Local Atomic Order Research Articles

Influences of Strain Rate on Copper Nanowire in Tension

This study uses molecular dynamics simulations with an embedded-atom method (EAM) potential to investigate the effects of strain rate on a copper wire in tension at the nanoscale. By averaging the atomic stresses over the entire system, the stress-strain curves for the nanowire were predicted. The results show that an elastic region clearly exists in the early deformation of the copper nanowire and the elastic modulus is about two times of the value measured at the macroscale. The yield stress of the nanowire increases with the strain rate. Moreover, the evolution of the crystal structure was investigated in terms of the radial distribution function and the local atomic order was also examined by the common neighbour analysis. The dislocations and recrystallization processes inside the nanowire have been observed. The simulated results reveal the process of the fracture of copper nanowire in tension.

Reverse Monte Carlo simulations of an amorphous Se 0.90S 0.10 alloy produced by mechanical alloying combining XRD and EXAFS data

The local atomic order of an amorphous Se 0.90S 0.10 alloy produced by Mechanical Alloying was studied by reverse Monte Carlo simulations combining X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) data obtained on Se K edge. From the simulations structural properties such as average interatomic distances and average coordination numbers were determined and compared with the values extracted using only XRD data in the simulations. In addition, the bond-angle distribution functions Θ ijℓ (cos θ) were obtained, and they indicate the existence of Se n clusters in the alloy and also the substitution of Se atoms by S atoms in some of these clusters.

Structural and Magnetic Properties of Fe50Cr50Alloys Prepared by Mechanical Alloying Method

Fe50Cr50 metastable alloys were prepared by the mechanical alloying method with milling periods of 1, 2, 4, 6, 12 and 24 hours, respectively. The structural evolution was analyzed by the extended x-ray absorption fine structure (EXAFS). In this work, the EXAFS analysis provided the local structural information around Fe central atom. The saturation magnetization was also measured by VSM. The magnetization decreased as the process mechanical alloying progressed. The magnetic property was related to the local structural variation as a function of processing time. The analysis showed that the diffusion Cr atoms into Fe clusters caused the reduction of magnetization. EXAFS analysis exhibited that the local ordering of magnetic atoms caused the magnetic ordering. Also, EXAFS analysis showed that the long range order of Fe atoms was destroyed completely in 24 hour milling.

Symmetry of (Na0.5R0.5)MO4 crystals (R = Gd, La; M = W, Mo)

Kinetic order-disorder phase transitions (space group space group \( I\bar 4 \) ↔ space group I41/a) have been considered for nonactivated and activated scheelite compounds (Na0.5Gd0.5)WO4 (NGW), (Na0.5Gd0.5)MoO4 (NGM), (Na0.5La0.5)WO4(NLW), and (Na0.5La0.5)MoO4 (NLM) synthesized by the Czochralski method and structural and growth sources of crystal dissymmetrization have been suggested. For NGW, it was shown that an increase in the difference between the content of Gd and Na in two positions of the structure with space group \( I\bar 4 \) and their ratio leads to an increase in the deviation from centrosymmetry. On the basis of available literature data and our results, it was demonstrated that the degree of order depends on the initial composition of the reaction mixture, crystal growth and cooling rates, activator concentrations, and postgrowth treatment conditions. The inconsistency between X-ray diffraction data and asynchronous second harmonic generation studies was explained by the possibility of formation of centrosymmetric superstructures and/or local ordering of atoms.

Local atomic order in apparently disordered crystals: what we can learn from pair distribution function

Local atomic order in apparently disordered crystals: what we can learn from pair distribution function

Analysis of 3D structures of platinum nanoparticles by high energy X-ray diffraction and reverse Monte Carlo simulations

In this paper, the atomic-scale three-dimensional structure of Pt nanoparticles were determined using high energy X-ray diffraction techniques and reverse Monte Carlo simulations. It was found that dendrimer encapsulated Pt nanoparticles are more ordered than those stabilized by long-chain alkanes. Furthermore, Pt nanoparticles measured in ambient dry conditions exhibit higher local atomic order compared to Pt nanoparticles measured in aqueous solutions. The atomic-scale structural information presented here can offer a more comprehensive understanding of the structure–property relationship for Pt nanoparticles and can be further used in the design of nanoparticles with improved properties.

Investigation on vibrational and structural properties of amorphous [formula omitted] alloys produced by mechanical alloying by Raman spectroscopy, X-ray diffraction, EXAFS and RMC simulations

The local atomic order of amorphous Se 1− x S x alloys, x = 0.20 , 0.30 , produced by mechanical alloying were studied by Raman scattering, X-ray diffraction, EXAFS and reverse Monte Carlo simulations of their total structure factors and EXAFS oscillations on Se K edge. The results obtained were compared to those found for other Se 1− x S x , x = 0 , 0.10 alloys, and the behavior of Raman modes, average coordination numbers and interatomic distances and bond-angle distribution functions as sulphur concentration increases were determined.

Structural details of Ge‐rich and silver‐doped chalcogenide glasses for nanoionic nonvolatile memory

AbstractWe are reporting our results of Raman and X‐ray diffraction (XRD) studies on amorphous Ge46S54 thin films and the films after silver photodiffusion. Based on the Raman scattering studies, a structural model for amorphous Ge46S54 is suggested including the formation of single Ge–S chains with a vibrational mode at 410 cm−1. The result of XRD measurement indicates that there exists a medium‐range order with about 6 Å´ even at such Ge‐rich composition. After the introduction of silver, the medium‐range order is lost and there was a change in the diffraction curve indicative of the change in the local atomic order. The experimental results are explained in terms of our structural model, in connection with the application for fast switching memory devices.

NIMROD: The Near and InterMediate Range Order Diffractometer of the ISIS second target station

NIMROD is the Near and InterMediate Range Order Diffractometer of the ISIS second target station. Its design is optimized for structural studies of disordered materials and liquids on a continuous length scale that extends from the atomic, upward of 30 nm, while maintaining subatomic distance resolution. This capability is achieved by matching a low and wider angle array of high efficiency neutron scintillation detectors to the broad band-pass radiation delivered by a hybrid liquid water and liquid hydrogen neutron moderator assembly. The capabilities of the instrument bridge the gap between conventional small angle neutron scattering and wide angle diffraction through the use of a common calibration procedure for the entire length scale. This allows the instrument to obtain information on nanoscale systems and processes that are quantitatively linked to the local atomic and molecular order of the materials under investigation.

Study of the Local Environment of Mn Ions Implanted in GaSb

The first attempts to establish an implantation process leading to formation of ferromagnetic inclusions inside the GaSb matrix are presented. Gallium antimonide containing ferromagnetic MnSb precipitations is considered as a promising material for novel spintronic applications. It is possible to obtain such inclusions during the molecular beam epitaxy (MBE) growth. However, for commercial application it would be also important to find an optimal way of producing this kind of inclusions by Mn ions implantation. In order to achieve this goal, several parameters of implantation and post annealing procedures were tested. The ion energy was kept at 10 keV or 150 keV and four different ion doses were applied, as well as various annealing conditions. The analysis of X-ray absorption spectra allowed to estimate the local atomic order around Mn atoms. Depending on the implantation energy and annealing processes, the manganese oxides or manganese atoms located in a heavily defected GaSb matrix were observed. The performed analysis helped in indicating the main obstacles in formation of MnSb inclusions inside the GaSb matrix by Mn ion implantation.

Local atomic order in low Pt-content nanocatalysts investigated in situ by XAS

The unique features of X-ray absorption spectroscopy allow investigations of nanosized catalysts for fuel cells under working conditions. We present the results of an experiment carried out on a low Pt content electrocatalyst supported by a mesoporous heteropolyacid salt and used at the cathode of a proton exchange membrane fuel cell (PEMFC). The analysis of the EXAFS signal at the Pt L3-edge indicates that upon operating the fuel cell a substantial oxygen desorption from the Pt catalytic surface occurs followed by a tangible structural change on the Pt local order.

The ratio of interstitial to substitutional site occupation by Mn atoms in GaAs estimated by EXAFS

The location of Mn atoms in the MBE-grown layers of Ga 1− x Mn x As is correlated with all important physical properties of the final material, therefore, it is the subject of many studies. It is known that in the as-grown MBE samples the Mn atoms occupy substitutional and interstitial positions but the proportion between these sites is not easy to find. A powerful tool for this kind of study is XAS as it probes the local atomic order and the electronic structure. The EXAFS data analysis was performed considering superposition of possible Mn locations. This allowed for determination of the distribution of Mn between those two lattice sites.

Vibrational, optical and structural studies of an amorphousSe0.90S0.10 alloy produced by mechanical alloying

The local atomic order of an amorphousSe0.90S0.10 alloy produced by mechanical alloying was studied by x-ray diffraction and extendedx-ray absorption fine structure (EXAFS) data obtained at three temperatures,T = 300, 200 and 30 K. From the cumulant analysis of the EXAFS data, structural properties suchas average interatomic distances, average coordination numbers, Debye–Waller factors andanharmonicity, given by the third cumulant, were obtained. The results found indicate thatthere is alloying at an atomic level, and Se–S pairs are more disordered and distorted thanSe–Se ones due to the milling process.

Local atomic order in the vicinity of Cu 2 dumbbells in TbCu 7-type YCu 6.576 studied by Bragg and total scattering techniques

The crystal structure of YCu 6.576, a hyperstoichiometric substitution variant of the CaCu 5-structure type, was shown to contain Cu 2 dumbbells replacing Y-atoms in a fraction s = 0.19 in the formula Y 1− s Cu 5+2 s . The structure type TbCu 7 is assigned to the disordered structure ( P6/ mmm, a = 4.96 Å, c = 4.15 Å). The local order in the vicinity of the Cu 2-dumbbells has been derived from total scattering X-ray synchrotron data via refinement of the atomic pair distribution function (PDF). The coordinating Cu 6-hexagon around the dumbbell site shows a shrinkage of 0.33(1) Å w.r.t. the equivalent environment of the Y-atom. No adjacent Y-atoms are substituted but a hexagonal arrangement of Cu 2-dumbbells, separated at least by the vectors √3 a and √3 b exists in layers perpendicular to c. The stacking along [001] is random and can be modeled locally from the PDF data consistent with both ab- and abc-type stacking. Therefore, the local order is comparable to that in Ni 17Th 2- and Th 2Zn 17-structure types. X-ray single crystal analysis and Rietveld refinements of neutron- and X-ray powder data (Bragg diffraction data) of YCu 6.576 are consistent with the findings from local crystallography (total scattering data) answering a long-standing question in disordered CaCu 5-type based intermetallic structure variants.

Local atomic order and optical properties in amorphous and laser-crystallized GeTe

Abstract In this work we study the role of short-range order changes upon amorphization on the optical properties of GeTe – a prototype phase change material employed for optical data storage. It is found that the profound change in the absorption is due to changes in the matrix elements of the optical transitions. The importance of the local distortions in the crystalline phase for the optical absorption are revealed as well. Modifying the degree of the distortions has a significant impact on the optical properties of the crystalline state and should therefore become a promising instrument to improve material properties for storage applications. Furthermore we study the effect of electron–electron and electron–hole interactions on the optical properties. This is achieved by evaluating many-body effects in the crystalline phase through a GW correction of eigenvalues and the solution of the Bethe–Salpeter equation. To cite this article: W. Welnic et al., C. R. Physique 10 (2009).

EXAFS study of local atomic order about iodine in thyroxine, rat, human and sheep thyroids

Radioactive (125)I emits short-range Auger electrons and represents a human health risk when incorporated in thyroglobulin of the thyroid. Quantitative evaluation of this risk can only be realised if local atomic order about iodine in the thyroid is known. Here, extended X-ray absorption fine structure (EXAFS) has been used to probe the local structure about iodine in pure thyroid hormone, thyroxine. These data are consistent with a model where iodine is bound to a single iodinated carbon ring linked to an oxygen atom, similar to a previously published model for monoiodotyrosine, a major iodinated residue in thyroglobulin. Several structural models for the local environment of iodine from rat, human and sheep have been tested and these data are found to be compatible with a slightly modified environment with respect to that found for thyroxine. The best-fit models include the following three components: (i) iodine covalently bonded to a tyrosine ring, as found for thyroxine; (ii) iodine bonded quasi-covalently to a carbonyl ligand in partially filled (50%) sites; (iii) partially filled sites (50-40%) of carbonyl ligands, with oxygen at van der Waals distances from iodine. Advantages of using Fourier-filtered EXAFS for complex crystal structures are discussed.

Calcium orthophosphates: crystallization and dissolution.

Calcium orthophosphates: crystallization and dissolution.

Atomic order in magnetic Mn inclusions in Si crystals: XAS and TEM studies

A Si (100) crystal implanted with Mn+ ions, exhibiting ferromagnetic properties, was studied with XAS and TEM to examine the local atomic order around Mn atoms. The advantage of the XAS technique was its elemental selectivity, which allowed extracting information on the atomic surroundings of Mn atoms even at very low concentrations of this element, incapable of producing a signal in X-ray diffraction. It is very important to find out what is responsible for the ferromagnetism of this new class of materials. The knowledge of the location of Mn atoms in the Si crystal lattice is crucial in developing models of ferromagnetic interactions. The performed studies have proven beyond doubt that Mn atoms are not located randomly in the Si crystalline matrix but form Mn–Si clusters immersed in a strained Si matrix. Assuming the atomic order and dimensions of the cluster found through EXAFS and HRTEM, we have reproduced the electronic structure of Mn atoms by modeling the XANES spectrum in agreement with the experimental one.

X-ray Spectroscopic and Magnetic Investigation of C:Ni Nanocomposite Films Grown by Ion Beam Cosputtering

The nearest-neighbor coordination and electronic structure in C:Ni(∼30 at.%) nanocomposite films grown by ion beam cosputtering in the temperature range of room temperatue (RT) to 500 °C are investigated by the means of extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge spectroscopy (XANES) and X-ray photoelectron spectroscopy (XPS). The obtained results are correlated with the composite nanostructure published elsewhere and magnetic properties determined by the means of X-ray magnetic circular dichroism (XMCD) and superconducting quantum interference device (SQUID) magnetometry. A combined use of EXAFS, XANES, and XPS shows that a carbidic Ni phase exhibiting only local atomic ordering is formed at low growth temperatures (≤200 °C), while ordered carbidic Ni phase forms at ∼300 °C. Further increase in growth temperature results in the formation of face-centered cubic (fcc) Ni with a high degree of crystallinity. On the other hand, Ni incorporation strongly promotes the formation of carbon structures with the prominent peak in C K-edge XANES spectra positioned at 288.5 eV in the whole growth temperature range. The magnetic measurements show no magnetic response for the films grown at RT to 200 °C, superparamagnetic behavior for the film grown at 500 °C with >90% of the Ni atoms in metallic state, and a weak magnetic response for the film grown at 300 °C, indicating the presence of Ni-rich regions within carbon containing Ni nanoparticles with ∼3% of Ni atoms in metallic state.

Modeling the atomic structure of an amorphous Co 57Ti 43 alloy produced by mechanical alloying using RMC simulations

The local atomic order of an amorphous Co 57Ti 43 alloy produced by Mechanical Alloying was studied by synchrotron x-ray diffraction and modeled through reverse Monte Carlo (RMC) simulations of its x-ray total structure factor. Average coordination numbers and average interatomic distances between first neighbors were calculated using the g i j ( r ) functions obtained from the RMC simulations. The values found were compared to data obtained recently using the Extended X-ray Absorption Fine Structure (EXAFS) technique. The results obtained from the simulations corroborated the shortening in the Co–Ti average interatomic distance indicated by the EXAFS analyses, and average coordination numbers and average interatomic distances obtained from both methods agree with each other within error bars, except for the Ti–Ti average interatomic distance. In addition, the bond-angle distribution functions Θ i − j − k ( cos θ ) were obtained from the simulations and compared to the Θ i − j − k ( cos θ ) functions obtained for an amorphous Ni 60Ti 40 alloy.