Reverse Monte Carlo Modeling Research Articles

Local cation order and ferrimagnetism in compositionally complex spinel ferrites

We present an exploration of a family of compositionally complex cubic spinel ferrites featuring combinations of Mg, Fe, Co, Ni, Cu, Mn, and Zn cations, systematically investigating the average and local atomic structures, chemical short-range order, magnetic spin configurations, and magnetic properties. All compositions result in ferrimagnetic average structures with extremely similar local bonding environments; however, the samples display varying degrees of cation inversion and, therefore, differing apparent bulk magnetization. Additionally, first-order reversal curve analysis of the magnetic reversal behavior indicates varying degrees of magnetic ordering and interactions, including potentially local frustration. Finally, reverse Monte Carlo modeling of the spin orientation demonstrates a relationship between the degree of cation inversion and the spin collinearity. Collectively, these observations correlate with differences in synthesis procedures. This work provides a framework for understanding magnetic behavior reported for “high-entropy spinels,” revealing many are likely compositionally complex oxides with differing degrees of chemical short-range order—not meeting the community established criteria for high or medium entropy compounds. Moreover, this work highlights the importance of reporting complete sample processing histories and investigating local to long-range atomic arrangements when evaluating potential entropic mixing effects and assumed property correlations in high entropy materials.

Ferroelectric Ordering in Nanosized PbTiO3.

The insight into the three-dimensional configuration of ferroelectric ordering in ferroelectric nanomaterials is motivated by the application of the development of functional nanodevices and the structural designing. However, the atomic deciphering of the spatial distribution of ordered structure remains challenging for the limitation of dimension and probing techniques. In this paper, a neutron pair distribution function (nPDF) was utilized to analyze the spontaneous polarization distribution of zero-dimensional PbTiO3 nanoparticles in three dimensions, via the application of reverse Monte Carlo (RMC) modeling. The comprehensive identification with transmission electron microscopy verified the linear characteristics of polarization along the c-axis in the main body, while electric polarization distribution on the surface was enhanced abnormally. In addition, the correlation of dipole vectors extending to three unit cells below the surface is retained. This work shows an application of the micro/macroscale information to effectively decode the polarization structure of nanoferroelectrics, providing new views of designing nanoferroelectric devices.

On the reliability of using reverse Monte Carlo simulations to construct the atomic structure model of metallic glasses

Reverse Monte Carlo (RMC) simulations have been widely utilized to generate three-dimensional structural models for amorphous materials. One practical case is the modeling of atomic-level structure in metallic glasses, based on, for example, X-ray scattering and extended x-ray absorption fine structure (EXAFS) data. However, to what extent this approach can faithfully mimic the atomic structure of real metallic glasses has not been quantitatively evaluated. Using the “known” configurations from molecular dynamics simulations as the benchmark, we evaluate the reliability of RMC modeling with the total structure factor and EXAFS as the input for RMC fitting. The results indicate that the RMC-generated configuration lacks accuracy in reconstructing the local atomic packing, which tends towards the most random (and thus more energetically unfavored) state that can reproduce the reference data. To help guide towards a physically stable and meaningful atomic configuration, we advise the incorporation of additional constraints and validation check points.

Structure of an amorphous calcium carbonate phase involved in the formation of Pinctada margaritifera shells.

Some mollusc shells are formed from an amorphous calcium carbonate (ACC) compound, which further transforms into a crystalline material. The transformation mechanism is not fully understood but is however crucial to develop bioinspired synthetic biomineralization strategies or accurate marine biomineral proxies for geoscience. The difficulty arises from the simultaneous presence of crystalline and amorphous compounds in the shell, which complicates the selective experimental characterization of the amorphous fraction. Here, we use nanobeam X-ray total scattering together with an approach to separate crystalline and amorphous scattering contributions to obtain the spatially resolved atomic pair distribution function (PDF). We resolve three distinct amorphous calcium carbonate compounds, present in the shell of Pinctada margaritifera and attributed to: interprismatic periostracum, young mineralizing units, and mature mineralizing units. From this, we extract accurate bond parameters by reverse Monte Carlo (RMC) modeling of the PDF. This shows that the three amorphous compounds differ mostly in their Ca-O nearest-neighbor atom pair distance. Further characterization with conventional spectroscopic techniques unveils the presence of Mg in the shell and shows Mg-calcite in the final, crystallized shell. In line with recent literature, we propose that the amorphous-to-crystal transition is mediated by the presence of Mg. The transition occurs through the decomposition of the initial Mg-rich precursor into a second Mg-poor ACC compound before forming a crystal.

Stochastic atomic modeling and optimization with fullrmc

Understanding materials' atomic structure with a high level of confidence and certainty is often regarded as a very arduous and sometimes impossible task, especially for newer, emerging technology materials exhibiting limited long-range order. Nevertheless, information about atomic structural properties is very valuable for materials science and synthesis. For non-crystalline amorphous and nanoscale materials, using conventional structural determination methods is impossible. Reverse Monte Carlo (RMC) modeling is commonly used to derive models of materials from experimental diffraction data. Here, the latest developments in the fullrmc software package are discussed. Despite its name, fullrmc provides a very flexible modeling framework for solving atomic structures with many methods beyond RMC. The stochastic nature of fullrmc allows it to explore all possible dimensions and degrees of freedom for atomic modeling and create statistical solutions to match measurements. Differing versions of fullrmc are provided as open source or for cloud computing access. The latter includes a modern web-based graphical user interface that incorporates advanced computing and structure-building modules and machine-learning-based components. The main features of fullrmc are presented, including constraint types, boundary conditions, density shape functions and the two running modes: stochastic using a Monte Carlo algorithm and optimization using a genetic algorithm. Capabilities include tools for statistical, mesoscopic and nanoscopic approaches, atomic or coarse-grained models, and smart artificial-intelligence-ready loss functions.

Reverse Monte Carlo Modeling for Catalyst Nanoparticles in Polymer Electrolyte Fuel Cells By High-Energy X-Ray Diffraction Measurement

Polymer electrolyte fuel cells (PEFCs) consist of a gas diffusion layer (GDL) and an electrode on each side, and a polymer electrolyte membrane (MEA) between the electrodes. The main components of the MEA are catalysts, electrolyte membranes and electrodes. In this study, the local atomic structures of catalytic nanoparticles in MEA were investigated by high-energy X-ray diffraction measurement and reverse Monte Carlo (RMC) modeling. The purpose of this study is to show guidelines for improving the performance of the PEFCs that contribute to industry by clarifying the differences in each catalyst structure and linking them with catalyst performance such as catalytic activity and catalyst durability.In this conference, we report the structural work on the platinum nanoparticles-supported carbon catalyst TEC10V30E (manufactured by TANAKA KIKINZOKU KOGYO K.K.). Although this sample is a commercial product, it is important to investigate its structure because it can be used as a standard sample for evaluating various catalyst samples in the future.RMC modeling based on the structure factor S(Q) obtained from high-energy X-ray diffraction measurement is suitable to determine the 3-dimensional (3D) atomic structures of nanoparticles. High-energy X-ray diffraction measurements were performed using a two-axis diffractometer at the beamline BL04B2 in SPring-8; the energy was used 61.4 keV corresponding to 0.202 Å in wavelength. RMC simulations were performed to obtain structure information from RMC-generated 3D configuration models using RMC_POT software in the case of non-periodic boundary conditions for experimental structure factor data. Figure 1 shows the experimental structure factor of the Pt nanoparticles in TEC10V30E and the RMC-simulated structure factor. For the Pt nanoparticle studied here, the initial configuration of the RMC simulations is a spherical shape with 3.8 nm in diameter based on the fcc bulk crystal structure. The agreement between the experiment and the RMC-simulated S(Q) is satisfactory. The 3D structure model of the Pt nanoparticle obtained by RMC simulations is shown in Fig. 1. It was found that the deviation from the lattice position was larger on the surface than on the inside in the obtained structure model.This research approach is expected to accelerate the creation of new catalysts for fuel cells by providing local atomic-scale structural information. It is also important to create the database of atomic arrangement structure data that is the basis of material design and material research.In the meeting, data recorded from actual catalyst samples will also be presented, including those from the MIRAI fuel cell (manufactured by Toyota Motor Corp.).This work was performed under the NEDO FC-Platform project. Figure 1

Characterization of the Local Atomic Structure of ALD Coated Interfaces

To accelerate technological innovations using atomic layer deposition (ALD) coatings, we need to establish better understanding of structure-property relationships for ALD films. Historically, the ALD community has had difficulty connecting the atomic structure of ALD films with their performance, largely because of the significant challenge in determining the atomic-scale structure of ultrathin ALD films that are often amorphous, polycrystalline, or defective. In this work, we describe a series of recent efforts that collectively aim to improve understanding of the atomic structure of ALD coatings and inform structure-property understanding. These efforts employ experimental measurements including (a) inert-transfer XPS, (b) in-situ synchrotron high energy X-ray diffraction, and (c) cryogenic electron diffraction. Of particular focus in this talk is the use of X-ray and electron diffraction measurements in combination with pair distribution function (PDF) analysis and reverse Monte Carlo (RMC) structural modeling. We describe a newly developed approach employing alternating RMC and molecular statics steps (RMC-MS) that improves the physical accuracy of model structures derived from experimental diffraction data with minimal additional computational cost over conventional RMC modeling. We also describe efforts using localized cryogenic electron diffraction and PDF analysis (cryo-ePDF) with a ≤ 5 nm spot size to measure atomic structure at ALD interfaces. Together, these advances allow us to quantify differences in the atomic structure as a function of position through the depth of ALD films. A simple two-phase model comprised of bulk and interfacial layers with distinct atomic structure features is consistent with our measurements for both aluminum oxide and zinc oxide ALD coatings. The approaches we report yield atomic structure models that can be used to inform computational studies of the properties if ALD films and will aid in the selection and modification of ALD coating chemistries to address technological needs.

Relationship between atomic structure and excellent glass forming ability in Pd[formula omitted]Ni[formula omitted]Cu[formula omitted]P[formula omitted] metallic glass

To understand the relation of the glass-forming ability (GFA) to the local atomic configurations of a Pd42.5Ni7.5Cu30P20 (PNCP) metallic glass having the best GFA at present, the local structures were investigated by combining data obtained from anomalous X-ray scattering, X-ray and neutron diffraction, and applying reverse Monte Carlo modeling. By comparing the results of PNCP with Pd40Ni40P20 (PNP) and Pd40Cu40P20 (PCP) having a slightly and much worse GFAs, respectively, characteristic features were observed in the hyper-ordered atomic structures. Firstly, the concentration inhomogeneity of Ni/Cu in PNCP is larger than that of Ni in PNP and Cu in PCP. Secondly, a Voronoi tessellation showed that the fraction of pure icosahedral arrangements around the Cu atoms increases significantly in PNCP by adding icosahedral-preferred Ni atoms in PCP. Finally, a persistent homology (PH) analysis reveals the largest intermediate-size Cu PH rings in PNCP among the PH rings in these Pd-based BMGs. The structural heterogeneity for the excellent GFA of PNCP would be considered by an incompatible mixture of specific Pd-P configurations and icosahedral clusters around the secondary Ni and Cu metals.

Structural study of Li2S-GeS2 glasses: Ge[sbnd]S network and local environment of Li

The static structures of (Li2S)x(GeS2)100-x glasses (x = 0, 20, 30, 40 and 50) were investigated by using a combination of diffraction measurements and simulation techniques. The X-ray diffraction results show that the host GeS network is formed by linkages of GeS4 tetrahedra. The three-dimensional structure models of GeS network were generated by using the X-ray diffraction data and reverse Monte Carlo (RMC) simulation. The RMC models show that the number of edge-sharing GeS4 tetrahedra decreases as Li2S content but its ratio to the number of connections between GeS4 tetrahedra remains substantially constant irrespective of the composition. The Li ion sites in the GeS network structures were determined by molecular dynamics (MD) simulation with reference to the previously reported neutron diffraction data. The MD results show that each Li ion is located in a deformed octahedral site consisting of bridging and non-bridging sulphur atoms.

Reverse Monte Carlo analysis of NaI-LiI solid electrolyte based on the neutron total scattering data

In the present study, the origin of Li+ ion conduction in an Li+/Na+ mixed solid electrolyte, 9NaI∙LiI, was investigated from the perspective of atomic configuration. The co-existence of Li+ and Na+ ions in solid electrolytes is known to result in a deterioration in conductivity, known as the mixed alkali effect. While dominant Li+ conduction has been reported in Li+-doped NaI where 10 mol% of Li+ ions are substituted for the host Na+ ions in NaI, the structural origin of the Li+ conduction in NaI is not well understood. From the results of reverse Monte Carlo (RMC) modeling based on the neutron total scattering data, it was revealed that the position of the Li+ is significantly off-centered from the ideal LiI6 octahedra despite the symmetrical distribution of the Na+ ions in the cation sites. The peculiar distribution of Li+ was explained by the large mismatch between the size of the Li+ ion and the lattice volume of NaI. These off-centered Li+ ions were shown to result in void spaces in the LiI6 octahedra. It is likely that the free volume in the LiI6 octahedra provides the hopping sites for the neighboring Li+ ions. Since the ionic radius of Na+ ions is larger than that of Li+ ions, the spaces in the LiI6 and NaI6 octahedra were not sufficient for Na+ hopping. One of the factors which contributed to the different ion conduction mechanism for Li+ and Na+ in the Li+/Na+ mixed solid electrolytes was shown to be the different local structures around Li+ and the host Na+ in Na compounds.

Evolving Differentiated Local Polar Displacement and Relaxor Behavior in Pb(Mg1/3Nb2/3)O3–PbTiO3 Perovskites

Lead-based relaxor ferroelectrics feature exceptional dielectric and electromechanical properties. The inherent local structural inhomogeneity remains poorly characterized and impedes the understanding of their unique electrical behavior. Herein, the local structure of a technologically important 0.75Pb(Mg1/3Nb2/3)O3–0.25PbTiO3 relaxor is studied as a function of temperature, by employing neutron total scattering combined with newly advanced reverse Monte Carlo modeling. The statistical analysis of local cationic polar displacement vectors demonstrates that a prevailing order/disorder feature and strong displacive behavior are observed in Pb and Ti, but both weak components are observed in Nb/Mg. This leads to trifurcated polar behavior across the feature temperatures. Importantly, these distinct cationic polar vectors are correlated and present a microscopic picture, where sporadic correlated orthorhombic polar clusters are embedded in a long-range rhombohedral polar matrix at low temperature. Their fraction extends with increasing temperature, presenting strong ferroelectricity with dielectric relaxation macroscopically. Furthermore, the long-range rhombohedral polar ordering is disrupted by temperature, forming free-oriented polar clusters with reducing electric dipole length, gradually losing their macroscopic polarization, and resulting in a diffuse phase transition behavior. Altogether, this offers a fundamental basis in terms of local polar fluctuations in inferring the nature of the unique properties of relaxor ferroelectrics.

Investigations on superconductivity in an equi-atomic disordered Hf-Nb-Ta-Ti-V high entropy alloy

High entropy alloys (HEA) are expected to show unusual superconducting behavior due to chemical disorder along with highly distorted lattice. Herein, we report the synthesis, structure, and detailed superconducting properties of a new and nearly equi-atomic single phase disordered superconducting high entropy alloy with nominal composition Hf20Nb20Ta20Ti20V20. Rietveld refinement of the Synchrotron X-ray diffraction (SXRD) confirms the DFT predicted single-phase BCC structure in the HEA with lattice parameter a = 3.32 Å. Reverse monte carlo (RMC) modelling of the diffraction data evince the local atomic disorder and a considerable lattice distortion overriding the crystal structure due to significant variations in bond length and co-ordination numbers. Microstructural characterization using energy-dispersive X-ray spectroscopy (XEDS) equipped SEM shows dendritic morphology of the as solidified alloy with incomplete partition of the constituent elements. Transport, magnetization and low temperature specific heat measurement established that the current HEA is a moderately coupled type-II superconductor with a Tc of 5.0 K, lower critical field Hc1(0)=19mT and upper critical field Hc2(0) = 6.63T.

Local Atomic Configuration in Pristine and A-Site Doped Silver Niobate Perovskite Antiferroelectrics.

Antiferroelectrics have attracted increasing research interests in recent years due to both their great potential in energy storage applications and intriguing structural characteristics. However, the links between the electrical properties and structural characteristics of distorted perovskite antiferroelectrics are yet to be fully deciphered. Here, we adopt local-structure methods to elucidate the nanoscale atomic structure of AgNbO3-based antiferroelectrics and their structural evolution upon La doping. The local structural features including interatomic distance distributions and atomic displacements have been analyzed using neutron small-box pair distribution function (PDF) refinement in conjunction with large-box Reverse Monte Carlo modelling. Our results highlight the correlation of cation displacements in AgNbO3 and its disruption by the incorporation of La, apparently in corroboration with the observed anomalous dielectric properties. Spatial ordering of cation vacancies is observed in La-doped AgNbO3 samples, which coordinates with oxygen octahedral tilting to relieve lattice strain. These results provide renewed insights into the atomic structure and antiferroelectric phase instabilities of AgNbO3 and relevant perovskite materials, further lending versatile opportunities for enhancing their functionalities.

Implementation for coping with sample and instrument effects for reverse Monte Carlo modelling of total scattering data

Implementation for coping with sample and instrument effects for reverse Monte Carlo modelling of total scattering data

Implementation for coping with sample and instrument effects for reverse Monte Carlo modelling of total scattering data

Implementation for coping with sample and instrument effects for reverse Monte Carlo modelling of total scattering data

Structural order in (As2S3)x(GeS2)1−x glasses

Structural order in the chalcogenide glasses of (As2S3)x(GeS2)1−x (x = 0.0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) system is examined in terms of the parameters of local atomic structure as a function of composition x, obtained using high-resolution Raman spectroscopy, high-energy synchrotron X-ray diffraction, extended X-ray absorption fine structure spectroscopy and reverse Monte-Carlo modeling of diffraction data. As a result of the research carried out it is revealed that the structural order of As-rich (x > 0.4) and Ge-rich (x < 0.4) glasses is organized by the main As−S and Ge−S structural motifs based on pyramidal AsS3 and tetrahedral GeS4 units linked by =As−S−As= and Ge−S−Ge structural configurations, respectively; while for the intermediate compound with x = 0.4 the structural network seems to be better homogeneous on the nanoscale due to appearance of Ge−S−As= mixed structural configurations resulting in misbalance between corner-shared and edge-shared tetrahedral units in comparison with their predicted ratio for binary GeS2 glass and the structure of this alloy is similar to the structure of the stoichiometric glass Ge18.2As18.2S63.6 (i.e., x = 0.455) consisting of a coner-shared network of homogeneously mixed GeS4 tetrahedra and AsS3 pyramids. Based on the structural studies, it is also established that the balance between corner-shared and edge-shared GeS4 tetrahedra in the glass backbone of the investigated GeS2-based glasses seems to be responsible for the interconnectivity between two speculative Raman modes at 370 and 430 cm−1. Compositional changes in studied glasses result in the evolution of the observed Raman bands. Such dependences of characteristic constituent Raman bands’ intensities showed that (As2S3)x(GeS2)1−x samples contain different nanophases whose concentration is changing along chosen compositional cross-section.

Local structure and dynamics of tungsten oxide-based glasses: insights from concurrent neutron diffraction and Compton scattering

In this work, following our previous work on molybdate glasses, we employ a combination of neutron diffraction and neutron Compton scattering, augmented by ab initio harmonic lattice dynamics and Reverse Monte Carlo modelling to characterise the force-constant disorder in the tungsten oxide-based glasses. Specifically, we discuss the correlations between the average interatomic force constant magnitudes inferred from neutron Compton scattering and the glass formation ability, measured in terms of the value of the glass transition temperature, as well as the average bond-lengths and interatomic distances obtained from diffraction data analysis. Moreover, we provide a comparative analysis of the widths of force-constant distributions of individual atomic species in glasses and their precursor metal oxides based on the distributions of the widths of nuclear momentum distributions. Furthermore, we assess the degree of softening of atom-projected vibrational densities of states induced by the force-constant disorder in the glasses.

Local Structure Properties of Hydrogenated and Nonhydrogenated Amorphous In–Ga–Zn–O Thin Films Using XAFS and High-Energy XRD

The strong effect of impurity hydrogen (H) on subgap electronic states in amorphous In–Ga–Zn–O thin-film transistors (a-IGZO TFTs) was confirmed using standard (STD) and ultrahigh-vacuum (UHV) sputtering systems with different base pressures of 10–4 and 10–7 Pa, respectively. However, comprehensive studies of the atomic-scale structure have yet to be reported. We investigated the correlations between the atomic-scale structure, the electronic state, and the impurity hydrogen content in a-IGZO by high-energy X-ray diffraction (HEXRD) coupled with reverse Monte Carlo (RMC) modeling and X-ray absorption fine structure (XAFS) spectroscopy. XAFS probed the distribution of unoccupied electronic states above the Fermi level and the local coordination structure around the In, Ga, and Zn atoms. A possible contribution for H from voids in the a-IGZO films was observed by the HEXRD and RMC configuration models. In contrast, the STD 3% film has many voids, which are occupied by impurity H. The proportion of lower coordinated M-O (M = Zn, Ga, In) structures is increased because of the H in voids. We revealed that this electronic and atomic-scale structure of the a-IGZO TFTs, which results in enhanced TFT characteristics, can be stabilized by the hydrogen-passivated defects resulting from STD sputtering with an optimum oxygen flow rate ratio of 3%.

Local Structures of Soft Carbon and Electrochemical Performance of Potassium-Ion Batteries.

Due to climate variation and global warming, utilization of renewable energy becomes increasingly imperative. Rechargeable potassium-ion batteries (PIBs) have lately attracted much attention due to their earth-abundance and cost-effectiveness. Because soft carbon materials are cheap, abundant, and safe, extensive feasible research studies have indicated that they could become promising anode materials for PIBs. In spite of gaining achievements, fundamental questions regarding effects of the basic structure unit inside soft carbon on potassium storage potential have not been sufficiently addressed yet. Here, a series of soft carbon pyrolyzed from 900 to 2900 °C were systematically and quantitatively characterized by combining Raman spectroscopy, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, X-ray pair distribution function analysis, and advanced evaluation of wide-angle X-ray scattering data. All these characterizations reveal structural details of soft carbon with increasing pyrolysis temperature. Our results show that the potassium storage behavior, especially the potential plateau is closely correlated to non-uniformity in interlayer distance and defect concentration in soft carbon, which is further confirmed by reverse Monte Carlo (RMC) modeling and density functional theory calculation. On the basis of these results, optimizing strategies are discussed to design an advanced soft carbon anode. This work provides significant insights into the structure engineering of soft carbon for high-performance rechargeable PIBs.

Atomic-level characterization of free volume in the structure of Cu67Zr33 amorphous alloy

The structure of Cu67Zr33 amorphous alloy was investigated in terms of packing density and free volume by using neutron, x-ray diffraction and reverse Monte Carlo (RMC) modelling. The RMC model was analysed by a method of decomposing the three-dimensional atomic configuration into fundamental polyhedral units (termed as ‘holes’ referencing the Bernal’s works) of which faces are all triangles consisting of chemical bonds. Not only tetrahedral and octahedral holes but also other larger holes were identified. Moreover, the atomic packing fractions and free volumes in the respective polyhedral holes were evaluated with reference to those for the corresponding crystal structures. The results show that the distribution of free volumes for the larger holes can be described by the exponential function assuming that there are no energetic interactions between each other. On the other hand, the local structural fluctuations due to densely and loosely packed tetrahedral holes were observed, leading to the negative free volume spaces.