Cristian Mocuta

In situ synchrotron wide-angle X-ray diffraction investigation of fatigue cracks in natural rubber.

Natural rubber exhibits remarkable mechanical fatigue properties usually attributed to strain-induced crystallization. To investigate this phenomenon, an original experimental set-up that couples synchrotron radiation with a homemade fatigue machine has been developed. Diffraction-pattern recording is synchronized with cyclic loading in order to obtain spatial distributions of crystallinity in the sample at prescribed times of the mechanical cycles. Then, real-time measurement of crystallinity is permitted during uninterrupted fatigue experiments. First results demonstrate the relevance of the method: the set-up is successfully used to measure the crystallinity distribution around a fatigue crack tip in a carbon black filled natural rubber for different loading conditions.

Structural investigation of the dynamics of the NiO(111) surface by GIXS

Abstract The behaviour of the NiO(111) surface has been investigated by grazing incidence X-ray scattering (GIXS). We show on single crystals with an improved crystalline quality that NiO(111) is always p(2×2) reconstructed. After air annealing the structure is close to the predicted octopolar reconstruction; after in situ annealing and oxidation it is very different. The transformation of the surface is continuous with temperature but seems to be only based on two possible atomic configurations.

Trace elemental imaging of rare earth elements discriminates tissues at microscale in flat fossils.

The interpretation of flattened fossils remains a major challenge due to compression of their complex anatomies during fossilization, making critical anatomical features invisible or hardly discernible. Key features are often hidden under greatly preserved decay prone tissues, or an unpreparable sedimentary matrix. A method offering access to such anatomical features is of paramount interest to resolve taxonomic affinities and to study fossils after a least possible invasive preparation. Unfortunately, the widely-used X-ray micro-computed tomography, for visualizing hidden or internal structures of a broad range of fossils, is generally inapplicable to flattened specimens, due to the very high differential absorbance in distinct directions. Here we show that synchrotron X-ray fluorescence spectral raster-scanning coupled to spectral decomposition or a much faster Kullback-Leibler divergence based statistical analysis provides microscale visualization of tissues. We imaged exceptionally well-preserved fossils from the Late Cretaceous without needing any prior delicate preparation. The contrasting elemental distributions greatly improved the discrimination of skeletal elements material from both the sedimentary matrix and fossilized soft tissues. Aside content in alkaline earth elements and phosphorus, a critical parameter for tissue discrimination is the distinct amounts of rare earth elements. Local quantification of rare earths may open new avenues for fossil description but also in paleoenvironmental and taphonomical studies.

CoO(111) surface study by surface X-ray diffraction

Abstract We present a structural investigation of the polar (111) surface of CoO single crystals by grazing incidence X-ray diffraction. The surface is found to be always covered by a ∼50 A-thick spinel Co3O4 layer that was quantitatively characterized. Attempts to remove this layer by UHV annealings or Ar+ bombardments lead to the formation of metallic Co clusters on the surface that transform again in Co3O4 by annealing under oxygen.

Comparative study of the mechanical properties of nanostructured thin films on stretchable substrates

Comparative studies of the mechanical behavior between copper, tungsten, and W/Cu nanocomposite based on copper dispersoid thin films were performed under in-situ controlled tensile equi-biaxial loadings using both synchrotron X-ray diffraction and digital image correlation techniques. The films first deform elastically with the lattice strain equal to the true strain given by digital image correlation measurements. The Cu single thin film intrinsic elastic limit of 0.27% is determined below the apparent elastic limit of W and W/Cu nanocomposite thin films, 0.30% and 0.49%, respectively. This difference is found to be driven by the existence of as-deposited residual stresses. Above the elastic limit on the lattice strain-true strain curves, we discriminate two different behaviors presumably footprints of plasticity and fracture. The Cu thin film shows a large transition domain (0.60% true strain range) to a plateau with a smooth evolution of the curve which is associated to peak broadening. In contrast, W and W/Cu nanocomposite thin films show a less smooth and reduced transition domain (0.30% true strain range) to a plateau with no peak broadening. These observations indicate that copper thin film shows some ductility while tungsten/copper nanocomposites thin films are brittle. Fracture resistance of W/Cu nanocomposite thin film is improved thanks to the high compressive residual stress and the elimination of the metastable beta-W phase.

Diffractive refractive optics: the possibility of sagittal focusing in Laue-case diffraction.

The sagittal deviation of a Laue-diffracted X-ray beam caused by the inclination of an exit crystal surface with respect to an entrance crystal surface has been studied both theoretically and experimentally. The use of this effect for sagittal focusing of X-ray synchrotron radiation diffracted by a Laue crystal is suggested. The focusing is based on the refraction effect due to the parabolic profile of an exit or/and entrance surface. The crystal is not bent. In order to achieve a reasonable focusing distance, the crystal should be cut asymmetrically. The experiment was performed at beamline BM5 at the ESRF.

Strontium speciation in archaeological otoliths

Fish otoliths (“ear stones”) are major environmental indicators used in ecology and fisheries sciences. Otoliths consist of a biomineral material containing an organically-templated mineral calcium carbonate, normally aragonite, in which strontium is incorporated at trace to minor levels depending on water chemistry and individual physiology. Sr content and fluctuations inform on the life histories of ancient specimens and provide data for palaeoenvironmental reconstructions. Identifying the impact of post-mortem alteration is a critical question to assure the reliability of such work. A central parameter for the reliability of Sr content as a palaeoenvironmental proxy is whether the mode of incorporation can be considered as stable and homogenous at the microscale in otoliths over thousands of years. In addition, it is important to know whether a different kind of speciation of Sr is observed, especially at the outer surface of the sample in contact with the soil and local environment. Here, a novel combination of synchrotron microscale point analyses and raster-scanning X-ray absorption spectroscopy is implemented and used for the first time to study otoliths at different length scales, spanning from millimetres down to micrometres. Strontium is found in substitution for calcium in aragonite in all our analyses of five Holocene otoliths and their three modern counterparts; the first set of samples from the Peruvian coast, up to 11 000 years old, are studied for their potential as palaeoenvironmental proxies. The chemical environment of strontium in otoliths is independent of content of this element, location in the otolith, species, and archaeological age. This is shown with a high lateral resolution (about 10 μm) over wide fields of view, as a way to consolidate macro-scale approaches. To our best knowledge, this work is the first report of the chemical environment of strontium in ancient otoliths. Our work opens the way to new approaches to validate palaeoenvironmental studies of biocarbonate paleoproxies.

Fast pole figure acquisition using area detectors at the DiffAbs beamline - Synchrotron SOLEIL

Errors in the article by Mocuta, Richard, Fouet, Stanescu, Barbier, Guichet, Thomas, Hustache, Zozulya & Thiaudiere [J. Appl. Cryst. (2013), 46, 1842–1853] are corrected.

Electrolytes at interfaces: accessing the first nanometers using X-ray standing waves

Ion-surface interactions are of high practical importance in a wide range of technological, environmental and biological problems. In particular, they ultimately control the electric double layer structure, hence the interaction between particles in aqueous solutions. Despite numerous achievements, progress in their understanding is still limited by the lack of experimental determination of the surface composition with appropriate resolution. Tackling this challenge, we have developed a method based on X-ray standing waves coupled to nano-confinement which allows the determination of ion concentrations at a solid-solution interface with a sub-nm resolution. We have investigated mixtures of KCl/CsCl and KCl/KI in 0.1 mM to 10 mM concentrations on silica surfaces and obtained quantitative information on the partition of ions between bulk and Stern layer as well as their distribution in the Stern layer. Regarding partition of potassium ions, our results are in agreement with a recent AFM study. We show that in a mixture of KCl and KI, chloride ions exhibit a higher surface propensity than iodide ions, having a higher concentration within the Stern layer and being on average closer to the surface by ≈1-2 Å, in contrast to the solution water interface. Confronting such data with molecular simulations will lead to a precise understanding of ionic distributions at aqueous interfaces.

Selected Synchrotron Radiation Techniques

Synchrotron radiation is produced when highly energetic charged particles are deviated in a magnetic field. The generated spectrum consists of a large energy range from infrared to gamma X-rays. Harder X-ray ranges require larger synchrotron storage rings. Recent rings offer simultaneously energy tunability for photons with high brightness, high photon fluxes, and low divergence [1]. Photon energy tunability and variable polarization provide chemical, electronic structure and/or magnetic sensitivity. The tunable penetration depth using glancing incident and/or exit scattering angles enables the study of buried interfaces. Importantly, X-ray techniques, especially at high photon energies, can be used with various sample environments including (and not limited to) high pressure, ...