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Journal of synchrotron radiation 28(5), 28 (2021). doi:10.1107/S1600577521007335, The European XFEL delivers up to 27000 intense (>1012 photons) pulses per second, of ultrashort (≤50 fs) and transversely coherent X-ray radiation, at a maximum repetition rate of 4.5 MHz. Its unique X-ray beam parameters enable groundbreaking experiments in matter at extreme conditions at the High Energy Density (HED) scientific instrument. The performance of the HED instrument during its first two years of operation, its scientific remit, as well as ongoing installations towards full operation are presented. Scientific goals of HED include the investigation of extreme states of matter created by intense laser pulses, diamond anvil cells, or pulsed magnets, and ultrafast X-ray methods that allow their diagnosis using self-amplified spontaneous emission between 5 and 25 keV, coupled with X-ray monochromators and optional seeded beam operation. The HED instrument provides two target chambers, X-ray spectrometers for emission and scattering, X-ray detectors, and a timing tool to correct for residual timing jitter between laser and X-ray pulses., Published by Wiley-Blackwell, [S.l.]

Journal of synchrotron radiation 28(3), 688 - 706 (2021). doi:10.1107/S1600577521002551, The high-precision X-ray diffraction setup for work with diamond anvil cells (DACs) in interaction chamber 2 (IC2) of the High Energy Density instrument of the European X-ray Free-Electron Laser is described. This includes beamline optics, sample positioning and detector systems located in the multipurpose vacuum chamber. Concepts for pump–probe X-ray diffraction experiments in the DAC are described and their implementation demonstrated during the First User Community Assisted Commissioning experiment. X-ray heating and diffraction of Bi under pressure, obtained using 20 fs X-ray pulses at 17.8 keV and 2.2 MHz repetition, is illustrated through splitting of diffraction peaks, and interpreted employing finite element modeling of the sample chamber in the DAC., Published by Wiley-Blackwell, [S.l.]

Small angle x-ray scattering (SAXS) is a well established technique to detect nanometer scale structures in matter. In a typical setup, this diagnostic uses a detector with a direct line of sight to the scattering target. However, in the harsh environment of high intensity laser interaction, intense secondary radiation and high-energy particles are generated. Such a setup would therefore suffer a significant increase of noise due to this background, which could eventually prevent such measurements. In this paper, we present a novel tool consisting of a mosaic graphite crystal that works as a mirror for the SAXS signal and allows us to position the detector behind appropriate shielding. This paper studies the performance of this mirror both by experiment at the European XFEL (X-Ray Free-Electron Laser Facility) laboratory and by simulations.

Successive crystallization of amorphous Cr-Zr-O thin films, formation of the (Cr,Zr)2O3/(Zr,Cr)O2 nanocomposites and the thermally induced changes in the hexagonal crystal structure of metastable (Cr,Zr)2O3 were investigated by means of in situ high-temperature synchrotron diffraction experiments up to 1100 °C. The thin films were deposited at room temperature by using reactive ion beam sputtering, and contained 3–15 at.% Zr. At low Zr concentrations, chromium-rich (Cr,Zr)2O3 crystallized first, while the crystallization of zirconium-rich (Zr,Cr)O2 was retarded. Increasing amount of zirconium shifted the onset of crystallization in both phases to higher temperatures. For 3 at.% of zirconium in amorphous Cr-Zr-O, (Cr,Zr)2O3 crystallized at 600 °C. At 8 at.% Zr in the films, the crystallization of (Cr,Zr)2O3 started at 700 °C. At 15 at.% Zr, the Cr-Zr-O films remained amorphous up to the annealing temperature of 1000 °C. Metastable hexagonal (Cr,Zr)2O3 accommodated up to ~ 3 at.% Zr. Excess of zirconium formed tetragonal zirconia, which was stabilized by chromium.

The combination of powerful optical lasers and an x-ray free-electron laser (XFEL) provides unique capabilities to study the transient behavior of matter in extreme conditions. The high energy density science instrument (HED instrument) at the European XFEL will provide the experimental platform on which an unique x-ray source can be combined with various types of high-power optical lasers. In this paper, we highlight selected scientific examples together with the associated x-ray techniques, with particular emphasis on femtosecond (fs)-timescale pump–probe experiments. Subsequently, we present the current design status of the HED instrument, outlining how the experiments could be performed. First user experiments will start at the beginning of 2018, after which various optical lasers will be commissioned and made available to the international scientific community.

Complementary in situ X-ray photoelectron spectroscopy (XPS), X-ray diffractometry, and environmental scanning electron microscopy are used to fingerprint the entire graphene chemical vapor deposition process on technologically important polycrystalline Cu catalysts to address the current lack of understanding of the underlying fundamental growth mechanisms and catalyst interactions. Graphene forms directly on metallic Cu during the high-temperature hydrocarbon exposure, whereby an upshift in the binding energies of the corresponding C1s XPS core level signatures is indicative of coupling between the Cu catalyst and the growing graphene. Minor carbon uptake into Cu can under certain conditions manifest itself as carbon precipitation upon cooling. Postgrowth, ambient air exposure even at room temperature decouples the graphene from Cu by (reversible) oxygen intercalation. The importance of these dynamic interactions is discussed for graphene growth, processing, and device integration.

International audience; In order to optimize materials for phase change random access memories (PCRAM), the effect of Ge doping on Ga-Sb alloy crystallization was studied using combined in situ synchrotron x-ray techniques, electrical measurements, and static laser testing. The present data emphasize that the crystallization temperature can be increased up to 390 degrees C with subsequent higher thermal stability of the amorphous phase; phase segregation is evidenced with GaSb, Sb, and Ge phases that crystallize in a two-step crystallization process. The Ge-doped GaSb films exhibit a larger electrical contrast as compared to undoped GaSb alloy (up to x 100). The optical contrast measured by laser testing is shown to follow the mass density change variations upon crystallization, with a negative contrast (higher value in amorphous state) whatever Ge-doping levels. In situ x-ray reflectivity measurements show that zero mass density change can be achieved by low Ge-doping. Ge-doped GaSb alloys look promising since a phase change material with zero mass density change and higher crystallization temperature satisfactorily fulfills the specifications for reliable PCRAM cells in terms of endurance and data retention. (C) 2016 AIP Publishing LLC.

In this work we investigated the structural re-organization of thin nanocrystalline Pt films in the temperature range between 250 °C and 400 °C by in-situ XRD, GIXRD and XRR synchrotron experiments. A re-orientation of (111) atomic planes and a relaxation of residual stress occurs. After heating up, Bragg peak fringes can be observed in the diffractograms. They are a direct proof that the Pt films are built of (111) columnar grains which essentially reach the whole film thickness of about 40 nm. During isothermal annealing a relaxation of the dispersion parameter of the atomic planes takes place which is associated with an activation energy of (0.4 ± 0.1) eV.

Three magnesium-based alloys, Mg 90 Ni 10 , Mg 80 Ni 10 Y 10 and Mg 85 Cu 5 Ni 5 Y 5 , were prepared by melt-spinning and compared regarding their hydrogen desorption properties. Their hydrogen desorption kinetics after activation and hydrogenation was investigated by thermogravimetry at different temperatures in the range from 150 °C to 250 °C. It was found that Mg 80 Ni 10 Y 10 exhibits a much faster desorption kinetics in comparison to Mg 90 Ni 10 and Mg 85 Cu 5 Ni 5 Y 5 of up to 1.3 wt.%-H 2 /min. The corresponding crystal phase transformations were investigated in detail by in situ synchrotron X-ray diffraction. It was found that the kinetics of hydrogenation is controlled by different reaction pathways for Mg 90 Ni 10 , Mg 80 Ni 10 Y 10 and Mg 85 Cu 5 Ni 5 Y 5 .

Actuation frequencies in thermally triggered Shape Memory Alloy (SMA) thin films are limited by the slow heat transport into/out of the films. Carbon Nanotubes (CNTs) are known to exhibit an exceptionally high thermal conductivity. Thus, we propose to thermally contact SMA films with CNTs to increase SMA actuation frequencies by enhanced heat transport through the CNTs. The basic requirement for this envisaged nanotube application is to obtain CNT forest growth on a SMA material while retaining a reversible martensitic transformation, as required for Shape Memory Effect exploitation. We show how such growth can be achieved on thin films of the SMA material NiTi. Future work is needed to measure thermal properties and obtainable cycling frequencies of CNT-SMA structures.

In situ synchrotron diffraction experiments were performed to study the behaviour of the cathode material LiNi0.8Co0.2O2 at high cycle numbers. The battery-cell was cycled 140 times between 4.2 and 2.7 V. The initial cycle and several cycles at proceeded degradation of the cell were investigated. In between the cell was cycled ex situ. The evolution of the lattice parameters during cycling was determined for the different stages of degradation by means of Rietveld refinement. Comparing the initial cycle of the cell with the higher cycle numbers, the mechanism of lithium insertion/extraction does not change significantly. But, the range in which the lattice parameters change during one cycle becomes smaller. This can be attributed to interface layer formation and to slightly less lithium exchange in the same voltage window.

The electrochemical performance of LiCoPO4-based electrodes was studied in dependence on synthetic conditions. The role of particle size distribution is illustrated. Structural changes occurring at lithium extraction–insertion were explored by ex situ and in situ synchrotron XRD measurements. For the samples annealed at temperature lower than 600 °C the amorphisation of the cathode material was identified. For a high-temperature sample the two-phase character of electrochemical reaction was confirmed. The character of structural changes observed at charge and discharge is different: the two-phase reaction LiCoPO4–“CoPO4” occurs during charging, while discharge induces a strong increase of the cell volume for the Li-poor phase.

An electrochemical in-situ cell for diffraction studies of battery materials has been developed. The cell works in transmission geometry with sample rotation, and the performance was characterized by experiments on the cathode material LiMn2O4. Several charge and discharge cycles can be studied over a few days without any misfunction of the cell. The quality of the obtained data allows for full structure refinements by the Rietveld method. The whole set-up is about 1 mm thin and consists of an aluminum piston as cathode current collector with a thickness below 0.1 mm in direct contact with a pellet of active cathode material, carbon black and binder. The lithium anode is separated from the cathode by a glass-fibre, soaked with electrolyte. The very good time and angular resolution of this set-up reveals three distinct spinel phases for different charging states of LiMn2O4, prepared by subsolidus reaction.

Typically, wavelength instabilities at synchrotron beams are largely due to thermomechanical phenomena at the primary-beam monochromators and fluctuations of the synchrotron orbit. Although they are small, they may have a negative influence on some kinds of diffraction experiments, for example in thermal expansion studies. Using a wavelength calibration procedure is a natural way of solving or at least reducing this problem. Diamond can be used as a calibration material because of its known low-thermal expansion and low X-ray absorption. Low-temperature X-ray powder diffraction measurements were carried out at the powder diffractometer at the B2 beamline at Hasylab/DESY (Hamburg). The cryostat ensured a good temperature stability and accuracy. Unit-cell parameters for cubic (spinel-type) silicon nitride, c-Si 3 N 4 , and for copper indium selenide, CuInSe 2 , were determined in the temperature range from 14 up to 302 K. An improvement of the data quality due to elimination of the wavelength fluctuation effect is demonstrated.

The synchrotron powder diffraction beamline B2 at HASYLAB/DESY is described. The beamline is capable of high-resolution powder diffraction as well as time-resolved studies and offers several sophisticated ancillary equipments for special applications. A newly developed image-plate system allows for kinetic studies with good resolution in the minutes range. Numerous sample environments allow for various standard applications including structure solution, kinetic studies and in situ observations under flexible and well defined conditions. Representative examples are shown for these setups, which are also supported for experiments of external users.

Close control over the active catalyst phase and hence carbon nanotube structure remains challenging in catalytic chemical vapor deposition since multiple competing active catalyst phases typically co-exist under realistic synthesis conditions. Here, using in-situ X-ray diffractometry, we show that the phase of supported iron catalyst particles can be reliably controlled via the addition of NH3 during nanotube synthesis. Unlike polydisperse catalyst phase mixtures during H2 diluted nanotube growth, nitrogen addition controllably leads to phase-pure γ-Fe during pre-treatment and to phase-pure Fe3C during growth. We rationalize these findings in the context of ternary Fe-C-N phase diagram calculations and, thus, highlight the use of pre-treatment- and add-gases as a key parameter towards controlled carbon nanotube growth.

Besides crystallization time and temperature, the mass density change upon crystallization is a key parameter governing the reliability of phase change random access memory. Indeed, few percentages density change induces considerable mechanical stress in memory cells, leading to film delamination with subsequent electrical failures. This letter presents an extensive study of density change upon crystallization in a series of Ga-Sb thin films with various antimony contents. The mass density of the films is precisely determined by x-ray reflectivity in both their amorphous and crystalline states. The variations of the density in crystalline and amorphous films according to the Sb content found to cross with a zero-density change for 70 at. % Sb. The peculiar behavior of Ga-Sb thin films upon crystallization may be linked to their stress state and mechanical properties.

Anin situcell designed for synchrotron X-ray diffraction during electrochemical cycling of battery materials is presented. The cell operates in transmission mode and can be oscillated with the dedicated sample holder to improve powder statistics. It allows fast data acquisition, provides hermetic sealing and high-quality diffraction data, and is easy to assemble, since a commercially available Swagelok junction is used as the cell body. Diffraction data from the cycling of a battery using LixNi0.8Co0.2O2(0 ≤x≤ 1) as the cathode material are presented.

Carbon diffusion barriers are introduced as a general and simple method to prevent premature carbon dissolution and thereby to significantly improve graphene formation from the catalytic transformation of solid carbon sources. A thin Al2O3 barrier inserted into an amorphous-C/Ni bilayer stack is demonstrated to enable growth of uniform monolayer graphene at 600 °C with domain sizes exceeding 50 μm, and an average Raman D/G ratio of

The crystallization of stoichiometric GaSb thin films was studied by combined in situ synchrotron techniques and static laser testing. It is demonstrated that upon crystallization, GaSb thin films exhibit an unusual behaviour with increasing thickness and concomitant decreasing mass density while its electrical resistance drops as commonly observed in phase change materials. Furthermore, beyond GaSb amorphous-to-crystalline phase transition, an elemental segregation and a separate crystallization of a pure Sb phase is evidenced.

The combination of powerful optical lasers and an x-ray free-electron laser (XFEL) provides unique capabilities to study the transient behaviour of matter in extreme conditions. The high energy density science instrument (HED instrument) at the European XFEL will provide the experimental platform on which an unique x-ray source can be combined with various types of high-power optical lasers. In this paper, we highlight selected scientific examples together with the associated x-ray techniques, with particular emphasis on femtosecond (fs)-timescale pump–probe experiments. Subsequently, we present the current design status of the HED instrument, outlining how the experiments could be performed. First user experiments will start at the beginning of 2018, after which various optical lasers will be commissioned and made available to the international scientific community.

We study the Fe-catalyzed chemical vapor deposition of carbon nanotubes by complementary in situ grazing-incidence X-ray diffraction, in situ X-ray reflectivity, and environmental transmission electron microscopy. We find that typical oxide supported Fe catalyst films form widely varying mixtures of bcc and fcc phased Fe nanoparticles upon reduction, which we ascribe to variations in minor commonly present carbon contamination levels. Depending on the as-formed phase composition, different growth modes occur upon hydrocarbon exposure: For γ-rich Fe nanoparticle distributions, metallic Fe is the active catalyst phase, implying that carbide formation is not a prerequisite for nanotube growth. For α-rich catalyst mixtures, Fe3C formation more readily occurs and constitutes part of the nanotube growth process. We propose that this behavior can be rationalized in terms of kinetically accessible pathways, which we discuss in the context of the bulk iron–carbon phase diagram with the inclusion of phase equilibrium lines for metastable Fe3C. Our results indicate that kinetic effects dominate the complex catalyst phase evolution during realistic CNT growth recipes.

Low-temperature (∼450 °C), scalable chemical vapor deposition of predominantly monolayer (74%) graphene films with an average D/G peak ratio of 0.24 and domain sizes in excess of 220 μm2 is demonstrated via the design of alloy catalysts. The admixture of Au to polycrystalline Ni allows a controlled decrease in graphene nucleation density, highlighting the role of step edges. In situ, time-, and depthresolved X-ray photoelectron spectroscopy and X-ray diffraction reveal the role of subsurface C species and allow a coherent model for graphene formation to be devised.

Synchrotron based combined in situ x-ray diffractometry and reflectometry is used to investigate the role of vacancies for the relaxation of residual stress in thin metallic Pt films. From the experimentally determined relative changes of the lattice parameter $a$ and of the film thickness $L$ the modification of vacancy concentration and residual strain was derived as a function of annealing time at $130\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. The results indicate that relaxation of strain resulting from compressive stress is accompanied by the creation of vacancies at the free film surface. This proves experimentally the postulated dominant role of vacancies for stress relaxation in thin metal films close to room temperature.

We present a detailed study of processes and interactions occurring during the Fe-catalyzed chemical vapor deposition of carbon nanotubes on metallic Ta supports. In situ X-ray photoemission spectroscopy and X-ray diffraction show that the Fe catalyst increases the reactivity of Ta toward oxidation and carbide formation, whereas Ta promotes the reduction of Fe. This causes an unusual temperature dependence of carbon nanotube growth, where at low temperatures (similar to 550 degrees C) vertically aligned forests of carbon nanotubes with ohmic contacts grow readily on metallic Ta, whereas at high temperatures (>600 degrees C) nanotube growth is sparse because of the diffusion of Fe away from the surface through grain boundaries of in situ formed polycrystalline Ta(2)O(5). The Fe-Ta model system highlights general material selection criteria for nanotube applications that require a conductive support.

Morphotropic PZT ceramics are the state of the art materials for ferroelectric actuators. Essential performance parameters for these materials are strain and hysteresis. On a microscopic scale the strain provided by an electric field is due to two different mechanisms. The piezoelectric effect causes an elongation of the unit cells, whereas domain switching changes their crystallographic orientation by aligning the polarization axis towards the field direction. A method is outlined to estimate the contribution of the two mechanisms to total strain by combining macroscopic strain measurements and X-ray diffraction (XRD) data. Results from macroscopic measurements of remanent and unipolar strain with the corresponding data on texture, derived from in situ synchrotron radiation XRD patterns, are analyzed and evaluated by a semi-empirical approach. The method was applied to six morphotropic, LaSr doped PZT materials of different Zr/Ti ratios. Results are discussed with respect to the differences between the materials.

LiCoPO4 samples were synthesized by two different techniques (high-temperature solid-state reaction and lower-temperature synthesis using NH4CoPO4·H2O as precursor) and tested as cathode materials for 5-V lithium batteries. An irreversible lithium deinsertion was observed for the high-temperature sample. In contrast, the application of lower-temperature synthesis led to a significant improvement of the lithium storage reversibility. Different delithiation mechanisms in LiCoPO4 were found for the samples obtained by different synthetic techniques. The nature of capacity fading during cycling of the cells is discussed.

Combined in situ X-ray scattering techniques using synchrotron radiation were applied to investigate the crystallization behavior of Sb-rich Ga-Sb alloys. Measurements of the sheet resistance during heating indicated a reduced crystallization temperature with increased Sb content, which was confirmed by in situ X-ray diffraction. The electrical contrast increased with increasing Sb content and the resistivities in both the amorphous and crystalline phases decreased. It was found that by tuning the composition between Ga:Sb = 9:91 (in at.%) and Ga:Sb = 45:55, the change in mass density upon crystallization changes from an increase in mass density which is typical for most phase change materials to a decrease in mass density. At the composition of Ga:Sb = 30:70, no mass density change is observed which should be very beneficial for phase change random access memory (PCRAM) applications where a change in mass density during cycling is assumed to cause void formation and PCRAM device failure.

Hydrogen plasma pretreatment is used to enforce the growth of vertically-aligned carbon nanotube forests on TiN substrates. The evolution of the substrate, catalyst, and nanotubes are studied by in situ and ex-situ photoemission and X-ray diffraction in order to understand the growth mechanism. We find that TiN retains its crystallographic structure and its conductivity during plasma pretreatment and nanotube growth, which is confirmed by electrical measurements. Plasma pretreatment is found to favor the growth of nanotube forests by root growth, as it binds the catalyst nanoparticles more strongly to the substrate than thermal pretreatment. We find that plasma pretreatment time should be limited, otherwise poor or no growth is found. VC

The growth of high density vertically aligned carbon nanotube forests on conductive CoSi2 substrate layers is characterized by in situ x-ray photoemission spectroscopy and x-ray diffraction. We use in situ silicidation to transform as loaded, low conductivity CoSi supports to highly conductive CoSi2 during nanotube growth. These cobalt silicide films are found to be stable against oxidation and carbide formation during growth and act as an excellent metallic support for growth of aligned nanotubes, resembling the growth on the insulating Fe/Al2O3 benchmark system. The good catalytic activity is attributed to interfacial reactions of the Fe catalyst particles with the underlying CoSi2 support. We obtain ohmic conduction from the support layer to the carbon nanotube forest.