Local Structural Properties Research Articles

Influence of dispersity on local physical, structural, magnetic and thermodynamic properties of bcc Fe and its binary alloys

The model was proposed for iron nano-particles (and solid solutions based on bcc-Fe) containing a core and a surface layer coherent with each other in the form of a two-phase stratifying parts (“surface” and bulk) system with different local physical properties. Using the results of measurements of the experimental temperature dependence of the heat capacity and the application of the Debye models for both the “core”-phase and the surface layer (“surface”-phase) as two-phase heat capacity components for Fe nanoparticles, it was shown by calculations that the local elastic modulus for the surface layer is nano-particles are approximately 2 times smaller than for the “core”-phase of the nano-particle iron. The purpose of the research is to establish the relationship between local physical properties (bulk modulus of elasticity, static displacements of iron atoms surrounding an impurity atom, magnetic moments) for the core part and the surface layer of nanoparticles. From the condition of local mechanical equilibrium it was shown that static displacements of Fe atoms around an impurity atom in various coordination spheres on the surface of nanoparticles of Fe-rich binary BCC solutions of Fe - (Cr, V, Mo, W) with nano-particles increase significantly (compared to static displacements of Fe atoms “core” of nano-particles).

Bounds on the Spectral Radius of Digraphs from Subgraph Counts

The spectral radius of a directed graph is a metric that can only be computed when the structure of the network is completely known. However, in many practical scenarios, it is not possible to exactly retrieve the whole structure of the network; hence, the exact value of the spectral radius is not computable. Even in these scenarios, it is typically possible to extract local structural properties of a network using, for example, graph crawlers. In this paper, we develop a novel measure-theoretic framework to upper and lower bound the spectral radius of a directed graph using local structural information, in particular, using the counts of a collection of small subgraphs or motifs. Our framework is based on recent results relating the multivariate moment problem with semidefinite programming. Using these results, we develop a hierarchy of (small) semidefinite programs whose solutions provide upper and lower bounds on the spectral radius of a directed graph using, solely, subgraph and motif counts. We numerically validate the quality of our bounds using both random and real-world directed graphs.

Local process-dependent structural and mechanical properties of extrusion blow molded high-density polyethylene hollow parts

Although applied for several decades, production of hollow plastic parts by extrusion blow molding (EBM) is still over-dimensioned. To overcome this issue, a thorough investigation of the process-structure-property relationship is required. In this study, the local process-structure-property relationship for high-density polyethylene EBM containers is analyzed with differential scanning calorimetry and dynamic mechanic analysis microindentation. Local process-dependent crystallinity and complex modulus data at various processing conditions are supplemented with wide-angle X-ray diffraction and transmission electron microscopy (TEM). The crystallinities and the complex moduli clearly show lower values close to the mold side than at the inner side and the middle of the cross-section, which reflects the temperature gradient during processing. Additionally, the orientation of the polymer chain (c-axis) reveals a low level of biaxiality with a slight tendency towards transverse direction. The biaxiality increases for low mold temperature and high draw ratio. Finally, biaxiality is confirmed with TEM, which reveals no preferred lamellar orientation.

A second-order output spectrum based method for detecting bolt-loosening fault in a satellite-like structure

Bolt-loosening faults frequently exist in industrial engineering structures since these bolted structures are often subjected to vibrations or the like in their service process. In this paper, a novel method based on the second-order output spectrum (SOOS) is proposed to detect potential bolt-loosening faults in a complex satellite-like structure. In this method, a general multi-degree-of-freedom (MDOF) model simulating bolt-loosening faults induced non-linearities and inherent boundary or material non-linearities by non-linear forces is built to describe the non-linear behaviour of the structure, and then a local damage indicator is derived for bolt-loosening fault detection through a local tuning approach (LTA) which tunes local structural properties. Results of experimental cases demonstrate that the state of bolted joint in the satellite-like structure with inherent non-linearities can be estimated by this novel SOOS based method effectively and reliably.

Electronic and magnetic properties of structural defects in SrTiO3(Co)

The synthesis conditions of SrTiO3(Co) samples in which cobalt predominantly enters the A or B sites of the perovskite ABO3 structure are found. EXAFS studies show that the Co impurity at the A site is off-center and is displaced from the site by 1.0 Å. XANES studies reveal two predominant oxidation states of Co: Co2+ at the A site and Co3+ at the B site. First-principles calculations of a number of possible cobalt-containing structural defects reveal defects whose properties are compatible with the experimentally observed Co oxidation state, its local structure, magnetic, electrical, and optical properties.

Exotic Transverse-Vortex Magnetic Configurations in CoNi Nanowires.

The magnetic configurations of cylindrical Co-rich CoNi nanowires have been quantitatively analyzed at the nanoscale by electron holography and correlated to local structural and chemical properties. The nanowires display grains of both face-centered cubic (fcc) and hexagonal close packed (hcp) crystal structures, with grain boundaries parallel to the nanowire axis direction. Electron holography evidences the existence of a complex exotic magnetic configuration characterized by two distinctly different types of magnetic configurations within a single nanowire: an array of periodical vortices separating small transverse domains in hcp-rich regions with perpendicular easy axis orientation and a mostly axial configuration parallel to the nanowire axis in regions with fcc grains. These vastly different domains are found to be caused by local variations in the chemical composition modifying the crystalline orientation and/or structure, which give rise to change in magnetic anisotropies. Micromagnetic simulations, including the structural properties that have been experimentally determined, allow for a deeper understanding of the complex magnetic states observed by electron holography.

Structural and dynamic properties of aluminosilicate melts: a molecular dynamics study

In the present work, the structural and dynamic properties of aluminosilicates (Al2O3)x-(SiO2)1−x (AS) as a function of the Al2O3 concentration x are studied by means of molecular dynamics simulations. Firstly, the parametrization of the Born–Mayer–Huggins type potential developed recently for the more general CaO-Al2O3-SiO2 ternary system is assessed. Comparison of local structural properties, such as the x-ray structure factor, partial pair-correlation functions, distributions of coordination numbers and bond angles, as well as the dynamics through the viscosity and self-diffusion coefficients to experimental data and other molecular dynamics simulations found in the literature, shows that this potential is transferable to AS melts for all compositions and is more reliable than other empirical potentials used so far. The evolution of viscosity with temperature in stable liquid and undercooled regions is studied in the whole composition range and results show a progressive increase of the fragility with increasing Al2O3 content correlated to that of local structural entities like the triply bonded oxygen (TBO), AlO5 and AlO6.

Identification of barium-site substitution of BiFeO3–Bi0.5K0.5TiO3 multiferroic ceramics: X-ray absorption near edge spectroscopy

In this work, the effects of barium substitution on the local structure, dielectric and magnetic properties of the polycrystalline ceramics 0.6BiFeO3–0.4(Bi0.5K0.5)TiO3 (0.6BFO–0.4BKT) system was investigated. A solid-state reaction technique was used to synthesize the materials with barium (Ba) doping of 1, 3, 5, 7, and 10 mol%. XRD analysis reveals the coexistence between tetragonal and rhombohedral phases of single-phase perovskite in pure 0.6BFO–0.4BKT and the rhombohedral reach phase was found with increasing Ba content. XANES simulations indicate that the majority of Ba atoms occupy A-site in BKT lattice of Ba-doped 0.6BFO-0.4BKT, the oxidation state of Fe, Ti, and Ba ions are +3, +4 and+2, respectively. At 5 mol% of Ba doping content, the dielectric measurement shows the morphotropic phase boundary (MPB) and the maximum value of ferromagnetic characteristic were observed, indicating an optimum composition, properties and production conditions.

A new approach for coupled modelling of the structural and thermo-physical properties of molten salts. Case of a polymeric liquid LiF-BeF2

The (Li,Be)Fx fluoride salt is an ionic liquid with complex non-ideal thermodynamic behaviour due to the formation of short-range order. In this work, we explore the relationship between local structure, thermo-physical and thermodynamic properties in this system using a multidisciplinary approach that couples molecular dynamics simulations using the Polarizable Ion Model (PIM) and thermodynamic modelling assessment using the CALPHAD method. The density, thermal expansion, viscosity, thermal conductivity, molar and mixing enthalpies and heat capacity of the (Li,Be)Fx melt are extracted from the polarizable ionic interaction potentials and investigated across a wide range of compositions and temperatures. The agreement with the available experimental data is generally very good. The local structure is also examined in detail, in particular the transition between a molecular liquid with Li+, BeF42− and F− predominant species at low BeF2 content, and a polymeric liquid at high BeF2 content, with the formation of polymers (Be2F73−, Be3F104−, Be4F135−, etc.), and finally of a three-dimensional network of corner-sharing tetrahedrally coordinated Be2+ cations for pure BeF2. Based on the available experimental information and the output of the MD simulations, we moreover develop for the first time a coupled structural-thermodynamic model for the LiF-BeF2 system based on the quasi-chemical formalism in the quadruplet approximation, that provides a physical description of the melt and reproduces (in addition to the thermodynamic data) the chemical speciation of beryllium polymeric species predicted from the simulations.

Spin-crossover in the iron(II) complex based on dihydro-bis(pyrazolyl)borate and 1,10-phenanthroline-5,6-dione

A novel mixed-ligand Fe(II) complex based on dihydro-bis(pyrazolyl)borate and 1,10-phenanthroline-5,6-dione Fe[(H2B(Pz)2]2[phen-dione] was synthesized. Its local atomic structure and magnetic properties were characterized by means of X-ray absorption and Mössbauer spectroscopies in the temperature range of 8–300 K. According to the experimental data, the transition of iron ions from high-spin to low-spin configuration is observed in the Fe[(H2B(Pz)2]2[phen-dione] complex when the temperature decreases. The obtained experimental data are in a good agreement with the results of quantum chemical calculations.

An investigation of local structures and EPR spectra for Cu2+ and VO2+ in biochar

Electron paramagnetic resonance (EPR) technology had been adopted to analyze the structures and properties of Cu2+ and VO2+ in Biochar (BG1 and BG2), but the local structural properties have not been clarified up to now. In present work, three possible structures (orthorhombically, rhombically and tetragonally elongated octahedra) are proposed for Cu2+ in BG1 and BG2 samples, and the former seems most reasonable in view of the best agreement between the theoretical EPR parameters based on the perturbation formulae and the experimental data. As for the VO2+ center, unlike the previous assignment of the tetragonally elongated [VO5]6− cluster based on the simple formulae of EPR parameters, presently proposed tetragonally compressed [VO6]8− cluster and the high order perturbation formulae of the EPR parameters yields good agreement with the observed values. This point is also supported by the d-d optical absorption 2B1g →2B2g for tetragonally compressed octahedral 3 d1 systems.

First-principles molecular dynamics simulations on the local structure and thermo-kinetic properties of molten magnesium chloride

MgCl2 is a salt of considerable interest for the concentrating solar thermal power (CSP) system with advantages of high operating temperature and low cost. While the value of first-principles molecular dynamics (FPMD) simulations in investigating molten salt has been illustrated in many studies, there have been very few studies on molten MgCl2 from a first-principles method. In this work, FPMD simulations are employed to investigate the local coordination structure, as well as its structural dynamics, and thermo-kinetic properties of molten MgCl2. The basic thermo-kinetic properties of density, diffusivity, and ion conductivity are calculated for pure MgCl2 at multiple temperatures. The partial radial distribution function, partial structure factor, probability distribution of coordination numbers, and bond angle distribution are also derived from FPMD simulations. FPMD simulations are found to give satisfactory structural and transport characteristics compared with experiments. This study shows that the four-fold coordinated Mg2+ cations exist in pure MgCl2 and the Mg2+ first coordination shell has more octahedral geometry with anion vacancies. In conclusion, this work shows that FPMD simulations provide an effective method for exploring molten salts, which can be used in future research on Mg-based salts.

Local metallic and structural properties of the strongly correlated metal LaNiO3 using 8Li β–NMR

We report $\beta$-detected NMR of ion-implanted $^{8}$Li in a single crystal and thin film of the strongly correlated metal LaNiO$_{3}$. In both samples, spin-lattice relaxation measurements reveal two distinct local metallic environments, as is evident from $T$-linear Korringa $1/T_{1}$ below 200 K with slopes comparable to other metals. A small, approximately temperature independent Knight shift of $\sim 74$ ppm is observed, yielding a normalized Korringa product characteristic of substantial antiferromagnetic correlations, but, we find no evidence for a magnetic transition from 4 to 310 K. Two distinct, equally abundant $^{8}$Li sites is inconsistent with the widely accepted rhombohedral structure of LaNiO$_{3}$, but cannot be simply explained by either of the common alternative orthorhombic or monoclinic distortions.

Identification of influencers in complex networks by local information dimensionality

The identification of influential spreaders in complex networks is a popular topic in studies of network characteristics. Many centrality measures have been proposed to address this problem, but most have limitations. In this paper, a method for identifying influencers in complex networks via the local information dimensionality is proposed. The proposed method considers the local structural properties around the central node; therefore, the scale of locality only increases to half of the maximum value of the shortest distance from the central node. Thus, the proposed method considers the quasilocal information and reduces the computational complexity. The information (number of nodes) in boxes is described via the Shannon entropy, which is more reasonable. A node is more influential when its local information dimensionality is higher. In order to show the effectiveness of the proposed method, five existing centrality measures are used as comparison methods to rank influential nodes in six real-world complex networks. In addition, a susceptible–infected (SI) model and Kendall’s tau coefficient are applied to show the correlation between different methods. Experiment results show the superiority of the proposed method.

Spectral Graph Forge: A Framework for Generating Synthetic Graphs With a Target Modularity

Community structure is an important property that captures inhomogeneities common in large networks, and modularity is one of the most widely used metrics for such community structure. In this paper, we introduce a principled methodology, the Spectral Graph Forge, for generating random graphs that preserves community structure from a real network of interest, in terms of modularity. Our approach leverages the fact that the spectral structure of matrix representations of a graph encodes global information about community structure. The Spectral Graph Forge uses a low-rank approximation of the modularity matrix to generate synthetic graphs that match a target modularity within user-selectable degree of accuracy, while allowing other aspects of structure to vary. We show that the Spectral Graph Forge outperforms state-of-the-art techniques in terms of accuracy in targeting the modularity and randomness of the realizations, while also preserving other local structural properties and node attributes. We discuss extensions of the Spectral Graph Forge to target other properties beyond modularity, and its applications to anonymization.

InitDAE: Computation of consistent values, index determination and diagnosis of singularities of DAEs using automatic differentiation in Python

InitDAE is a prototype written in Python that computes consistent initial values of differential–algebraic equations (DAE), determines their index and a related condition number that permits the diagnosis of singularities. The algorithm for the consistent initialization uses a projector based constrained optimization approach and the inherent differentiations are provided by automatic differentiation (AD), using AlgoPy. Consequently, a detailed description of the local structural properties of the DAE becomes possible using the SVD. InitDAE has been conceived for academic purposes and is well-suited for examples of moderate size. In this article we give an overview of the algorithm, show actual features and discuss future possibilities, in particular the integration with Taylor series methods.

Temperature dependence of structural, dynamical, and electronic properties of amorphous Bi2Te3: an ab initio study

Bismuth telluride (Bi2Te3) has garnered significant interest in thermoelectric applications and three-dimensional topological insulators due to its unique electronic, transport, and thermal properties. Bi2Te3 and Sb2Te3 chalcogenide compounds have the same crystal structure. While Sb2Te3 has been shown to be a prototypical phase change memory (PCM) compound along the pseudobinary tie-line of Ge-Sb-Te alloys, whether Bi2Te3 can also exhibit PCM functionality is still not well established. In this work, a systematic study on the structural, dynamical, and electronic properties of amorphous Bi2Te3 during the quenching process has been performed by using ab initio molecular dynamics simulations. Pair correlation function, coordination number, bond-angle distribution functions, and a novel atomistic cluster alignment method are used to explore the structural characteristics of Bi2Te3 as a function of temperature. Our study shows that there are many distorted octahedral clusters in amorphous Bi2Te3. In comparison with the local structures in Sb2Te3, we found that the degree of distortion of the octahedrons in the Bi2Te3 system is smaller than that in Sb2Te3 system. Moreover, the changes in the dynamical properties of Bi2Te3 from liquid to glassy state are also explored. The approximate range of liquid-to-glass transition temperature is determined to be between 673 and 723 K. The electronic properties of Bi2Te3 and Sb2Te3 are also analysed by density-of-states and Bader charge calculations, both of them in glass state are semiconductors. Our studies provide useful insights into the local structure and dynamical properties of Bi2Te3 at the atomistic level during the fast cooling process, and suggest that the compound can be a candidate for PCM materials.

Remarkable active-site dependent H2O promoting effect in CO oxidation

The interfacial sites of supported metal catalysts are often critical in determining their performance. Single-atom catalysts (SACs), with every atom contacted to the support, can maximize the number of interfacial sites. However, it is still an open question whether the single-atom sites possess similar catalytic properties to those of the interfacial sites of nanocatalysts. Herein, we report an active-site dependent catalytic performance on supported gold single atoms and nanoparticles (NPs), where CO oxidation on the single-atom sites is dramatically promoted by the presence of H2O whereas on NPs’ interfacial sites the promoting effect is much weaker. The remarkable H2O promoting effect makes the Au SAC two orders of magnitude more active than the commercial three-way catalyst. Theoretical studies reveal that the dramatic promoting effect of water on SACs originates from their unique local atomic structure and electronic properties that facilitate an efficient reaction channel of CO + OH.

Simultaneous scanning near-field optical and X-ray diffraction microscopy for correlative nanoscale structure-property characterization.

A multimodal imaging instrument has been developed that integrates scanning near-field optical microscopy with nanofocused synchrotron X-ray diffraction imaging. The instrument allows for the simultaneous nanoscale characterization of electronic/near-field optical properties of materials together with their crystallographic structure, facilitating the investigation of local structure-property relationships. The design, implementation and operating procedures of this instrument are reported. The scientific capabilities are demonstrated in a proof-of-principle study of the insulator-metal phase transition in samarium sulfide (SmS) single crystals induced by applying mechanical pressure via a scanning tip. The multimodal imaging of an in situ tip-written region shows that the near-field optical reflectivity can be correlated with the heterogeneously transformed structure of the near-surface region of the crystal.

Molecular dynamics simulation on local structure and thermodynamic properties of molten ternary chlorides systems for thermal energy storage

Molten alkali chlorides mixtures are potential thermal energy storage materials. The structure and thermodynamic properties of LiCl-NaCl-KCl systems with different KCl concentrations were investigated by molecular dynamics simulation. The force field was verified by experimental data for the density and viscosity, and its precision was compared with the first-principle-based molecular dynamics simulation results. The densities obtained from rigid ion model were in satisfactory agreement with experimental values with a minimum error of 4.9% underestimation. The discrepancy in the structural information obtained by the two methods was no more than 1%. The local structure and thermodynamic properties were simulated across a full range of KCl concentration from 900 to 1200 K. The stability of cations coordination was analyzed by cage-out correlation function. The calculated self-diffusion coefficients, shear viscosity, heat capacity, and thermal conductivity were in good agreements with the available experimental data. The results showed that addition of KCl strengthened the correlation between unlike charged ions pairs, which was verified by the stability analysis. The stability of the first coordination shell determined the shear viscosities. Change in the structure affected the thermodynamic properties.