism changes to homogenous deformation above glass transition temperature (Tg). The higher strength and thermal stability of the TFMG is associated with a much higher oxygen concentration in the thin film compared to the BMG. The enhanced mechanical performance of the TFMG compared to the BMG highlights the potential of oxygen in the thermal stability characteristics and deformation capacity of Zr-based thin film metallic glasses.

Local atomic configurations in mechanically alloyed amorphous (FeCoNi)70Ti10B20 powders

The atomic structure of amorphous (FeCoNi)70Ti10B20 alloy synthesized by mechanical alloying was investigated using high energy synchrotron X-ray diffraction and inverse Monte Carlo simulations of pair distribution functions. Empirical potential structure refinement indicates a chemical short-range order at the length scales of 2.1–2.5 Å via local atomic arrangements forming deformed bcc-like clusters. The structural model obtained was described by bond lengths, coordination numbers, and bond angle distribution functions determined for the first neighbor atoms by x-ray scattering supplemented with 3D Monte Carlo simulations.

Assembly Engineering of Rh Atoms on CoAl-Layered Double Hydroxide Nanosheets for Boosting Alkaline Water Splitting

Layered double hydroxides (LDHs), especially Co-based LDHs, are regarded as a class of promising electrocatalysts toward overall water splitting, due to their good performance in oxygen evolution reaction (OER). However, inferior activity toward hydrogen evolution reaction (HER) hinders their scale-up applications. Here, a one-step hydrothermal strategy is proposed to fabricate 3D self-supported Rh-incorporated CoAl LDH (Rh/CoAl LDH) nanosheet arrays as bifunctional catalysts for overall water splitting. The thickness and lateral size of CoAl LDHs depend strongly on the introduced content of Rh atoms. Importantly, Rh incorporation distorts the local atomic arrangement owing to different atom sizes on the CoAl laminate. The strong interaction between Rh atoms and CoAl LDHs dramatically boosts the HER activity by facilitating HER kinetics and improves the OER performance simultaneously. The Rh/CoAl-300 LDH electrode with optimized composition only requires an overpotential of 71 mV to reach 10 mA/cm2 for HER and 390 mV to achieve 100 mA/cm2 for OER. The alkaline electrolyzer assembled by bifunctional Rh/CoAl-300 LDH electrodes only needs a cell voltage of 1.54 V at an overall water splitting of 10 mA/cm2.

Efficient overall water-splitting enabled by tunable electronic states of vanadium-substituted P–Co3O4

Despite the fact that phosphorus (P) partly substituted Co3O4 (P–Co3O4) has been considered as an intriguing electrocatalyst toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), the modulation of electronic property using hetero atoms to improve activities of P-rich Co3O4 catalyst remains an elusive goal. Herein, we demonstrate that the vanadium (V) doping provides the most promising strategy to significantly regulate the electronic states of P–Co3O4, thus yielding an outstanding overall water-splitting performance. A small cell voltage of 1.594 V is required to deliver the current density of 10 mA/cm2, largely outperforming the RuO2||Pt/C catalyst. It is further revealed that the added V5+ with empty 3d orbital can act as electron acceptors, which weakens the coordination of Co–O band. As a result, the electrons are inclined to transfer to the V through the O ligand, rather than to Co. We then report that the V doping could modulate the local electronic configuration and atomic arrangement of bonded Co and adjacent O atoms to enhance the electrons’ delocalization capacity of Co atoms for high electrical conductivity, as well as effectively accelerate the formation of key H∗ intermediates toward fast HER kinetics. These findings represent a new strategy for regulating the electronic properties of catalysts and are expected to open new opportunities for the development of high-performance catalysts.

Pair distribution function analysis of nano-object assemblies

The atomic pair distribution function (aPDF) analysis technique, also known as the total scattering method, which considers both Bragg and diffuse scattering, has been used extensively to probe local atomic arrangements in crystalline and disordered materials. In contrast, there have been limited applications of the PDF in self-assembled nanomaterials, which represent a class of materials built from nanoscale objects, such as nano-colloids, micelles and proteins. As distinguished from atoms, nano-objects have polydispersity in size and shape, and such form-factor effects complicate the application of PDF analysis to nano-systems. Herein, the application of the PDF is extended to spherical nano-object assemblies and the formulae for the nano-PDF (nPDF) are derived, showing some differences from the aPDF. By numerical simulations, the properties of the nPDF (peak broadening and pattern profile) are studied systematically as a function of structural features, such as nano-object parameters (size and size polydispersity) and assembly structural features (size, shape, structure type and lattice disorder), and of data processing parameters (q cut-off and `missing' data in ultra-small-angle regions). The nPDF analysis method is found to provide an effective route to revealing not only nanoscale but also mesoscale structural properties, for example the morphology of a nano-assembly.

Unveiling the local structure of the amorphous metal text {Fe}_{(1-x)}text {Zr}_x combining first-principles-based simulations and modelling of EXAFS spectra

Amorphous alloys exhibit useful properties such as the excellent soft magnetic behaviour of Fe-based metallic glasses. The detailed structure of amorphous text {Fe}_{(1-x)}text {Zr}_x with x = 0.07, 0.10, and 0.20 is in this work explored through a synergetic combination of atomistic simulations and experimental characterisation. Thin-film samples were investigated using X-ray diffraction and extended X-ray absorption fine structure (EXAFS), while the corresponding atomic structures were simulated using an efficient first-principles-based method called stochastic quenching (SQ). The simulated local atomic arrangements are investigated by constructing the radial- and angular-distribution functions, as well as by Voronoi tesselation. The radial distribution functions are then used to construct a model to fit simultaneously the experimental EXAFS data of multiple samples with different compositions, creating a simple yet accurate description of the atomic structures valid for any composition in the range x = 0.07 to 0.20, using a minimal number of free parameters. This approach significantly improves the accuracy of the fitted parameters and allows us to relate the compositional dependence of the amorphous structures with the magnetic properties. The proposed EXAFS fitting process can be generalised to other amorphous systems, contributing to the understanding of structure-property relationships and the development of amorphous alloys with tailored functional properties.