Analysis Of X-ray Absorption Fine Structure Data Research Articles

Systematic investigation of relativistic effects in EXAFS data analysis

Modern XAFS (x-ray absorption fine structure) data analysis is based on accurate multiple-scattering (MS) calculations of the x-ray absorption cross section, usually carried out solving the nonrelativistic Schr\"odinger equation for complex effective optical muffin-tin potentials describing the scattering of the atoms. The introduction of relativistic effects in extended XAFS (EXAFS) multiple-scattering calculations has been described in several papers and shown to be important for heavy atoms. However, few examples of applications and detailed studies of relativistic effects were given so far. In this work, we have performed a systematic investigation of relativistic corrections in systems of increasing atomic number, using a reliable simulation scheme recently developed and based on the incorporation of a pseudo-Schr\"odinger equation effectively replacing the Dirac relativistic form. Calculations have been put to a test in 12 different pure-element condensed-state systems, with the atomic number ranging from $Z=10$ for crystalline Ne to $Z=90$ for crystalline Th. The importance of accounting for relativistic effects has been highlighted for elements with $Z\ensuremath{\gtrsim}60$, as ones for which relativistic corrections for amplitudes of calculated XAFS MS signals exceed $10%$. The size of relativistic effects for calculated higher-order XAFS signal (with respect to the dominant single-scattering first-neighbor signal) has been shown, taking as example the ${L}_{3}$-edge spectra of crystalline Au and Pb. The size of relativistic effects for the $K$ and ${L}_{3}$ edges has been also evaluated, showing a slight increase of relativistic corrections for the ${L}_{3}$ edge. The improvement in the accuracy of XAFS simulations has been demonstrated comparing the results obtained for structural refinements of the ${L}_{3}$ edge of crystalline Au at 300 K.

MiXAFS: a program for X-ray absorption fine-structure data analysis.

A new program called miXAFS for the analysis of X-ray absorption fine-structure (XAFS) data is presented. miXAFS can analyze the XAFS functions simultaneously for all measured X-ray absorption edges of the constituent elements in a sample under the constraints for the structural parameters over the edges. The program provides a surface plot of the R-factor as a function of two structural parameters, which is useful to validate the optimized structural parameters. The structural parameters can be obtained from the XAFS data in a few steps using the setting file and batch process. The program, which is coded in MATLAB and freely available, runs on Macintosh and Windows operating systems. It has a graphical user interface and loads experimental data and XAFS functions in a variety of ASCII data formats.

Local structure analysis around the Nd center in a ternary catalyst comprising Nd(vers)3, Al((i)Bu)3 and Al((i)Bu)2Cl by XAFS.

Lanthanide-based catalysts are highly active for isoprene polymerization in hexane. In this paper, a ternary catalyst consisting of neodymium neodecanoate {Nd(vers)3}, Al((i)Bu)3 and Al((i)Bu)2Cl was studied by using X-ray-absorption fine-structure (XAFS) technique. A sealed and moisture-proof liquid sample cell with adjustable thickness was designed for Nd LIII-edge XAFS measurements. Based on the XAFS data analysis, detailed structure changes around the Nd center were obtained. It was found that the Nd(vers)3 molecules formed an oligomer structure in hexane solution with two Nd-O subshells (5O @ 2.39 Å and 5O @ 2.54 Å) around the Nd center. The alkylation process by adding Al((i)Bu)3 to the hexane solution of Nd(vers)3 partially destroyed the aggregation degree of Nd(vers)3 molecules in hexane solution. Al((i)Bu)3 ligands were bonded to the Nd center by Nd-C bonding. With the Nd : Al ratio increasing from 1 : 2.5 to 1 : 10, the O neighbors around Nd decreased from 4 to 2 but with an unchanged Nd-O bond length of 2.38 Å, and the C neighbors around Nd were kept at ca. 4 with Nd-C bond lengths in the range of 2.57-2.58 Å. The Nd-O bonds can be further replaced by Nd-C bonds during the aging process. The chlorination process by adding Al((i)Bu)2Cl to the mixture solution of Nd(vers)3 and Al((i)Bu)3 restrained intensively the agglomeration of Nd(vers)3 molecules in hexane solution. Al((i)Bu)2Cl ligands were bonded to the Nd center by Nd-Cl bonds. There were about 3-4 C neighbors at 2.58 Å, 2 Cl neighbors at 2.87 Å, and 2 Al next-neighbors at 3.14 Å around the Nd center. After allowing the ternary catalyst to stand for 5 days, the coordination numbers of Nd-C and Nd-Cl were all stabilized to 3 without bond length changes, and partial single Cl(-) anions were also bonded to the Nd center. All these structural details and their change tendency demonstrate that the decrease of aggregation degree of Nd(vers)3 molecules in hexane solution can improve the catalytic activity of the ternary lanthanide-based catalyst system.

Study of a neutron irradiated reactor pressure vessel steel by X-ray absorption spectroscopy

Reactor pressure vessel (RPV) reference steel samples submitted to neutron irradiations followed by thermal annealing were investigated by X-ray absorption fine structure (XAFS) spectroscopy. Several studies revealed that Cu and Ni impurities can form nanoclusters. In the unirradiated sample and in the only-irradiated sample no significant clustering is detected. In all irradiated and subsequently annealed samples increases of Cu and Ni atom densities are recorded around the absorber. Furthermore, the density of Cu and Ni atoms determined in the first and second shells around the absorber is found to be affected by the irradiation and annealing treatment. The comparison of the XAFS data at Cu and Ni K-edges shows that these elements reside in arrangements similar to bcc Fe. However, the local irradiation damage yields vacancy fractions which were determined from the analysis of XAFS data with a precision of ∼5%.

Sensitivity of Zn<sub>1-x</sub>Cd<sub>x</sub> Te to Tera-Hertz Signals Enhanced by Structural Orderings

A better efficiency of sensing tera-hertz signals (T-rays) depends on a better crystalline structure which is obtained by replacing a small amount of Cd on Zn sites of ZnTe crystals. Zn1-xCdxTe (ZCT) crystals were fabricated with an advanced vertical Bridgeman method and the structural properties of the ZCT crystals were studied with high resolution x-ray diffraction (HRXRD) and x-ray absorption fine structure (XAFS) measurements. The Rietveld refinement to the HRXRD data showed that the ZCT crystals at x ~ 0.05 were slightly less disordered than the ZnTe crystals. These results were confirmed by the XAFS data analysis. The better structural orderings can be directly related to the sensing and emitting efficiencies of T-ray by the ZCT crystals.

Atomic structure of Al88Y7Fe5 metallic glass

The structure of Al88Y7Fe5 metallic glass has been investigated by differential scanning calorimetry, x-ray powder diffraction, and x-ray absorption fine structure (XAFS) techniques. The amorphous alloy crystallizes according to the following scheme: amorphous→α-Al+residual amorphous→α-Al+Al3Y+AlFeY. The atomic structure of the amorphous alloy was modeled by reverse Monte Carlo methods. The resulting structure indicates that the prepeak (at 1.42Å−1) in the static structure factor S(Q) is caused by contributions of distinct Y-Y, Y-Fe pairs. From the analysis of our XAFS spectra at the Fe and Y K edge the following characteristics of the local structure are found: The interatomic distance between Fe and coordinated Al atoms in the amorphous state is significantly (8.9%) shorter than the sum of the nominal metallic-state radii. The average coordination number is anomalously 36% reduced compared to the value derived from the dense-random-packing (DRP) model, using again the nominal metallic-state radii. On the other hand, the Y-Al distance as well as the number of Al atoms coordinating Y is close to the values predicted by the DRP model. These anomalous changes around the Fe atoms indicate a strong interaction between Fe and Al, which corroborate a covalent bonding. In connection with the values obtained from the XAFS data analysis, the effective atomic radii in the amorphous phase have been calculated. Consequently, applying them to the calculation of the atomic size factor (λ=0.1076) confirms that Al88Y7Fe5 follows the rule of atomic size ratio for glass formation.

Statistical errors in X-ray absorption fine-structure data analysis

General criteria for performing a rigorous statistical error determination in X-ray absorption fine-structure data analysis are formulated. The equivalence of the residual function increment approach ( lambda : R(( lambda ))<Rmin+ chi p,0.952) with more rigorous Monte Carlo procedures is demonstrated in the case of the analysis of the Br2 spectrum. The possible existence of nonlinear effects in parameter space is discussed and a specific example referring to the S-C=O molecule case is illustrated. Practical suggestions are given to improve data collection procedures and to select appropriate fitting parameters in the analysis.