Partial Interatomic Distances Research Articles

Changes in the chemical compound and local atomic structure of ultrathin surface layers of Fe due to implantation of argon and oxygen ions

In this work, we consider the effect of irradiation in Ar+ → O+ and O+ → Ar+ sequences on changes in the chemical compound, type of chemical bond and the local atomic structure of ultrathin (~20 nm) iron surface layers. Investigation of the chemical compound and the type of chemical bond of the ion-modified surface was carried out by the XPS (X-ray photoelectron spectroscopy) and Auger electron spectroscopy. The study of changes in the local atomic structure was carried out by an XAFS-like method — electron energy loss fine structure (EELFS) spectroscopy. The parameters of the local atomic environment of oxygen and iron — partial interatomic distances, dispersion parameters, and coordination numbers — were obtained by analyzing atomic pair correlation functions. Analysis of experimental data showed that the two-stage irradiation of iron significantly changes the chemical compound and local atomic structure of the initial surface. This leads to formation of an oxide layer of greater depth than in the case of irradiation with only oxygen ions.

Structure evolution and entropy changes of Ga0,7Bi0,3 liquid alloy

ABSTRACTStructure of Ga0,7Bi0,3 has been studied by means of X-ray diffraction method reverse Monte–Carlo simulations at different temperatures. Temperature dependences of structure factors and parameters obtained from them are analysed. Most probable interatomic distances, mean and mean root square distances have been calculated from pair correlation functions and used for estimation of thermal expansion coefficients at short-range-order scale. It is shown that untypical behaviour of these coefficients is related to free volume evolution from critical point to higher temperatures. Based on the temperature dependencies of partial interatomic distances and coordination numbers, as well as free volume distribution, a temperature interval of transition of a melt from a microheterogeneous to a microhomogeneous state was established.

Local atomic structure of solid solutions with overlapping shells by EXAFS: The regularization method

Abstract The regularization method of solving ill-posed problem is used to determine five partial interatomic distances on the basis of combined two EXAFS spectra. Mathematical algorithm and experimental results of the EXAFS analysis for Ni c Zn 1− c O ( c = 0.0, 0.3, 0.5, 0.7, 1.0) solid solutions with the rock salt (rs) crystal structure are discussed. Samples were synthesized from the binary oxide powders at pressure of 7.7 GPa and temperatures 1450–1650 K. The measurements were performed using synchrotron facilities (Russian Research Centre “Kurchatov Institute”, Moscow). The Ni and Zn K absorption spectra were recorded in transmission mode under room temperature. It is shown, the ideal rock salt lattice is distorted and long-range order exists only in the average (Vegard law). In order to determine coordinates ions for the solid solution with rock salt structure, we used the Pauling model. The simulation is performed for 343,000 cluster of oxide ions. The distribution functions for ions (Ni O, Ni Ni, Ni Zn, Zn Zn, Zn O, O O) depending on the distance are obtained. The width of the Gaussian distribution function is determined by the difference of the radii of the metal ions. The results are consistent with the data both X-ray diffraction and the EXAFS spectroscopy.

Electron energy loss spectroscopy equation for spectra with overlapping oscillations and its solution by a regularization method

Abstract The integral equation of EELFS spectra with the overlapping oscillations for two-component systems is proposed. To solve this equation Tikhonov’s regularization method with the iterative procedure is applied. The atomic pair correlation functions of the investigated objects are obtained from the experimental EELFS spectra of Cu50Ni50 and Cu50Mn50 alloys by using Tikhonov’s regularization method. The experimental results agree well with the known crystallographic data (partial interatomic distances, coordination numbers and Debye–Waller factors).

Interatomic distances for overlapping shells in disordered systems: model-less approach

The regularization method of solving ill-posed problem is used to determine five partial interatomic distances from combined two EXAFS spectra without building any model of atomic structure. The mathematical procedure and the experimental results of the EXAFS analysis for NicZn1−cO (c = 0.0, 0.3, 0.5, 0.7, 1.0) solid solutions with the rock salt (rs) crystal structure synthesized from the binary oxide powders are discussed. The measurements were performed using synchrotron facilities (Russian Research Centre "Kurchatov Institute", Moscow). The Ni and Zn K absorption spectra were recorded in transmission mode at room temperature. It was shown the ideal rock salt lattice is distorted and long-range order exists only at the average (Vegard law).

Electron energy loss spectroscopy for quantitative analysis of the local atomic structure of superthin oxide films on the surface of 3d-metals

Abstract In this paper we present a theoretical description of the extended energy loss fine structure (EELFS) formation, and derive simple analytical formulas for the EELFS spectra calculation. Methods of calculating the amplitude and intensity of electron transitions of different multipolarity on excitation of an atomic core level by an electron impact are proposed, and the corresponding test calculations are carried out. The M 2,3 EELFS spectra of pure 3d-metals (Fe, Ni, Cu) and stoichiometric oxide films (Fe 2 O 3 , NiO and CuO), as well as the K EELFS spectra of oxygen, obtained with a JEOL JAMP - 10 Scanning Auger microprobe are presented. A technique of extracting the normalized oscillating parts from experimental EELFS data, based on the use of the calculated electron transition intensities is suggested. The atomic pair correlation functions of the investigated objects are obtained from the experimental EELFS data with allowance for multipolarity of the processes of excitation of the atomic core level by electron impact. The experimental results agree well with the known crystallographic data (partial interatomic distances, coordination numbers and Debye–Waller factors).

Variable angle XAFS study of multilayer nanostructure: Determination of selective concentration profile and depth-dependent partial atomic distributions

We propose a new method for studying multilayer structure using angle resolved extended x-ray absorption fine structure (EXAFS) measurements. The linear integral equation describing a connection between the fluorescence intensity for spectrum of element C, the incident beam energy E, the incident angle ϕ and the concentration profile p(z,C) has been derived. It is a Fredholm integral equation of the first kind, it belongs to the class of ill-posed problems and for solution it needs special methods. We use the regularization method. For determining the depth-dependent partial interatomic distances we use angle resolved EXAFS data. The effectiveness of the method has been tested during numerical simulation on the model crystalline three-layer with BCC structure: Cr/Fe/Cr.

High resolution in EXAFS data analysis of multilayer nanostuctures

Abstract The regularization method of solving ill-posed problem is used to determine three partial interatomic distances from one EXAFS spectrum. The mathematical procedure and the experimental results of the EXAFS analysis for multilayer FeNi/V nanostructures are discussed. The measurements were performed using ESRF facilities (BM line 26A, Grenoble, France). The Fe, Ni and V edge absorption spectra were recorded in fluorescence and transmission modes at room temperature for foils Fe, Ni, V and for multilayer nanostructures [Fe 82 Ni 18 (5ML)/V(6ML)] 25 , [Fe 82 Ni 18 (10ML)/V(6ML)] 25 , films Fe 82 Ni 18 , Fe, Ni, V (1000 A thickness) in fluorescence mode only. All film samples were prepared at University Uppsala (Sweden). Peculiar features of the method are high resolution for overlapping in r -space shells and high accuracy in the determination of partial interatomic distances.

New possibilities of the EXAFS spectroscopy in study of the local atomic structure of multilayer nanoheterostructures

New possibilities of studying the local atomic structure of multilayer nanoheterostructures by EXAFS spectroscopy have been considered. It is shown that this method makes it possible to determine the average partial interatomic distances for overlapping coordination spheres with a small error (∼0.01 A). As an example, the experimental results for a sample of a (FeNi)/V multilayer nanoheterostructure.

Spectroscopy of extended electron energy loss fine structures for quantitative analysis of the local atomic structure in ultrathin oxide films on the surface of 3d-metals

The work presents the theoretical description of formation processes of extended electron energy loss fine structure (EELFS) spectra. Simple analytical formulas are obtained to calculate EELFS spectra. Methods are proposed to compute amplitudes and intensities of electronic transitions of different multipolarity in electron-impact excitation of the inner shell of an atom. The corresponding test calculations are performed. We present experimental M 2,3 EELFS spectra of 3d-metals obtained from clean surfaces of Fe, Ni, and Cu and ultrathin stoichiometric oxide films of Fe2O3, NiO, and CuO as well as K EELFS spectra of oxygen from thin stoichiometric oxide films. Techniques are proposed to extract normalized oscillating parts from experimental EELFS data based on the use of calculated intensities of the corresponding electronic transitions. Atomic pair correlation functions are obtained for the objects under study from experimental EELFS data with regard to multipolarity of electron impact excitation of the atomic inner shell. Experimental results correspond well to known crystallographic data (partial interatomic distances, coordination numbers, and Debye-Waller factors).

Short range order in disordered Ni–Mn alloys by EXAFS

Abstract A new method of determination of the nearest-neighbor (1NN) coordination numbers for binary disordered alloys is proposed. The method is reduced to the solution of the system of linear algebraic equations for the partial coordination numbers. As input data we use 1NN partial interatomic distances obtained independently for each EXAFS spectrum in binary alloy and the Debye–Waller factors. Coordination numbers and interatomic distances for 75 and 80 at% Ni–Mn alloys in ordered and disordered states are presented.

Local structure in Ge–Si solid-state solutions by combined Ge and Si EXAFS

Abstract A new method of studying local structure in disordered binary systems by combined EXAFS is proposed. This consists of determining partial interatomic distances by solution of the system of integral equations describing two EXAFS spectra. This information is used as input data for the determination of partial coordination numbers. The problem in this case is reduced to the solution of the well-conditioned system of linear algebraic equations. We apply this method to obtain local structure information in Ge–Si solid-state solutions.

STUDY OF ALUMINUM-OXYGEN BINARY SYSTEMS: DETERMINATION OF PARTIAL INTERATOMIC DISTANCES

This paper reports on an Auger-electron study of the oxidation of the perfect surface of polycrystalline Al in residual gas atmosphere at a partial pressure of oxygen of 10 -10 torr. Under supervacuum conditions, layer-by-layer oxidation of the aluminum surface takes place along with stabilization of the chemical composition of the oxide film. Oxygen can penetrate into the oxidation region not only by deposition from the residual atmosphere but also by other mechanisms. After the chemical composition of the oxide film was stabilized, its local atomic structure was studied by Al and O K-edge extended electron energy loss fine structure (EELFS) spectroscopy. The estimated partial interatomic distances for Al–Al, Al–O, O–Al, and O–O atomic pairs permitted us to conclude that an oxide-like phase is formed in the subsurface layers of aluminum oxidized under supervacuum conditions and to construct a model of the local atomic structure of this oxide-like phase.

Local Structures for Ni–Mn Alloys

A new method to determine of the nearest neighbor (1NN) coordination numbers for binary disordered alloys is proposed. The method is reduced to the solution of the system of linear algebraic equations for the partial coordination numbers. As input data we use 1NN partial interatomic distances obtained independently for each EXAFS spectrum in binary alloy and the Debye-Waller factors. Coordination numbers and interatomic distances for 75 and 80 at.% Ni-Mn alloys in ordered and disordered states are presented.

Application of the electron energy loss fine structure technique to the investigation of the Al [sbnd]Ag binary system

Abstract In this study, Electron Energy Loss Fine Structure (EELFS) spectra were recorded over Al K and Ag M4,5 edges for the Al-20% Ag alloy after high temperature aging and for pure components of the alloy. Radial distribution functions (RDF) were obtained by Fourier transform and with the regularization method. A comparison between the experimental RDFs and model ones shows the decomposition state of the solid solution. After high temperature aging, the binary alloy becomes an aluminum matrix with zones enriched with silver atoms. The Al Al interatomic distance in the binary system is equal to that in pure aluminum. Ag Ag and Ag Al partial interatomic distances agree well with the distances in the γ-phase of Ag2Al.