Diffraction instruments using filtering by one or several analyser crystals exist since the 1980s and 1990s at synchrotron radiation sources, but, due to its low efficiency, this filtering is little used on laboratory sources. In order to overcome this limitation, the efficiency of a small diffraction filtering multi-analyzer block (MAD block) realized with a `single-crystal-comb' curved on a rigid support is demonstrated here. The geometry of this curved surface is logarithmic spiral and is optimized to allow multi-filtering over a relatively important diffraction angular range and to be also applicable over an X-ray spectral range. The efficiency of such a small rigid-compact MAD block consisting of this single-crystal-comb generating 20-50 Si(111) single-crystal blades, associated with a block of Soller collimators, is demonstrated. The angle between each crystal is 0.1°, so the measurement range of the comb is 2-5°. The geometry of this system has been optimized for operation with a synchrotron X-ray source over an energy range of 22â keV to 46â keV and could be used with laboratory X-ray sources (Agâ Kα1, 22.1â keV). This MAD block complements and exploits the qualities of the `photon-counting' detectors which have very low intrinsic noise. Their joint efficacy is supported by powder pattern measurements of a LaB6 reference sample and of several heterogeneous samples of cultural heritage materials, carried out at 22â keV on the D2AM beamline at the ESRF. Their signal-to-noise ratio is excellent (1000/1) and allows the detection thresholds of the measurements (from 3-1% to 0.1%) to detect minor phases in the studies of `real' heterogeneous materials to be drastically improved.
Diffraction and spectroscopy instruments using a filtering process with several analyser crystals have existed for about 30 years at synchrotron radiation sources, but they are difficult to use on laboratory sources. Several diffraction multi-filtering systems for powder diffraction experiments have been studied and optimized, in order to show the relevance, simplicity and efficiency of their implementation. Optical filter systems containing one or many diffracting elements, precisely positioned in a rigid manner on a logarithmic spiral surface and having a stability that allows high resolution and high sensitivity to powder diffraction experiments, have been developed. After having tested prototypes with various geometries, we present in particular the realization of a small rigid-compact multi-analyser comb that allows 20-50 measurements on synchrotron radiation sources to be filtered in parallel, but also and especially that can be adapted on laboratory X-ray sources (Agâ Kα1) to increase by an order of magnitude the intensities and resolutions of the measurements. Such a rigid-compact multi-analyser block can advantageously be associated with `photon-counting' 1D and 2D detectors in order to drastically improve the detection thresholds of powder diffraction measurements to better than 0.1%, which allows the detection/quantification/analysis of minor phases in studies of `real' complex materials.
X-ray tomography is a non-destructive three-dimensional imaging/microanalysis technique selective to a wide range of properties such as density, chemical composition, chemical states and crystallographic structure with extremely high sensitivity and spatial resolution. Here the development of in situ high-pressure high-temperature micro-tomography using a rotating module for the Paris-Edinburgh cell combined with synchrotron radiation is described. By rotating the sample chamber by 360°, the limited angular aperture of ordinary high-pressure cells is surmounted. Such a non-destructive high-resolution probe provides three-dimensional insight on the morphological and structural evolution of crystalline as well as amorphous phases during high pressure and temperature treatment. To demonstrate the potentials of this new experimental technique the compression behavior of a basalt glass is investigated by X-ray absorption tomography, and diffraction/scattering tomography imaging of the structural changes during the polymerization of C60 molecules under pressure is performed. Small size and weight of the loading frame and rotating module means that this apparatus is portable, and can be readily installed on most synchrotron facilities to take advantage of the diversity of three-dimensional imaging techniques available at beamlines. This experimental breakthrough should open new ways for in situ imaging of materials under extreme pressure-temperature-stress conditions, impacting diverse areas in physics, chemistry, geology or materials sciences.
The collapsing of C60 into polycrystalline diamond has been studied after nonhydrostatic pressurization at ambient temperature using x-ray scattering computed tomography. Using this selective structural probe we provide evidence of concentric coexistence of "compressed graphite" (d(00l)â¼3.09-3.11 Å), sp2-graphitelike phase (d(00l)â¼3.35-3.42 Å), and sp3-like amorphous carbon surrounding polycrystalline diamond (aâ¼3.56-3.59 Å). The so-called "compressed graphite" exhibits a collapsed c axis and is textured with disordered layers. This latter phase is better described as a short interlayered carbon phase with buckled sp2-sp3 layers with possible interlayer bonding. Additionally, our 3D maps of phase distribution and of the residual stress retained in the polycrystalline diamond phase support the importance of stressed synthesis conditions for diamond formation.
A high efficiency multichannel collimator (MCC) device has been developed at the high pressure beamline ID27 of the European Synchrotron Radiation Facility to drastically reduce the x-ray background from the sample environment in the Paris-Edinburgh press. The main technical difficulty, which resides in the minimum slits size achievable using the classical mono-bloc design, has been resolved using an original concept based on a set of independent slits. Then, a very small slit size of 50 µm was manufactured resulting in a great improvement of the signal to background ratio. In addition, the transfer function of the MCC has been measured using the x-ray diffusion signal of a metal doped glass and efficiently applied to correct the raw data. The potential of this new device is illustrated in two challenging examples: iron-sulfur liquid structures and C(60) polymerization process at high pressure and high temperature.
'Forbidden' Bragg reflections of iron orthoborate Fe(3)BO(6) were studied theoretically and experimentally in the vicinity of the iron K edge. Their energy spectra are explained as resulting from the interference of x-rays scattered from two inequivalent crystallographic sites occupied by iron ions. This particular structure property gives rise to complex azimuthal dependences of the reflection intensities in the pre-edge region as they result from the interplay of site specific dipole-quadrupole and quadrupole-quadrupole resonant scattering. Also evidenced is an anisotropic character of the absorption spectrum. Self-absorption correction to the diffraction data, as well as possible contributions of thermal vibrations and magnetic order, are discussed. Particular care is given to extracting clean spectra from the data, and it is demonstrated that excellent results can be obtained even from measurements that appear corrupted by several effects such as poor crystal quality and multiple scattering.
Here we show that the low temperature phase of magnetite is associated with an effective, although fractional, ordering of the charge. Evidence and a quantitative evaluation of the atomic charges are achieved by using resonant x-ray diffraction (RXD) experiments whose results are further analyzed with the help of ab initio calculations of the scattering factors involved. By confirming the results obtained from x-ray crystallography we have shown that RXD is able to probe quantitatively the electronic structure in very complex oxides, whose importance covers a wide domain of applications.
Diffraction anomalous fine-structure (DAFS) spectroscopy uses resonant elastic X-rays scattering as an atomic, shell and site-selective probe that provides information on the electronic structure and the local atomic environment as well as on the long-range-ordered crystallographic structure. A DAFS experiment consists of measuring the Bragg peak intensities as a function of the energy of the incoming X-ray beam. The French CRG (Collaborative Research Group) beamline BM2-D2AM (Diffraction Diffusion Anomale Multi-longueurs d'Onde) at the ESRF (European Synchrotron Radiation Facility) has developed a state-of-the-art energy scan diffraction set-up. In this article the requirements for obtaining reliable DAFS data are presented and recent technical achievements are reported.
Algorithms , Spectrometry, X-Ray Emission/instrumentation , Spectrometry, X-Ray Emission/methods , Synchrotrons/instrumentation , X-Ray Diffraction/instrumentation , X-Ray Diffraction/methods , Artifacts , Crystallography/instrumentation , Crystallography/methods , Equipment Design , Europe , Molecular Conformation , Quality Control , Reproducibility of Results , Sensitivity and Specificity
Resonant X ray scattering has been used to investigate charge localization on the octahedral iron atoms in magnetite below and above the Verwey temperature. We have measured the DAFS spectra of the 002 and 006 "forbidden" Bragg reflections permitted by the anisotropy of the iron anomalous scattering factor. We performed ab initio calculations which are in fair agreement with the experiment in the near edge region and demonstrate the sensitivity of the DAFS spectra to tiny structural and electronic changes. No change is observed, in the energy and azimuthal dependences, when the sample is cooled down below the Verwey temperature. Charge ordering can be definitely excluded and different charge localisation schemes discarded. Ab initio simulations, performed by using the refined crystallographic structure proposed for the room temperature phase, do not show a good agreement with the experiment in the extended region of the DAFS spectrum. This point is being investigated.
We use the anomalous x-ray diffraction technique to investigate the nature of the tantalum displacement pattern in the modulated phase of the charge-density-wave compound (TaSe4)2I. In addition to the known acousticlike modulation, we find the first direct evidence for the condensation of opticlike Ta displacements along the metallic chains corresponding to an LLSS pattern of long and short in-chain Ta-Ta distances (Ta-tetramerization modes). This result confirms a previous model in which the interaction of the electronically coupled optic modes with long-wavelength acoustic shear modes leads to the condensation of a modulation of mixed character.
Resonant x-ray scattering was used to investigate electronic fluctuations of the octahedral iron atoms in magnetite. We measured the (002) and (006) "forbidden" x-ray diffraction reflections permitted by the anisotropy of the iron anomalous scattering factor. The energy and azimuthal angle dependencies of these reflections, and the polarization analysis, are shown and discussed. The results clearly show p and d iron empty states ordering in magnetite at room temperature. Moreover, the octahedral iron atoms are electronically equivalent in a time scale lower than 10(-16) sec. Therefore, magnetite should be considered as an itinerant magnet and not as a fluctuating mixed valence material.
A filtering technique to remove parasitic scattering from X-ray absorption spectra that are acquired in energy-dispersive mode has been developed and tested at the European Synchrotron Radiation Facility. The improved set-up removes small-angle scattering of the sample or the windows of sample cells which may spoil the energy resolution or reduce the intensity of prominent features in the absorption spectrum, such as the white line at the Pt L(III) edge. The sample is placed behind the curved monochromator and between two plane perfect crystals in the Bonse-Hart configuration. The dispersion of the Bonse-Hart double-crystal camera is matched to the dispersion of the curved monochromator by inclining the scattering planes of the two optical elements against each other.
The diffraction anomalous fine structure (DAFS) technique is applied to highly absorbing ;real' crystals of arbitrary shape containing several heavy atoms. A multiwavelength refinement procedure of DAFS signals is demonstrated on two different oxides, BaPt(4+)(1-2x)(Pt( 2+)(1-y)Ba( 2+)(y) )(x)O(3-3x) with x = 0.25, y approximately 0 and BaZnFe(6)O(11), which have complex crystallographic structures. In these compounds, anomalous scatterers are located in different crystallographic sites and thus a multiwavelength refinement is necessary to separate out the site-selective information. An accurate absorption correction procedure for small, highly absorbing single crystals necessary for the DAFS analysis of this kind of samples is also presented.
