X-ray Absorption Fine Structure Experiments Research Articles

Atomic and electronic structure of quasiperiodic and crystalline Mg-61 at.% Al alloys

Mg-Al alloys with composition Mg-61 at.% Al display two types of structure: a stable crystalline state (the so-called β-Mg2Al3) and a metastable one showing both quasiperiodicity and inflation symmetry. The atomic as well as electronic structures of stable crystalline β-Mg2Al3 and a metastable Mg39Al61 phase that exhibits quasiperiodicity and inflation symmetry have been studied thanks to x-ray spectroscopy techniques. Extended x-ray absorption fine-structure experiments performed above the Al and Mg K absorption edges have probed the local atomic order around Mg as well as Al atoms. X-ray emission and absorption spectroscopies have investigated occupied and unoccupied electronic states around Mg and Al. The local order has been found to be the same around Al atoms in both alloys, whereas around Mg atoms differences are seen for higher shells than first neighbours. This suggests that the same clusters must be involved in both phases and that the quasiperiodicity is connected with modification of the Mg atom environment forming the linkage between large clusters. On the other hand, the electronic distributions show differences from β-Mg2Al3 to the quasiperiodic phase that are consistent with the local order studies. A pseudogap is observed in the Mg and Al electronic structures which is more marked in the quasiperiodic phase than the crystalline alloy. By analogy to quasicrystals, we suggest the enhancement observed is due to the occurrence of an inflation mechanism in the arrangement of the clusters in the quasiperiodic alloy with respect to the β-Mg2Al3 crystal.

Time-resolved stopped-flow x-ray absorption fine structure system using synchrotron radiation for fast reactions in solution

A time-resolved stopped-flow x-ray absorption fine structure (XAFS) system was developed using synchrotron radiation as an x-ray source. The shortest sampling time for time-resolved measurements was on the order of 1 ms with the dead time of ∼5 ms, which was limited by the mixing procedure in the stopped-flow unit and by the drift velocity of the ions generated in the ionization chamber as a detector. The time-resolution ability and the quality of the obtained XAFS spectra were greatly improved in comparison with the laboratory stopped-flow XAFS apparatus previously constructed. The time-resolved XAFS measurements were performed for the reduction reaction of [Fe(CN)6]3− with L-ascorbic acid in an acidic aqueous solution. The rate constants obtained from the XAFS experiments were reasonable judging from the kinetic parameters previously reported.

Structural analysis of quasicrystals and approximants in AlCu(Ru, Fe) systems

Abstract Using extended X-ray absorption fine-structure experiments performed above the Ru, Cu and Fe K absorption edges, the local order around respectively Cu and Ru, Cu and Fe has been determined in icosahedral Al65Cu20Ru15 and in rhombohedra1 Al62·8Cu26Fe11·2. Strong resemblances are observed in the environments of Cu and of Ru or Fe atoms in these alloys and in icosahedral Al63Cu24·5Fe12·5. Chemical ordering between Al and Ru or between Al and Fe atoms has been found. Correlations with the electronic structure are made.

High-intensity x-ray line focal spot for laboratory extended x-ray absorption fine-structure experiments

Use of an intense and narrow linear-shaped x-ray source is essential to improve the quality of the extended x-ray absorption fine-structure measurements in a laboratory. To this end, an x-ray generator with a large rotating anode, designed for dedicated use of point-shaped focal spot, was modified to provide a linear-shaped focal spot of ∼0.1 mm in width at an exit angle of 6°. The maximum allowable current was 600 mA, which has never been realized so far using a conventional x-ray linear focal spot.

Signature of the local order in a perfect icosahedral AI–Pd–Mn quasicrystal

Abstract Using extended X-ray absorption fine-structure experiments performed above the Al, Pd and Mn K absorption edges, the local order around Al, Pd and Mn atoms has been determined in perfect icosahedral Al71Pd19Mn10. Chemical ordering between A1 and Pd or Mn atoms is confirmed. Moreover, the environment of Pd atoms appears unique and could be connected to the particular stability of this alloy.

ChemInform Abstract: Study by Extended X‐Ray Absorption Fine‐Structure Technique and Microscopy of the Chemical State of Yttrium in α‐Polycrystalline Alumina.

AbstractDer größte Teil des im Al2O3 als Dotierung (l mol‐%) vorhandenen Yttriums wird als Y3Al5O,2 ausgefällt.

Spin-polarized photoelectron diffraction.

The use of multiplet-split $s$ levels in magnetic atoms or ions for spin-polarized photoelectron diffraction or extended x-ray absorption fine-structure experiments is discussed. Over the range 50-500 eV, spin-dependent scattering factors for ${\mathrm{Mn}}^{2+}$ show exchange-induced asymmetries of as high as 8%, with significantly larger effects expected for forward scattering as compared to backscattering. Single-scattering-cluster photoelectron diffraction calculations for Mn $3s$ emission from antiferromagnetic KMn${\mathrm{F}}_{3}$ show net effects ranging from 8% at 50 eV to 1% at 400 eV.