Direct tomography imaging for inelastic X-ray scattering experiments at high pressure

Non-resonant inelastic X-ray scattering for discrimination of pigments

Inelastic X-ray scattering (IXS) spectroscopy has been used in many fields of solid-state physics and theoretical chemistry as an accurate and quantitative probe of elementary excitations. We show that non-resonant IXS spectra in the energy loss range below 100 eV exhibit a strong contrast across a wide range of commercially available pigments, opening new routes for their discrimination. These signatures combine plasmonic transitions, collective excitations and low energy absorption edges. We have performed IXS to discriminate different artists' pigments within complex mixtures and to quantitatively determine rutile and anatase polymorphs of TiO2. The integration of experimental data on pigment powders with suitable ab initio simulations shows a precise fit of the spectroscopic data both in the position of the resonances and in their relative intensity.

Hydration in aqueous NaCl

Atomistic details about the hydration of ions in aqueous solutions are still debated due to the disordered and statistical nature of the hydration process. However, many processes from biology, physical chemistry to materials sciences rely on the complex interplay between solute and solvent. Oxygen K-edge X-ray excitation spectra provide a sensitive probe of the local atomic and electronic surrounding of the excited sites. We used ab initio molecular dynamics simulations together with extensive spectrum calculations to relate the features found in experimental oxygen K-edge spectra of a concentration series of aqueous NaCl with the induced structural changes upon solvation of the salt and distill the spectral fingerprints of the first hydration shells around the Na⁺- and Cl^--ions. By this combined experimental and theoretical approach, we find the strongest spectral changes to indeed result from the first hydration shells of both ions and relate the observed shift of spectral weight from the post- to the main-edge to the origin of the post-edge as a shape resonance.

The rise of X-ray spectroscopies for unveiling the functional mechanisms in batteries

Synchrotron-based techniques have been key tools in the discovery, understanding, and development of battery materials. In this review, some of the most suitable X-ray spectroscopy related techniques employed for addressing diverse scientific cases connected to battery science are highlighted. Furthermore, current shortcomings, intrinsic limitations, and ongoing challenges of individual techniques are pointed out, providing an outlook of future trends that are relevant to the battery research community. In particular, the ongoing development of next generation synchrotrons, machine learning algorithms for data analysis and combined theoretical/experimental approaches will enhance the already powerful assets of these advanced spectroscopic methods.

New frontiers in extreme conditions science at synchrotrons and free electron lasers

Synchrotrons and free electron lasers are unique facilities to probe the atomic structure and electronic properties of matter at extreme thermodynamical conditions. In this context, 'matter at extreme pressures and temperatures' was one of the science drivers for the construction of low emittance 4th generation synchrotron sources such as the Extremely Brilliant Source of the European Synchrotron Radiation Facility and hard x-ray free electron lasers, such as the European x-ray free electron laser. These new user facilities combine static high pressure and dynamic shock compression experiments to outstanding high brilliance and submicron beams. This combination not only increases the data-quality but also enlarges tremendously the accessible pressure, temperature and density space. At the same time, the large spectrum of available complementary x-ray diagnostics for static and shock compression studies opens unprecedented insights into the state of matter at extremes. The article aims at highlighting a new horizon of scientific opportunities based on the synergy between extremely brilliant synchrotrons and hard x-ray free electron lasers.

X-ray back-diffraction: can we further increase the energy resolution by tuning the energy slightly below that of exact backscattering?

X-ray beams at energies tuned slightly below that of exact backscattering (extreme conditions, where X-ray back-diffraction is almost extinguished – called residual XBD) are better focused if the experiment is carried out at lower energies in order to avoid multiple-beam diffraction effects. Following previous work by the authors [Hönnicke, Conley, Cusatis, Kakuno, Zhou, Bouet, Marques & Vicentin (2014). J. Appl. Cryst. 47, 1658–1665], herein efforts are directed towards characterizing the residual XBD beam of an ultra-thin Si 220 crystal (UTSiXTAL) at ∼3.2 keV. To achieve the residual XBD condition the UTSiXTAL was cooled from 310 to 273 K. The results indicate that under this extreme condition the energy resolution can be further improved. Issues with the energy resolution measurements due to incoming beam divergence and the ultra-thin crystal flatness are discussed.

A versatile diamond anvil cell for X-ray inelastic, diffraction and imaging studies at synchrotron facilities

We present a new diamond anvil cell design, hereafter called mBX110, that combines both the advantages of a membrane and screws to generate high pressure. It enables studies at large-scale facilities for many synchrotron X-ray techniques and has the possibility to remotely control the pressure with the membrane as well as the use of the screws in the laboratory. It is fully compatible with various gas-loading systems as well as high/low temperature environments in the lab or at large scale facilities. The mBX110 possesses an opening angle of 85° suitable for single-crystal diffraction or Brillouin spectroscopy and a large side opening of 110° which can be used for X-ray inelastic techniques, such as X-ray Raman scattering spectroscopy, but also for X-ray emission, X-ray fluorescence, or X-ray absorption. An even larger opening of 150° can be manufactured enabling X-ray imaging tomography. We report data obtained with the mBX110 on different beamlines with single-crystal diffraction of stishovite up to 55 GPa, X-ray powder diffraction of rutile-GeO₂ and tungsten to 25 GPa and 280 GPa, respectively, X-Ray Raman spectra of the Si L-edge in silica to 95 GPa, and Fe Kβ X-ray emission spectra on a basalt glass to 17 GPa.

Resonant inelastic X-ray scattering of magnetic excitations under pressure

Resonant inelastic X-ray scattering (RIXS) is an extremely valuable tool for the study of elementary, including magnetic, excitations in matter. The latest developments of this technique have mostly been aimed at improving the energy resolution and performing polarization analysis of the scattered radiation, with a great impact on the interpretation and applicability of RIXS. Instead, this article focuses on the sample environment and presents a setup for high-pressure low-temperature RIXS measurements of low-energy excitations. The feasibility of these experiments is proved by probing the magnetic excitations of the bilayer iridate Sr₃Ir₂O₇ at pressures up to 12 GPa.

X-ray Raman spectroscopy of lithium-ion battery electrolyte solutions in a flow cell

The effects of varying LiPF₆ salt concentration and the presence of lithium bis(oxalate)borate additive on the electronic structure of commonly used lithium-ion battery electrolyte solvents (ethylene carbonate-dimethyl carbonate and propylene carbonate) have been investigated. X-ray Raman scattering spectroscopy (a non-resonant inelastic X-ray scattering method) was utilized together with a closed-circle flow cell. Carbon and oxygen K-edges provide characteristic information on the electronic structure of the electrolyte solutions, which are sensitive to local chemistry. Higher Li⁺ ion concentration in the solvent manifests itself as a blue-shift of both the π* feature in the carbon edge and the carbonyl π* feature in the oxygen edge. While these oxygen K-edge results agree with previous soft X-ray absorption studies on LiBF₄ salt concentration in propylene carbonate, carbon K-edge spectra reveal a shift in energy, which can be explained with differing ionic conductivities of the electrolyte solutions.

Pressure driven spin transition in siderite and magnesiosiderite single crystals

Iron-bearing carbonates are candidate phases for carbon storage in the deep Earth and may play an important role for the Earth's carbon cycle. To elucidate the properties of carbonates at conditions of the deep Earth, we investigated the pressure driven magnetic high spin to low spin transition of synthetic siderite FeCO₃ and magnesiosiderite (Mg_0.74Fe_0.26)CO₃ single crystals for pressures up to 57 GPa using diamond anvil cells and x-ray Raman scattering spectroscopy to directly probe the iron 3d electron configuration. An extremely sharp transition for siderite single crystal occurs at a notably low pressure of 40.4 ± 0.1 GPa with a transition width of 0.7 GPa when using the very soft pressure medium helium. In contrast, we observe a broadening of the transition width to 4.4 GPa for siderite with a surprising additional shift of the transition pressure to 44.3 ± 0.4 GPa when argon is used as pressure medium. The difference is assigned to larger pressure gradients in case of argon. For magnesiosiderite loaded with argon, the transition occurs at 44.8 ± 0.8 GPa showing similar width as siderite. Hence, no compositional effect on the spin transition pressure is observed. The spectra measured within the spin crossover regime indicate coexistence of regions of pure high- and low-spin configuration within the single crystal.

A large-solid-angle X-ray Raman scattering spectrometer at ID20 of the European Synchrotron Radiation Facility

An end-station for X-ray Raman scattering spectroscopy at beamline ID20 of the European Synchrotron Radiation Facility is described. This end-station is dedicated to the study of shallow core electronic excitations using non-resonant inelastic X-ray scattering. The spectrometer has 72 spherically bent analyzer crystals arranged in six modular groups of 12 analyzer crystals each for a combined maximum flexibility and large solid angle of detection. Each of the six analyzer modules houses one pixelated area detector allowing for X-ray Raman scattering based imaging and efficient separation of the desired signal from the sample and spurious scattering from the often used complicated sample environments. This new end-station provides an unprecedented instrument for X-ray Raman scattering, which is a spectroscopic tool of great interest for the study of low-energy X-ray absorption spectra in materials under in situ conditions, such as in operando batteries and fuel cells, in situ catalytic reactions, and extreme pressure and temperature conditions.

Miniature diamond anvils for X-ray Raman scattering spectroscopy experiments at high pressure

X-ray Raman scattering (XRS) spectroscopy is an inelastic scattering method that uses hard X-rays of the order of 10 keV to measure energy-loss spectra at absorption edges of light elements (Si, Mg, O etc.), with an energy resolution below 1 eV. The high-energy X-rays employed with this technique can penetrate thick or dense sample containers such as the diamond anvils employed in high-pressure cells. Here, we describe the use of custom-made conical miniature diamond anvils of less than 500 µm thickness which allow pressure generation of up to 70 GPa. This set-up overcomes the limitations of the XRS technique in very high-pressure measurements (>10 GPa) by drastically improving the signal-to-noise ratio. The conical shape of the base of the diamonds gives a 70° opening angle, enabling measurements in both low- and high-angle scattering geometry. This reduction of the diamond thickness to one-third of the classical diamond anvils considerably lowers the attenuation of the incoming and the scattered beams and thus enhances the signal-to-noise ratio significantly. A further improvement of the signal-to-background ratio is obtained by a recess of ∼20 µm that is milled in the culet of the miniature anvils. This recess increases the sample scattering volume by a factor of three at a pressure of 60 GPa. Examples of X-ray Raman spectra collected at the O K-edge and Si L-edge in SiO₂ glass at high pressures up to 47 GPa demonstrate the significant improvement and potential for spectroscopic studies of low-Z elements at high pressure.