A high‐pressure single‐crystal‐diffraction experimental system at 4W2 beamline of BSRF

Information on the structural evolution of materials under high pressure is of great importance for understanding the properties of materials exhibited under high pressure. High‐pressure powder diffraction is widely used to investigate the structure evolution of materials at such pressure. Unfortunately, powder diffraction data are usually insufficient for retrieving the atomic structures, with high‐pressure single‐crystal diffraction being more desirable for such a purpose. Here, a high‐pressure single‐crystal diffraction experimental system developed recently at beamline 4W2 of Beijing Synchrotron Radiation Facility (BSRF) is reported. The design and operation of this system are described with emphasis on special measures taken to allow for the special circumstance of high‐pressure single‐crystal diffraction. As an illustration, a series of diffraction datasets were collected on a single crystal of LaB₆ using this system under various pressures (from ambient pressure to 39.1 GPa). The quality of the datasets was found to be sufficient for structure solution and subsequent refinement.     

A new ultra‐high‐vacuum variable temperature and high‐magnetic‐field X‐ray magnetic circular dichroism facility at LNLS

X‐ray magnetic circular dichroism (XMCD) is one of the most powerful tools for investigating the magnetic properties of different types of materials that display ferromagnetic behavior. Compared with other magnetic‐sensitive techniques, XMCD has the advantage of being element specific and is capable of separating the spin and magnetic moment contributions associated with each element in the sample. In samples involving, for example, buried atoms, clusters on surfaces or at interfaces, ultrathin films, nanoparticles and nanostructures, three experimental conditions must be present to perform state‐of‐the‐art XMCD measurements: high magnetic fields, low temperatures and an ultra‐high‐vacuum environment. This paper describes a new apparatus that can be easily installed at different X‐ray and UV beamlines at the Brazilian Synchrotron Light Laboratory (LNLS). The apparatus combines the three characteristics described above and different methods to measure the absorption signal. It also permits in situ sample preparation and transfer to another chamber for measurement by conventional surface science techniques such as low‐energy electron diffraction (LEED), reflection high‐energy electron diffraction (RHEED), X‐ray photoelectron spectroscopy (XPS) and X‐ray photoelectron diffraction (XPD). Examples are given of XMCD measurements performed with this set‐up on different materials.     

30‐Lens interferometer for high‐energy X‐rays

A novel high‐energy multi‐lens interferometer consisting of 30 arrays of planar compound refractive lenses is reported. Under coherent illumination each lens array creates a diffraction‐limited secondary source. Overlapping such coherent beams produces an interference pattern demonstrating strong longitudinal functional dependence. The proposed multi‐lens interferometer was tested experimentally at the 100 m‐long ID11 ESRF beamline in the X‐ray energy range from 30 to 65 keV. The interference pattern generated by the interferometer was recorded at fundamental and fractional Talbot distances. An effective source size (FWHM) of the order of 15 µm was determined from the first Talbot image, proving the concept that the multi‐lens interferometer can be used as a high‐resolution tool for beam diagnostics.     

A new fullerene network phase obtained from C70 at high‐pressure and high‐temperature

Fullerene networks are an exciting class of materials that may display exceptional physical properties. A new C₇₀‐fullerene network phase, synthesized at high‐pressure, 7 GPa, and high‐temperature, 600 °C, is presented. Its structure, determined by Rietveld analysis of the X‐ray diffraction data combined with density functional theory modeling, consists of puckered polymerized layers where each molecule is bonded to three neighbors through 2 + 2 cycloaddition bonds, a new polymeric configuration unseen in other fullerene networks. This new C₇₀‐fullerene network structure adds to the previous zigzag structures, demonstrating the potential of C₇₀ molecules as building blocks to synthesize new carbon phases. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

A high‐pressure and controlled‐flow gas system for catalysis research

A high‐pressure gas rig for in situ catalytic reactions at X‐ray absorption spectroscopy beamline (BM26A) has been developed. The rig enables catalysts to be studied in a variety of cells under well controlled and industrially relevant operation conditions. A large variety of gas mixtures can be generated and pressures of up to 50 bar with dry gas and 20 bar with wet gas (steam) can be obtained. Analyses of reaction products can be performed using an on‐line mass spectrometer.     

Fluid dynamics analysis of a gas attenuator for X‐ray FELs under high‐repetition‐rate operation

Newtonian fluid dynamics simulations were performed using the Navier–Stokes–Fourier formulations to elucidate the short time‐scale (µs and longer) evolution of the density and temperature distributions in an argon‐gas‐filled attenuator for an X‐ray free‐electron laser under high‐repetition‐rate operation. Both hydrodynamic motions of the gas molecules and thermal conductions were included in a finite‐volume calculation. It was found that the hydrodynamic wave motions play the primary role in creating a density depression (also known as a filament) by advectively transporting gas particles away from the X‐ray laser–gas interaction region, where large pressure and temperature gradients have been built upon the initial energy deposition via X‐ray photoelectric absorption and subsequent thermalization. Concurrent outward heat conduction tends to reduce the pressure in the filament core region, generating a counter gas flow to backfill the filament, but on an initially slower time scale. If the inter‐pulse separation is sufficiently short so the filament cannot recover, the depth of the filament progressively increases as the trailing pulses remove additional gas particles. Since the rate of hydrodynamic removal decreases while the rate of heat conduction back flow increases as time elapses, the two competing mechanisms ultimately reach a dynamic balance, establishing a repeating pattern for each pulse cycle. By performing simulations at higher repetition rates but lower per pulse energies while maintaining a constant time‐averaged power, the amplitude of the hydrodynamic motion per pulse becomes smaller, and the evolution of the temperature and density distributions approach asymptotically towards, as expected, those calculated for a continuous‐wave input of the equivalent power.     

Elemental analysis by high‐energy electron excitation

The possibility of using high‐energy β‐particles (10²–10³ keV) to induce the emission of characteristic x‐rays from pure chemical elements, with important improvements with respect to conventional excitation methods, has been recently reviewed. An excitation procedure named BIXE (β‐induced x‐ray emission) is used for implementing a spectrometric technique along the lines developed for EPMA (electron probe microanalysis). We have found that by using BIXE it is possible to determine binary sample compositions of elements present at concentrations higher than 1%, by comparisons with reference samples to obtain calibration curves. Experience with EPMA shows that when ternary and higher order samples are analyzed, the use of reference samples is not enough and it is necessary to perform theoretical corrections to the relationship between the line intensity and the corresponding concentrations, as a suitable complement to the analytical procedure. Semi‐quantitative results are thus obtained with corrections applied through the ZAF method usually used in EPMA. In this work we concentrated in finding whether calibration curves as used in EPMA (where the electron beam is monochromatic) can be used in BIXE, where the electron beam is polychromatic (the β spectrum). We expect that BIXE can be developed as a spectrometric technique whose main advantages are that it is a low‐cost technique suitable for in situ studies and that the experimental arrangement and data acquisition and its evaluation are comparatively simple. Copyright © 2004 John Wiley & Sons, Ltd.     

Yb‐fiber‐based,high‐average‐power,high‐repetition‐rate,picosecond source at 2.1 µm

We report a high‐repetition‐rate picosecond fiber‐based source at 2.1 µm offering exceptional performance capabilities over existing lasers near this wavelength, providing high average power and efficiency together with excellent spectral, power and beam pointing stability, in high spatial beam quality. This new source is based on a near‐degenerate MgO:PPLN optical parametric oscillator (OPO) pumped by an Yb‐fiber laser at 1064 nm, and incorporating a diffraction grating for spectral control. The device provides as much as 7.1 W of average power at 2.1 µm for a pump power of 18 W at an extraction efficiency of 39.4% in pulses of 20 ps at 79.3 MHz. The output exhibits passive power stability better than 1% rms over 15 hours, and a beam pointing stability ∼40 µrad over 1 hour, in high spatial quality with M² ∼ 3.5. The output beam is linearly polarized and the pulse train has an amplitude stability better than 3.4% rms over 2 µsec. Radio‐frequency measurements of the output pulse train also confirm high temporal stability and low timing jitter, indicating that the source is ideal for variety of applications including pumping long‐wavelength mid‐infrared OPOs. Photograph shows the temperature‐controlled, 50‐mm‐long MgO:PPLN crystal inside the cavity, used as nonlinear gain medium in the picosecond source operating at 2.1 µm. The visible light is the result of non‐phase‐matched second harmonic generation of the pump beam in the MgO:PPLN crystal.

     

Probing high‐energy ion‐implanted silicon by micro‐Raman spectroscopy

The effect of ion implantation (4 MeV¹²C²⁺, 5 MeV¹⁶O²⁺, and 8 MeV²⁸Si²⁺) on [110] silicon wafers in channeling and random orientation is investigated by micro‐Raman spectroscopy. The profiles were measured using Scanning Electron Microscope (SEM) showing that the ions were penetrating deeper inside the wafer in the channeling case creating a 1–2 µm wide strongly modified region and agreeing with the d‐nuclear reaction analysis measurements. Micro‐Raman spectroscopy was employed for the assessment of the lattice damage, probing the side surface of the cleaved wafers at submicron step. The phonon modifications show strong lattice distortions in zones parallel to the front surface of the wafers and at depths, which agree with the results of the characterization techniques. In these strongly damaged zones, there is a substantial reduction in the phonon intensity, a small shift in wavenumber position, and a large increase in the phonon width. On the basis of a modification of the phonon confinement model that takes under consideration the laser beam profile, the reduction in intensity of scattered light, and the nanocrystallite size distribution from the simulation of the lattice displacements, the main characteristics of the Raman spectra could be reproduced for the random C and O implantations. The results indicate that at a critical doping level, the induced defects and lattice distortions relax by breaking the silicon single crystal into nanocrystallites, thus creating the observed zones of strongly distorted lattice. Copyright © 2014 John Wiley & Sons, Ltd.     

Periodic density functional theory study of the high‐pressure behavior of crystalline 7,2′‐anhydro‐β‐d‐arabinosylorotidine

In this work, a detailed study of the structural, electronic, and absorption properties of crystalline 7,2′‐anhydro‐β‐d ‐arabinosylorotidine (Cyclo ara‐O) in the pressure range of 0–350 GPa is performed by density functional theory calculations. The detail analysis of the crystal with increasing pressure shows that complex transformations occur in Cyclo ara‐O under compression. In addition, the b‐direction is much stiffer than the a‐ and c‐axis at 0–330 GPa, suggesting that the Cyclo ara‐O crystal is anisotropic in the certain pressure region. In the pressure range of 110–290 GPa, repeated formations and disconnections of covalent bonds in O7–O6* and C3–C6* occur several times, resulting in a new six‐atom ring that forms at 220, 270, and 290 GPa, while a five‐atom ring and seven‐atom ring form between two adjacent molecules at 300 and 340 GPa, respectively. Then, the analysis of the band gap and DOS (PDOS) of Cyclo ara‐O indicates that its electronic character has changed at 300 GPa into an excellent insulator, but the electron transition is much easier at 350 GPa. Moreover, the relatively high optical activity with the pressure increases of Cyclo ara‐O is seen from the absorption spectra, and two obvious structural transformations are also observed at 180 and 230 GPa, respectively. Copyright © 2016 John Wiley & Sons, Ltd.     

Single‐crystal diffraction at the Extreme Conditions beamline P02.2: procedure for collecting and analyzing high‐pressure single‐crystal data

Fast detectors employed at third‐generation synchrotrons have reduced collection times significantly and require the optimization of commercial as well as customized software packages for data reduction and analysis. In this paper a procedure to collect, process and analyze single‐crystal data sets collected at high pressure at the Extreme Conditions beamline (P02.2) at PETRA III, DESY, is presented. A new data image format called `Esperanto' is introduced that is supported by the commercial software package CrysAlis^Pro (Agilent Technologies UK Ltd). The new format acts as a vehicle to transform the most common area‐detector data formats via a translator software. Such a conversion tool has been developed and converts tiff data collected on a Perkin Elmer detector, as well as data collected on a MAR345/555, to be imported into the CrysAlis^Pro software. In order to demonstrate the validity of the new approach, a complete structure refinement of boron‐mullite (Al₅BO₉) collected at a pressure of 19.4 (2) GPa is presented. Details pertaining to the data collections and refinements of B‐mullite are presented.     

Particle‐in‐cell simulation of an optimized high‐efficiency multistage plasma thruster

Electric propulsion attracts increasing attention in contemporary space missions as an interesting alternative to chemical propulsion because of the high efficiency it offers. The High‐Efficiency Multistage Plasma thruster, a class of cusped field thruster, is able to operate at different anode voltages and operation points and thereby generate different levels of thrust in a stable and efficient way. Since experiments of such thrusters are inherently expensive, multi‐objective design optimization (MDO) is of great interest. Several optimized thruster designs have resulted from a MDO model based on a zero‐dimensional (0D) power balance model. However, the MDO solutions do not warrant self‐consistency due to their dependency on estimation from empirical modelling based on former experimental studies. In this study, one of the optimized thruster designs is investigated by means of particle‐in‐cell (PIC) analysis to examine the predicted performance characteristics with self‐consistent simulations. The 0D power balance model is used to develop additional diagnostics for the PIC simulations to improve the physics analysis. Using input parameters for the 0D power balance model from the PIC simulations allows further improvement for the design optimization.     

An electrochemical cell with sapphire windows for operando synchrotron X‐ray powder diffraction and spectroscopy studies of high‐power and high‐voltage electrodes for metal‐ion batteries

A new multi‐purpose operando electrochemical cell was designed, constructed and tested on the Swiss–Norwegian Beamlines BM01 and BM31 at the European Synchrotron Radiation Facility. Single‐crystal sapphire X‐ray windows provide a good signal‐to‐noise ratio, excellent electrochemical contact because of the constant pressure between the electrodes, and perfect electrochemical stability at high potentials due to the inert and non‐conductive nature of sapphire. Examination of the phase transformations in the Li_1–xFe_0.5Mn_0.5PO₄ positive electrode (cathode) material at C/2 and 10C charge and discharge rates, and a study of the valence state of the Ni cations in the Li_1–xNi_0.5Mn_1.5O₄ cathode material for Li‐ion batteries, revealed the applicability of this novel cell design to diffraction and spectroscopic investigations of high‐power/high‐voltage electrodes for metal‐ion batteries.     

In situ synchrotron high‐energy X‐ray diffraction analysis on phase transformations in Ti–Al alloys processed by equal‐channel angular pressing

Mixtures of 47‐Al and 53‐Ti powders (atomic %) have been consolidated using back pressure equal‐channel angular pressing starting with both raw and ball‐milled powders. In situ synchrotron high‐energy X‐ray diffraction studies are presented with continuous Rietveld analysis obtained upon a heating ramp from 300 K to 1075 K performed after the consolidation process. Initial phase distributions contain all intermetallic compounds of this system except Al, with distribution maxima in the outer regions of the concentrations (α‐Ti, TiAl₃). Upon annealing, the phase evolution and lattice parameter changes owing to chemical segregation, which is in favour for the more equilibrated phases such as γ‐TiAl, α₂‐Ti₃Al and TiAl₂, were followed unprecedentedly in detail. An initial δ‐TiH₂ content with a phase transition at about 625 K upon heating created an intermediate β‐Ti phase which played an important role in the reaction chain and gradually transformed into the final products.     

Ionic liquids based on the bis(trifluoromethylsulfonyl)imide anion for high‐pressure Raman spectroscopy measurements

Assembling a diamond anvil cell for high‐pressure measurements involves placing in a gasket hole the sample of interest, a pressure transmitting fluid, and a material for pressure calibration. In this communication, we propose the use of ionic liquids containing the bis(trifluoromethylsulfonyl)imide anion ([Tf₂N]^‐), [(CF₃SO₂)₂ N]^‐, as a simultaneous pressure transmitting and calibrant material for high‐pressure Raman spectroscopy measurements of solid samples that are not soluble in ionic liquids. The position of the characteristic Raman band of the [Tf₂N]^‐ anion at 740 cm⁻¹ exhibits linear frequency shift for pressures up to 2.5 GPa. High‐pressure Raman spectra of different ionic liquids containing the same anion indicate that the actual magnitude of the pressure‐induced frequency shift of the [Tf₂N]^‐ normal mode depends on the counterion, the typical shift being 4.2 cm⁻¹/GPa. Ionic liquids based on the [Tf₂N]^‐ anion are also good pressure transmitting mediums because hydrostatic condition is kept at high pressure, and no crystallization is observed up to 4.0 GPa. Copyright © 2012 John Wiley & Sons, Ltd.     

The high‐resolution diffraction beamline P08 at PETRA III

The new third‐generation synchrotron radiation source PETRA III located at the Deutsches Elektronen‐Synchrotron DESY in Hamburg, Germany, has been operational since the second half of 2009. PETRA III is designed to deliver hard X‐ray beams with very high brilliance. As one of the first beamlines of PETRA III the high‐resolution diffraction beamline P08 is fully operational. P08 is specialized in X‐ray scattering and diffraction experiments on solids and liquids where extreme high resolution in reciprocal space is required. The resolving power results in the high‐quality PETRA III beam and unique optical elements such as a large‐offset monochromator and beryllium lens changers. A high‐precision six‐circle diffractometer for solid samples and a specially designed liquid diffractometer are installed in the experimental hutch. Regular users have been accepted since summer 2010.     

Novel high‐resolution silicon drift detectors

Silicon drift detectors (SDDs) are used as energy‐dispersive detectors for x‐ray fluorescence analysis in commercial systems. Because of the low capacitance of the readout anode, achieved by the device topology and by the integration of the first FET on the chip, noise contributions are very small, allowing good energy resolution at low shaping times and high count rates. Typical energy resolution is better than 147 eV FWHM at 5.9 keV (Mn Kα), at ?10°C. This allows the chips to be cooled with a thermoelectric element, avoiding the use of liquid nitrogen. SDD chips are produced at MPI‐Halbleiterlabor in Munich with different geometries and areas. Recently, a new SDD has been developed which places the anode and the integrated JFET at the margin of the chip where it can easily be shielded from direct irradiation with the use of a collimator. The new layout allows the design of a readout anode with smaller area and therefore reduces the capacitance to values of about 120 fF compared with 200–250 fF with standard SDDs. The result is an improvement in energy resolution down to 128 eV at ?15°C. A second effect is the enhancement of the peak‐to‐background values to 6000 homogeneously across the active area of the detector. Copyright © 2004 John Wiley & Sons, Ltd.     

Raman G‐mode of single‐wall carbon nanotube bundles under pressure

Raman studies of nanotubes under pressure have been a lively area of research. However, the results are not always as expected and at times have not been adequately explained. One example of the diversity of the results is the higher energy Raman mode (the graphitic mode, GM) shift to higher wavenumber under pressure. Here we report a new high‐pressure Raman study showing that the effects of the variation in the tube diameters and the pressure transmitting medium are both crucial for understanding the outcomes of such high‐pressure experiments. Copyright © 2011 John Wiley & Sons, Ltd.     

Large‐aperture prism‐array lens for high‐energy X‐ray focusing

A new prism‐array lens for high‐energy X‐ray focusing has been constructed using an array of different prisms obtained from different parabolic structures by removal of passive parts of material leading to a multiple of 2π phase variation. Under the thin‐lens approximation the phase changes caused by this lens for a plane wave are exactly the same as those caused by a parabolic lens without any additional corrections when they have the same focal length, which will provide good focusing; at the same time, the total transmission and effective aperture of this lens are both larger than those of a compound kinoform lens with the same focal length, geometrical aperture and feature size. This geometry can have a large aperture that is not limited by the feature size of the lens. Prototype nickel lenses with an aperture of 1.77 mm and focal length of 3 m were fabricated by LIGA technology, and were tested using CCD camera and knife‐edge scan method at the X‐ray Imaging and Biomedical Application Beamline BL13W1 at Shanghai Synchrotron Radiation Facility, and provided a focal width of 7.7 µm and a photon flux gain of 14 at an X‐ray energy of 50 keV.