H. Tolentino

Incorporation of Ni into natural goethite: An investigation by X-ray absorption spectroscopy

Abstract Goethite (α-FeOOH) is abundant at the Earth’s surface and has the unusual capacity to adsorb and fix ions from migrating solutions. Understanding the mechanisms by which foreign elements are incorporated into natural goethite has implications for environmental and mining problems. X-ray absorption spectroscopy (XAS) was used to obtain structural information on the local environment around Ni in natural Ni-containing goethite (1.8-4.1 mol% Ni) from Vermelho lateritic deposit of Serra dos Carajás (Brazil) and in synthetic analogues. The data were collected at the LNLS XAS beam line at the Ni and Fe K-edges, at room temperature, and at the Ni K-edge at 8 K. Nickel was found in essentially the same environment in all natural and synthetic samples, with negligible thermal disorder. The coordination polyhedron is a tetragonal dipyramid of oxygen atoms showing that Ni preserves its usual local symmetry. This finding is compatible with a model in which substitution of Ni for Fe is accompanied by a proton capture resulting in NiO2(OH)4 octahedra. The polyhedral linkages are similar to that of pure a-FeOOH, consisting of four shared edges at about the same metal-metal distances, as in the pure mineral. The third and longest metal-metal distance is about 6% larger than the expected corner-sharing distance in the α-FeOOH structure, showing that incorporation of Ni locally distorts and opens the structure.

Inter-atomic distance contraction in thiol-passivated gold nanoparticles

Abstract Size-dependent inter-atomic distance contraction was investigated by Extended X-ray Absorption Fine Structure in thiol-capped gold nanoparticles. A slight nearest-neighbor distance reduction was observed as a function of particle diameter (2–4 nm range) but, for all samples, it was less than 1%. This value contrasts with the larger effect expected and found in other systems, especially for the smallest particles (2 nm) where it is twice as small. Our analysis also points out a short metal–ligand bond, suggesting a rather strong surface interaction. We interpreted these results as a passivant effect where the metal–ligand interaction partially compensates the expected lattice contraction.

A 2.3 to 25 keV XAS beamline at LNLS

The LNLS XAS beamline has been operating for external users since July 1997. Many facilities and improvements have been progressively added to it, extending the range of applications. Here, a technical description of the main beamline components is given, and results concerning important points, such as available flux at low and high energies, harmonic contamination, energy resolution and stability, are presented. Some key results are given to demonstrate the beamline performance and limitations. It is shown that the beamline can cover a large energy range, starting from the rather low energy of 2.3 keV up to 25 keV.

Aberration-free and harmonic-free optics for time-resolved X-ray absorption spectroscopy using synchrotron radiation

Abstract To obtain aberration-free optics, we tailored a new crystal shape which achieves an ellipsoidal profile. The theoretical shape has been designed using elastic equations and a ray-tracing technique. Cutting has been carried out and evaluation of the on-site performance has been achieved for a set of crystals. The size of the image turns out to be limited only by the demagnification factor and is restricted to about 300 μm. In order to get rid of the harmonic contamination and to get a reliable signal out of an absorbing sample we use a flat fused quartz mirror at grazing incidence as a low-pass filter. The performance of this filter has been studied experimentally by scanning the angle of the mirror relative to the beam and by a ray-tracing simulation. An adjustment of the grazing incident angle to reflect the fundamental without any harmonic contamination turns out to be an efficient and easy task in our current setup.

Energy-dispersive X-ray absorption spectroscopy at LNLS: investigation on strongly correlated metal oxides

An energy-dispersive X-ray absorption spectroscopy beamline mainly dedicated to X-ray magnetic circular dichroism (XMCD) and material science under extreme conditions has been implemented in a bending-magnet port at the Brazilian Synchrotron Light Laboratory. Here the beamline technical characteristics are described, including the most important aspects of the mechanics, optical elements and detection set-up. The beamline performance is then illustrated through two case studies on strongly correlated transition metal oxides: an XMCD insight into the modifications of the magnetic properties of Cr-doped manganites and the structural deformation in nickel perovskites under high applied pressure.

Local tetragonal distortion in La0.7Sr0.3MnO3 strained thin films probed by x-ray absorption spectroscopy

We report on an angular resolved x-ray absorption spectroscopy study of

Stability of Jahn Teller distortion in LaMnO3 under pressure: An x-ray absorption study

{\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}

Nanosecond-resolved XMCD on ID24 at the ESRF to investigate the element-selective dynamics of magnetization switching of Gd–Co amorphous thin film

thin films epitaxially grown by pulsed laser deposition on slightly mismatched substrates which induce tensile or compressive strains. XANES spectra give evidence of tetragonal distortion within the

Studies of Fe(II) and Fe(III)-DNA complexes by XANES spectroscopy.

{\mathrm{MnO}}_{6}

Quarter-Wave Plates and X-ray Magnetic Circular Dichroism on ID24 at the ESRF

octahedra, with opposite directions for tensile and compressive strains. Quantitative analysis has been done and a model of tetragonal distortion reflecting the strain has been established. EXAFS data collected in plane for tensile substrate confirm the change in the