Local Atomic Order Research Articles

X-ray diffraction and extended X-ray absorption fine-structure study of RMn2 hydrides (R = Y, Gd or Dy)

Structural properties of RMn2 hydrides (R = Y, Gd or Dy) have been studied by means of powder X-ray diffraction (XRD) at 300 K and extended X-ray absorption fine structure (EXAFS) at 300 K and low temperatures. It was found that at 300 K the lattice parameters increase continuously upon hydrogen absorption (except for DyMn2Hx with x < 1) and the cubic crystal structure is preserved up to x = 3.5 for all three systems. For the fully charged RMn2H4.3, the same type of rhombohedral distortion of the crystal lattice was found (space group, R3m). For the hydrogen concentration range 3.5 < x < 4.3, coexistence of cubic C15 and rhombohedral phases with various contents was found. The local atomic order was studied by means of the EXAFS method. It was found that except for DyMn2Hx with x < 2 the values of Mn-Mn and Mn-R interatomic distances at low temperature agree well with those obtained from XRD measurements and that the hydrides with x = 1 and 2 are locally highly disordered. To stress similarities between the three RMn2Hx systems a common schematic phase diagram was constructed. The method of preparation of single-phase hydrides was found.

The atomic structure of a ?=5[001]/(310) grain-boundary in an Al-5% Mg alloy by high-resolution electron microscopy

A symmetrical tilt Σ=5[001]/(310) grain-boundary and its surrounding matrix in an Al-5% Mg alloy, as prepared by cold rolling and annealing, have been studied by conventional transmission electron microscopy, high-resolution electron microscopy, and analytical electron microscopy. EDS measurements of the Mg concentration in the bulk and at grain boundaries indicate variations of Mg content ranging between 4 to 11 atomic percent. This variation is attributed to local ordering of Mg atoms in the alloy. HREM images show that the boundary runs parallel to the median of (310) and contains segments primarily composed of two types of structural unit. One unit contains seven atomic sites, while the other contains eight. In both types of unit, the grain-boundary coincident site lattice is continuous across the interface and exhibits periodicity in its core structure. The core structure, which is characterized by “kite-like” structural units containing seven atoms, conforms well with the structure of Σ=5 grain boundaries in pure FCC materials. The presence of atomic-scale ordering of Mg atoms along [001] of the boundary core is associated with the existence of the structural unit possessing eight atomic sites.

Determining medium range order by powder diffraction

The method of atomic pair-distribution function (PDF) analysis of powder diffraction data, applied to the study of short-to-medium range atomic order in crystalline solids, is described. In real crystalline materials local atomic order is sometimes significantly different from the long range crystallographic lattice structure, due to lattice defects or some other local effects. While various physical properties are affected by such short and intermediate range order, there is not a wealth of experimental methods that can effectively characterize the local atomic order. We suggest that the PDF method coupled with the real space analysis is one of the most effective tools for this purpose.

Nanostructured amorphous SiAu alloys — Structure and atomic correlations

Nanostructured amorphous Si 1− x Au x alloys ( 0.1 ⪕ x Au ⪕ 0.35 ) were produced by an inert gas condensation and in situ consolidation technique. Small-angle neutron scattering (SANS), density measurements and wide-angle neutron scattering (WANS) were employed to characterize the microstructure and local atomic order. The SANS results are interpreted in terms of a microstructure which consists of nanometre-sized amorphous grains embedded in a matrix of lower density, formed by grain boundaries and free volumes. The relative densities depend on the average particle size, but not on the alloy composition. In all samples, the SANS intensity decreases at low Q according to a power law − Q −3.37, which is attributed to a surface fractal structure of dimension D s = 2.63. WANS reveals two contributions: (a) a continuously decreasing part resulting from the presence of about 10 at.% hydrogen, and (b) a modulation of the intensity as a function of the scattering angle, characteristic of amorphous material. The total pair correlation function g( R) presents maxima corresponding to the first and second atomic neighbours. Nanostructured amorphous SiAu alloys exhibit a reduced short-range order compared with systems with dense random packing of hard spheres, e.g. metallic glasses, or continuous random network structures, e.g. covalent systems.

ReflEXAFS Spectroscopy of Thin Fe/Sc Multilayers

For two three-layer systems, Sc/Fe/Sc and Fe/Sc/Fe, the fine structure in specular reflected x rays above the Fe K-edge has been investigated experimentally. Computer simulation of ReflEXAFS formation on multilayered structures has been done. Estimates of each layer's thickness and density as well as of local atomic order in Fe layers have been obtained. The possibility of ultrathin-layer investigations (down to a few angstroms) has been analyzed.

ReflEXAFS spectroscopy of thin Fe/Sc multilayers

For two three-layer systems. Sc/Fe/Sc and Fe/Sc/Fe, the fine structure in specular reflected x rays above the Fe K-edge has been investigated experimentally. Computer simulation of ReflEXAFS formation on multilayered structures has been done. Estimates of each layer's thickness and density as well as of local atomic order in Fe layers have been obtained. The possibility of ultrathin-layer investigations (down to a few angstroms) has been analyzed.

Atomic-scale structures of liquid Sn, Ge and Si by reverse Monte Carlo simulations

The atomic-scale structures of liquid Si, Ge and Sn have been modelled by the reverse Monte Carlo method. A geometrical analysis of the model atomic configurations has been carried out in terms of bond angle distributions and spherical harmonics invariants. Similar geometrical features, including signatures of local tetrahedral order, have been found in the first coordination shells of all the liquid metals studied. A relation between the local atomic ordering in liquid Si, Ge and Sn, and that of the corresponding solids with the tetrahedral white-tin-type structure has been established. The influence of the local and medium-range atomic orderings in liquid Se, Ge and Sn on the shape of the respective structure factors has been discussed.

The diffuse-scattering method for investigating locally ordered binary solid solutions

Diffuse-scattering investigations comprise a series of maturing methods for detailed characterization of the local-order structure and atomic displacements of binary alloy systems. The distribution of coherent diffuse scattering is determined by the local atomic ordering, and analytical techniques are available for extracting the relevant structural information. An extension of such structural investigations, for locally ordered alloys at equilibrium, allows one to obtain pairwise interaction energies. Having experimental pairwise interaction energies for the various coordination shells offers one the potential for more realistic kinetic Ising modeling of alloy systems as they relax toward equilibrium. Although the modeling of atomic displacements in conjunction with more conventional studies of chemical ordering is in its infancy, the method appears to offer considerable promise for revealing additional information about the strain fields in locally ordered and clustered alloys. The diffuse-scattering methods for structural characterization and for the recovery of interaction energies are reviewed, and some preliminary results are used to demonstrate the potential of the kinetic Ising modeling technique to follow the evolution of ordering or phase separation in an alloy system.

Thermomagnetic annealing of amorphous CoZrM (M=Zr, Nb, Ti) and CoZrPt thin films Relationship between the relaxation process of the induced anisotropy K u and the local atomic order

The uniaxial magnetic anisotropy of amorphous Co 95- x Zr 5M x (M=Zr, Nb, Ti) and Co 91- x Zr 9Pt x thin films present large variations as a function of the pressure of the sputter gas. An adequate thermomagnetic annealing permitted to show that K u is the sum of two contributions K a u+ K s u, where K a u and K s u are the i nduced anisotropies related to the local anisotropy of the octahedral-icosahedral and trigonal-type clusters, respectively. Co 91- x Zr 9Pt x films exhibit a peculiar thermomagnetic relaxation that is explained by the reversibility of the pseudodipolar anisotropy and the thermal instability of trigonal-type clusters resulting from the substitution of Pt.

Thermodynamic modelling of the local atomic order in amorphous alloys based on silicon and germanium

Abstract The free-energy model (FEM) previously developed for the prediction of the bonding in amorphous covalent alloys is applied here to amorphous alloys based on Si and Ge. Predictions of the FEM are presented which indicate that H is preferentially bonded to Si and that dangling bonds appear preferentially on Ge atoms in amorphous Si x Ge y ,H z (a- Si x Ge y ,H z ) alloys, thus demonstrating that these experimentally observed effects need not be attributed to specific deposition conditions. In addition, it is predicted that O bonds preferentially to Si in a- Si x Ge y ,O z , alloys and that Ge will be more strongly oxidized when present in very dilute concentrations in a-SiO2.

Structural variations in strained crystalline multilayers

We present a computer simulation study of thin crystalline multilayers constructed from two fcc solids with differing lattice constants and binding energies. Initially the two solids have the same orientation, and the interface is perpendicular to the common [100] direction. We then minimize the energy of the system at zero temperature or equilibrate it at a finite temperature. Both materials are described by Lennard-Jones interatomic potentials. A novel technique for analyzing local atomic ordering, common neighbor analysis, is used to identify structural characteristics in these systems. As we gradually vary the lattice mismatch between the two solids, several structural changes are observed in the layers of smaller atoms after energy minimization. At a mismatch larger than 14%, the layers transform into the hep structure, while at smaller mismatches extended structural defects are generated. At elevated temperatures, the hcp structure is transformed back to fcc, and the structure defects disappear.

Structure and Properties of Strained Crystalline Multilayers

ABSTRACTWe present a computer simulation study of thin crystalline multilayers constructed from two FCC solids with differing lattice constants and binding energies. Both materials are described by Lennard-Jones interatomic potentials and initially have the same orientation and coherent interfaces. We have studied systems in which interfaces are perpendicular to the common [100] and [111] directions, respectively. A novel technique for analyzing local atomic ordering, Common Neighbor Analysis, is used to identify structural characteristics in these systems. We have found several structural changes in the layers of smaller atoms, including an FCC to HCP transition. In a system with (111) texture, a coherent interface to incoherent interface transformation is observed. Calculations of elastic constants of these multilayer structures show that elastic anomalies are associated with the structural variations.

High-resolution photoelectron spectroscopy study of (√3 × √3)R30°-Ag on Si(111)

High-resolution surface and bulk sensitive photoemission were used to study the Si2p core level from the (√3 × √3)R30°Ag/Si(111) surface. Four components of the Si2p levels are observed for this system. Their relative positions are found to be independent with respect to sample preparation, suggesting no net change in the local atomic order of the √3 domains. However, their relative intensities, measured for surfaces prepared at deposition temperatures of 350 and 530°C, show significant differences which are assigned to changes in the relative √3 domain sizes.

High-resolution photoelectron spectroscopy study of (√3 × √3)R30°-Ag on Si(111)

High-resolution surface and bulk sensitive photoemission were used to study the Si2p core level from the (√3 × √3)R30°-Ag/Si(111) surface. Four components of the Si2p levels are observed for this system. Their relative positions are found to be independent with respect to sample preparation, suggesting no net change in the local atomic order of the √3 domains. However, their relative intensities, measured for surfaces prepared at deposition temperatures of 350 and 530°C, show significant differences which are assigned to changes in the relative √3 domain sizes.

Caracterisation de l'ordre par diffraction d'electrons

CHARACTERIZATION OF THE ORDER BY ELECTRON DIFFRACTION: In the case of a solid with a local atomic order, we can consider only a statistical description of atomic positions. For a homogeneous phase, this description is given by the two-site occupancy correlation function, the three-site occupancy correlation function, ... etc. The local atomic order is investigated by experiments in which the solid is bombarded with a well-collimated monoenergetic beam of particles (here electrons). Starting from a model describing the force law between an electron and an atomic scatterer and from a model of the solid describing the distribution of atoms in space, a computation of elastically backscattered intensities can be carried out. The latter depends on geometrical parameters which are determined in such a way that one gets the best agreement between the results of calculation and the measurements. This work aims to provide the way of proceeding to calculate the diffuse intensity. The main difficulty in this task appears when we undertake to treat the problem of multiple scattering. We provide here a detailed description of a calculation corresponding to the case of the backscattering of electrons at a single crystal surface covered with a partially disordered atomic layer. Assuming that the coverage in adatoms is small and these adatoms are weak scatterers, we can neglect the multiple scattering events inside the layer. This simplifying assumption leads to an expression of the diffuse intensity in terms of: the Fourier transform on the 2D surface lattice of the pair correlation function; the renormalized transition matrices of the different kinds of adatoms. PACS numbers: 61.14.Dc, 68.10.Jy

Electronic structure of crystalline InP oxides

Tight-binding electronic structure calculations for all crystalline InP oxides are presented. X-ray-photoemission valence band spectra are compared with these results and the different contributions to each spectrum are analyzed. A systematic correlation with the local atomic order is found. The notion of oxidation number is analyzed in terms of the electronic structure, concluding that whatever the coordination for a given atom is, the oxidation number is the same.

Enthalpy relaxation and Curie temperature behavior in Fe(Ni,Co)Zr amorphous alloys

Annealing-induced changes in enthalpy relaxation and Curie temperature, T c, were examined for amorphous (Fe 0.85Ni 0.15) 90Zr 10 and (Fe 0.9Co 0.1) 90Zr 10 alloys. The compositional dependence of an endothermic reaction was also examined for amorphous (Fe 1− x Co x ) 90Zr 10 ( x = 0–1) alloys. A reversible endothermic reaction occurs above the annealing temperature, T a. The change in the magnitude of the endothermic peak, ΔC pmax , with T a shows a single stage with a peak at about T x − 400 K (i.e., the temperature that is 400 K lower than crystallization temperature). For (Fe 1− x Co x ) 90Zr 10 alloys, the endothermic reaction is the largest at x = 0.5 and disappearsat x = 0 and 1. The change in T c with T a is related to the increase in T c at T a ≌ T x − 400 K , followed by the decrease in T c at temperatures above T x − 400 K. A good correspondence was observed between the reversible endothermic enthalpy relaxation and the increase in T c and between the irreversible exothermic enthalpy relaxation and the decrease in T c. These results are explained as a consequence of the low-temperature endothermic reaction which is due to the local chemical ordering of FeNi and FeCo atoms with shorter relaxation times. The exothermic reaction is mainly due to the long-range topological change leading to the densification of amorphous structure with longer relaxation times.

Origin of tweed texture in the simulation of a cuprate superconductor

The origin of metastable tweed texture (microstructure) is studied by computer simulation. A two-dimensional model of 99*99 unit cells represents a layer with an oxygen deficit of the high-Tc superconductor YBa2Cu3O7- delta and exhibits the ferroelastic tetragonal-orthorhombic phase transition. The tweed texture is known to be important for flux pinning. In the model, the local ordering of the oxygen atoms produces long-range strain fields, which have been studied computationally by a molecular dynamics technique. The system has a strong tendency to form the tweed texture, as observed experimentally. Well above the structural phase transition temperature, the strain fluctuations show well developed embryos of the tweed texture, whose temperature dependence agrees with theoretical estimates obtained using the independent-site approximation. On quenching to below the transition temperature, the texture first becomes more regular in spacing and coarsens before the system orders macroscopically: the kinetics behaviour is quite different from the traditional model of nucleation and growth.

Local Atomic Ordering in Amorphous Fe-Based Alloys

Local Atomic Ordering in Amorphous Fe-Based Alloys

Local atomic order in amorphous III-V semiconductors by EXAFS and X-ray anomalous scattering

Short range ordering is compared in three amorphous III-V compounds: InP, GaP and GaAs. For a-InP and a-GaP, a high degree of chemical disorder is evident from an extended X-ray absorption fine structure (EXAFS) study. In the case of a-GaAs, the three partial pair distribution functions deduced from anomalous X-ray diffraction experiments indicate strong chemical ordering. A joint analysis of EXAFS and anomalous X-ray diffraction data reveals the presence of threefold coordinated As which is shown to play an important role in the relaxation of the network as demonstrated by the anomalous X-ray diffraction measurements.