Hard X-ray Diffraction Experiments Research Articles

First pump-probe-probe hard X-ray diffraction experiments with a 2D hybrid pixel detector developed at the SOLEIL synchrotron.

A new photon-counting camera based on hybrid pixel technology has been developed at the SOLEIL synchrotron, making it possible to implement pump-probe-probe hard X-ray diffraction experiments for the first time. This application relies on two specific advantages of the UFXC32k readout chip, namely its high frame rate (50 kHz) and its high linear count rate (2.6 × 106 photons s-1 pixel-1). The project involved the conception and realization of the chips and detector carrier board, the data acquisition system, the server with its specific software, as well as the mechanical and cooling systems. This article reports on in-laboratory validation tests of the new detector, as well as on tests performed at the CRISTAL beamline within the targeted experimental conditions. A benchmark experiment was successfully performed, showing the advantages of the pump-probe-probe scheme in correcting for drifts of the experimental conditions.

Influence of Nd or Ca addition on the dislocation activity and texture changes of Mg–Zn alloy sheets under uniaxial tensile loading

In situ hard X-ray diffraction experiments were carried out to investigate the dislocation slip activity of Mg-1 wt.% Zn-based alloys containing Nd or Ca during tensile loading. Diffraction patterns collected during tensile loading at 3 temperatures were analyzed using a convolutional multiple whole profile fitting procedure. High activation of nonbasal <a> and pyramidal <c+a> dislocations was found in the Nd- and Ca-containing alloys. The microstructure evolution after 10% deformation was examined by complementary EBSD measurements. The microstructure evolution was related to the differences in the initial texture and active deformation modes, according to the alloying and temperature. In-grain misorientation axes analysis obtained from the EBSD measurements confirms that the addition of Nd or Ca contributes to the higher activity of prismatic <a> slip. The high activity of prismatic <a> slip leads to a broadening of the basal poles perpendicular to the loading direction and a strengthening of the <101‾0> pole along the loading direction. The overall dislocation density evolution at elevated temperatures is controlled by dynamic recovery and dynamic recrystallization. These thermally activated mechanisms are retarded in the Nd- or Ca-containing alloys.

LAMBDA 2M GaAs—A multi-megapixel hard X-ray detector for synchrotrons

Synchrotrons can provide very intense and focused X-ray beams, which can be used to study the structure of matter down to the atomic scale. In many experiments, the quality of the results depends strongly on detector performance; in particular, experiments studying dynamics of samples require fast, sensitive X-ray detectors. “LAMBDA” is a photon-counting hybrid pixel detector system for experiments at synchrotrons, based on the Medipix3 readout chip. Its main features are a combination of comparatively small pixel size (55 μm), high readout speed at up to 2000 frames per second with no time gap between images, a large tileable module design, and compatibility with high-Z sensors for efficient detection of higher X-ray energies. A large LAMBDA system for hard X-ray detection has been built using Cr-compensated GaAs as a sensor material. The system is composed of 6 GaAs tiles, each of 768 by 512 pixels, giving a system with approximately 2 megapixels and an area of 8.5 by 8.5 cm2. While the sensor uniformity of GaAs is not as high as that of silicon, its behaviour is stable over time, and it is possible to correct nonuniformities effectively by postprocessing of images. By using multiple 10 Gigabit Ethernet data links, the system can be read out at the full speed of 2000 frames per second. The system has been used in hard X-ray diffraction experiments studying the structure of samples under extreme pressure in diamond anvil cells. These experiments can provide insight into geological processes. Thanks to the combination of high speed readout, large area and high sensitivity to hard X-rays, it is possible to obtain previously unattainable information in these experiments about atomic-scale structure on a millisecond timescale during rapid changes of pressure or temperature.

Optically induced lattice deformations, electronic structure changes, and enhanced superconductivity in YBa2Cu3O6.48.

Resonant optical excitation of apical oxygen vibrational modes in the normal state of underdoped YBa2Cu3O6+x induces a transient state with optical properties similar to those of the equilibrium superconducting state. Amongst these, a divergent imaginary conductivity and a plasma edge are transiently observed in the photo-stimulated state. Femtosecond hard x-ray diffraction experiments have been used in the past to identify the transient crystal structure in this non-equilibrium state. Here, we start from these crystallographic features and theoretically predict the corresponding electronic rearrangements that accompany these structural deformations. Using density functional theory, we predict enhanced hole-doping of the CuO2 planes. The empty chain Cu dy2-z2 orbital is calculated to strongly reduce in energy, which would increase c-axis transport and potentially enhance the interlayer Josephson coupling as observed in the THz-frequency response. From these results, we calculate changes in the soft x-ray absorption spectra at the Cu L-edge. Femtosecond x-ray pulses from a free electron laser are used to probe changes in absorption at two photon energies along this spectrum and provide data consistent with these predictions.

Characterizing short-range vs. long-range spatial correlations in dislocation distributions

Hard X-ray diffraction experiments have provided evidence of a strongly heterogeneous distribution of dislocation densities along the axis of cylindrical ice single crystals oriented for basal slip in torsion creep. The dislocation arrangements showed a complex scale-invariant character, which was analyzed by means of statistical and multifractal techniques. A trend to decreasing autocorrelation of the dislocation distribution was observed as deformation proceeds. At low strain levels, long-range spatial correlations control the distribution, but short-range correlations in relation with cross-slip progressively prevail when strain increases. This trend was reproduced by a model based on field dislocation dynamics, a theory accounting for both long-range elastic interactions and short-range interactions through transport of dislocation densities.

Lattice distortion and basal slip bands in deformed ice crystals revealed by hard X-ray diffraction

Hard X-ray diffraction experiments were performed on ice single crystals deformed in pure torsion. The torsion strain appears to be totally accommodated by geometrically necessary basal screw dislocations. Dislocation slip is concentrated on macroscopic basal slip bands, which are clearly seen on the prismatic diffraction lines. The slip band spacing is decreasing with the increase of the finite torsion strain. The deformation appears to result from the increase of the number of slip bands and not from the accumulation of dislocations within these bands. The elastic interaction between dislocations located in neighbouring slip bands is weak since they all have the same sign and move in the same direction.

Lattice distortion in ice crystals from the Vostok core (Antarctica) revealed by hard X-ray diffraction; implication in the deformation of ice at low stresses

Hard X-ray diffraction experiments have been carried out on ice monocrystals taken from the 3623 m long Vostok core (Antarctica). Strain gradients associated with the storage of geometrically necessary dislocations appear to be a general feature of the deformation microstructure of ice. The observed lattice distortion is related to the bending of the basal plane and the torsion of the lattice around the c-axis. The lattice distortion is shown to be compatible with the basal dislocations generally observed in ice crystals, supporting the assumption of deformation modes governed by basal slip and accommodated by recrystallization processes. The dependence of the ice viscosity on grain size in ice sheets appears to be compatible with these accommodation modes.

Strain gradients and geometrically necessary dislocations in deformed ice single crystals

Hard X-ray diffraction experiments were performed on ice single crystals deformed in torsion. This work shows the relationship between the density of geometrically necessary dislocations and strain gradients. The torsion strain appears to be totally accommodated by geometrically necessary basal screw dislocations.

Spatial texture distribution in thermomechanically deformed 2–14–1-based magnets

Quantifying the relationship between crystallographic texture and magnetic properties is highly desirable for engineering high-energy-product magnets. Such an undertaking is a challenge for melt-quenched magnets formed from Nd2Fe14B-based alloys because they possess a nanoscale microstructure complicated by the presence of multiple phases, both ferromagnetic and paramagnetic. Procedures for the evaluation of texture in permanent magnets often rely upon magnetic remanence measurements; however, such determinations are based on the use of the Stoner–Wohlfarth model, and are applicable only to assemblies of noninteracting particles, which nullifies their use in texture determinations of “exchange-spring” magnets. To overcome these inherent experimental difficulties, alternative techniques for the measurement of the bulk texture of permanent magnets are explored in this work. Crystallographic alignment was investigated in both neutron and hard x-ray diffraction experiments. Transmission synchrotron x-ray diffraction measurements of the rocking curve width as a function of both the vertical and horizontal positions were performed on a series of thermomechanically processed, melt-quenched magnets based on the Nd2Fe14B composition. The results of the hard x-ray diffraction experiments were verified by neutron diffraction work. A simple model based on a squared-Lorentzian distribution is used to obtain a quantitative estimation of the population of misoriented grains for a given rocking curve half-width. It is deduced that even the most deformed magnets still possess a significant degree of particle misalignment; over 50% of the constituent grains have a misorientation of at least 15° from the overall axial direction at the center of the magnet. The spatial distribution of the rocking curve widths indicates that the development of the c-axis texture is initiated in the center region of the magnet and progresses to the perimeter with increasing deformation. Samples with lower deformation levels possess more uneven texture distributions than do those with higher deformation levels, and the degree of orientation seems to develop rapidly near a deformation level of 50%. The results of this work are utilized to produce a phenomenological model, consistent with the findings of past researchers, of deformation based on mass transport under applied stress via a liquid grain-boundary phase. It is proposed that the micromagnetic structure consists primarily of relatively large clusters of exchange-coupled grains that result in lower-coercivity areas within the magnet. Recommendations are included for improving the production and processing of thermomechanically deformed melt-quenched magnets.