Combined X-ray absorption and X-ray diffraction under high pressure

ABSTRACT

X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) are two complementary structural techniques. Their combination improves the understanding of the effect of pressure on materials as illustrated by examples taken from studies on different types of materials (semiconductors, molecular solid, ferroelectric perovskite and gas mixture). The introduction of nanopolycrystalline diamonds anvils has extended XAS to high-energy edges with the possibility to use energy-scanning XAS beamlines where XRD can be performed in addition to XAS experiments.     

X-ray absorption spectroscopy studies in supercritical aqueous solutions

ABSTRACT

The past two decades have witnessed significant advances in the application of X-ray absorption spectroscopy (XAS) to the study of supercritical aqueous fluids. The data obtained using in situ XAS have provided insights into the stability and the structure of metal complexes that are fundamental to understanding natural and industrial hydrothermal processes. Important recent advances using XAS can be attributed to the use of new high temperature and pressure autoclaves designed specifically for the analysis of fluids at extreme temperatures and pressures, improved techniques for the acquisition of X-ray absorption spectra and molecular-level computational modelling used in association with XAS analysis. High-brilliance light sources have not only provided new opportunities for XAS investigations of supercritical fluids, but have also revealed the effects of beam-induced radiolysis of the same fluids. The advent of energy-dispersive and rapid-acquisition XAS holds promise for future studies of beam-induced radiolysis and of the kinetics associated with the formation of metal complexes in high-temperature fluids.     

Applications of nano-polycrystalline diamond anvils to X-ray absorption spectroscopy under high pressure

ABSTRACT

Recently, nano-polycrystalline diamond (NPD) anvils have been widely applied in high pressure research using X-ray absorption spectroscopy (XAS). The nanometer-sized polycrystallization in NPD anvils enables us to obtain glitch-free X-ray absorption spectra. This advantage of NPD anvils drastically improves the experimental conditions of XAS, which has previously used conventional single-crystal diamond (SCD) anvils. Distorted spectra due to the glitches from the SCD anvils have been an inevitable problem of XAS. This paper reviews recent studies of XAS and related spectroscopic techniques using the NPD anvils, which have mainly been performed on BL39XU of SPring-8. We demonstrate how NPD anvils are useful when using XAS for high pressure research.     

Solid and Liquid AgI at High Pressure and High Temperature: A X-ray Absorption Spectroscopy Study

We report about recent X-ray absorption spectroscopy (XAS) measurements on solid and liquid AgI under high pressure. The structural behaviour of AgI has been investigated to pressures P～4.3 GPa at room temperature and to P～1.8 GPa at 1100 K. The high temperature/high pressure conditions have been obtained by means of a large-volume press of the Paris-Edinburgh type, coupled with a 10 mm boron/epoxy biconical gasket. The absorption spectra have been collected in transmission mode, both at the K-edge of Ag and I, and the samples have been characterized in situ by energy scanning X-ray diffraction at fixed angles. Our XAS results for solid AgI are compatible with previous X-ray diffraction measurements. For liquid AgI, we observe a slight change in the intensity and a shift in the frequency of the XAS oscillations with respect to what obtained in the case of the ambient pressure liquid.     

High Pressure X-Ray Absorption and Diffraction Study of InAs

We have performed X-ray absorption (XAS) and diffraction (XRD) measurements at high pressure on samples of powdered InAs, up to 50 and 80 GPa, respectively. In the lower pressure range, our data are consistent with the following structural sequence: Zincblende M NaCl M Cmcm . The first order transition from the semiconducting Zincblende phase to the metallic NaCl phase is clearly seen by the shift in the absorption onset at the As K-edge and the strong modifications of the extended X-ray absorption fine structure (EXAFS) due to the changes in the local structure from a 4-fold to a 6-fold coordinated environment. XAS shows the high pressure phase to be locally site-ordered. The diffraction data, analized by Rietveld fitting, gives a volume discontinuity of j V/V 0 ～0.18 for the first order transition. There is no apparent volume discontinuity associated to the NaCl M Cmcm transition.     

X-ray absorption study of Cu,Zn SOD under high pressure

Abstract

We have investigated Cu, Zn Superoxide Dismutase (Cu, Zn SOD) metal sites at high pressure using X-ray absorption. XAS (X-ray Absorption Spectroscopy) gives information on local structure and it is particularly suited to metal site investigation. To the best of our knowledge, this is the first time that protein conformational states have been investigated using the high pressure XAS technique. Cu, Zn SOD catalyses the dismutation of toxic oxygen radicals produced in cells; this reaction occurs at the copper metal site. Structural changes around the copper, induced by pressure, can be directly related to protein substates. Their characterisation is thus important in the understanding of protein activity.

The high-pressure device was a Paris-Edinburgh large volume cell.

Experiments were performed on lyophilised Cu, Zn SOD between 0 and 48 kbar at the copper and zinc K-edges. The two metal local atomic environments have a different behaviour as pressure increases: copper exhibits a more flexible environment; on the contrary, zinc shows small structural modifications. We have identified a state, formed between 3 and 8 kbar, which is stable up to 48 kbar.     

Spectroscopic characterization of β-FeSi2 single crystals and homoepitaxial β-FeSi2 films by XPS and XAS

Chemical state analysis by a combination of X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) using synchrotron radiation is performed for β-FeSi₂ single crystals and homoepitaxial β-FeSi₂ films. The Si 2p XPS and Fe L-edge XAS spectra imply that the annealing at 1173 K to remove native oxide layers on the crystal induces the formation of FeSi in the surface. The formation of FeSi is also confirmed by Si K-edge XAS analysis. For the homoepitaxial β-FeSi₂ films grown on the crystals, the Si K-edge XAS spectra indicate that structurally homogeneous β-FeSi₂ films can be grown on the β-FeSi₂ single crystals when the substrate temperatures of 973 and 1073 K are applied for molecular beam epitaxy (MBE). Consequently, it is indicated that the combination of XPS and XAS using synchrotron radiation is a useful tool to clarify chemical states of β-FeSi₂ single crystals and homoepitaxial β-FeSi₂ films, which is important to reveal optimized growth conditions of homoepitaxial films.     

Structural study of adsorption of isonicotinic acid and related molecules on rutile TiO2(1 1 0) I: XAS and STM

X-ray absorption spectroscopy (XAS) and scanning tunneling microscopy (STM) have been used to study the absorption of monolayers of the pyridinecarboxylic acid monomers (isonicotinic acid, nicotinic acid, and picolinic acid) and benzoic acid on a rutile TiO₂(1 1 0) surface. We find that the pyridine and phenyl rings are oriented with their planes largely perpendicular to the surface. The azimuthal orientations are strongly influenced by adsorbate-adsorbate interactions, which in each case leads to at least two different molecular species. In order to reach this conclusion a detailed strategy has been developed for the interpretation of angle-dependent XAS data, which does not rely on any curve fitting procedures.     

A comparison between X-ray absorption spectroscopy and Bremsstrahlung Isochromat Spectroscopy: The empty states of Pd−Al alloys and Pd2Si

We present X-ray absorption spectra (XAS) obtained with synchrotron radiaton at the PdL _2,3 edges of Pd–Al alloys and Bremsstrahlung Isochromat Spectra (BIS) at 1,486.7 eV of Pd–Al alloys and of Pd₂Si. The XAS and BIS results for the alloys are very similar indicating that the effect of the core potential (e.g. many-body, electronhole excitation) is negligible. The BIS results on Pd₂Si show differences with respect to Pd–Al due to the more localized bonds. The comparison between XAS and BIS data in Pd₂Si is still an open problem and might indicate a more subtle role of the core-hole potential.     

Blueprint XAS: a Matlab‐based toolbox for the fitting and analysis of XAS spectra

Blueprint XAS is a new Matlab‐based program developed to fit and analyse X‐ray absorption spectroscopy (XAS) data, most specifically in the near‐edge region of the spectrum. The program is based on a methodology that introduces a novel background model into the complete fit model and that is capable of generating any number of independent fits with minimal introduction of user bias [Delgado‐Jaime & Kennepohl (2010), J. Synchrotron Rad. 17 , 119–128]. The functions and settings on the five panels of its graphical user interface are designed to suit the needs of near‐edge XAS data analyzers. A batch function allows for the setting of multiple jobs to be run with Matlab in the background. A unique statistics panel allows the user to analyse a family of independent fits, to evaluate fit models and to draw statistically supported conclusions. The version introduced here (v0.2) is currently a toolbox for Matlab. Future stand‐alone versions of the program will also incorporate several other new features to create a full package of tools for XAS data processing.     

Electronic structure of Alq3 and Liq using soft X-ray spectroscopy and density functional theory calculation

The electronic structure of aluminum tris-8-hydroxyquinoline (Alq₃) and 8-hydroquinolatolithium (Liq) was investigated using a combination of X-ray emission spectroscopy (XES), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and spectral simulation with density functional theory. The chemical bonding states of Alq₃ and Liq were analyzed using XPS core-level spectra. The band gaps of Alq₃ and Liq were measured by combining the XAS and XES results. Additionally, resonant and non-resonant XES were used to measure the density of states for O sites in the molecules.     

Thickness dependence of X-ray absorption and photoemission in Fe thin films on Si(0 0 1)

To investigate the initial growth of Fe films on Si(0 0 1) and the Fe/Si interface, Fe films at various thicknesses have been systematically studied by soft X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS). The Fe L edge XAS spectrum shows a strong thickness dependence with broader line-width for thinner films. Detailed analysis of the Fe absorption signal as a function of the thickness shows that the broad linewidth of Fe L edge XAS spectra is mostly contributed by the first Fe layer at the Fe/Si interface. In contrast to XAS, Fe 2p photoemission spectra for these films are identical. However, valence band photoemission also shows a strong thickness dependence. Comparing the valence band photoemission spectra of the thin Fe/Si(0 0 1) films with that of pure Si and the thickest Fe film, the difference spectra at all thicknesses show almost identical shape indicating the same origin: the Fe/Si interface. Thus, it is mainly the first Fe layer at Fe/Si layer that is reactive with the Si substrate changing its electronic structure.     

Coordination changes in crystalline and vitreous GeO2

Abstract

High pressure x ray absorption spectroscopy (XAS) has been performed up to 29 GPa on crystalline and amorphous GeO₂. The modification of the x ray absorption near edge structure (XANES) as well as the variation of the Ge-O distance, indicate that the coordination of Ge changes from 4 to 6 above 6.5 GPa. The transition is confirmed by Raman spectroscopy.     

Versatile variable temperature insert at the DEIMOS beamline for in situ electrical transport measurements

The design and the first experiments are described of a versatile cryogenic insert used for its electrical transport capabilities. The insert is designed for the cryomagnet installed on the DEIMOS beamline at the SOLEIL synchrotron dedicated to magnetic characterizations through X‐ray absorption spectroscopy (XAS) measurements. This development was spurred by the multifunctional properties of novel materials such as multiferroics, in which, for example, the magnetic and electrical orders are intertwined and may be probed using XAS. The insert thus enables XAS to in situ probe this interplay. The implementation of redundant wiring and careful shielding also enables studies on operating electronic devices. Measurements on magnetic tunnel junctions illustrate the potential of the equipment toward XAS studies of in operando electronic devices.     

X ray absorption spectroscopy investigation of phase transition in Ge,GaAs and GaP

Abstract

The phase transitions of GaAs, GaP and Ge under pressure have been investigated by x-ray absorption spectroscopy (XAS). At the onset of the transition the Debye-Waller factor increases and the x-ray absorption near edge structure (XANES) is progressively modified. A mixing of the low and high pressure phase can be determined by XAS as well as amorphization of the sample on pressure release.     

Resonant Inelastic X-ray Scattering: From band mapping to inter-orbital excitations

Resonant inelastic X-ray scattering (also known as resonant X-ray Raman spectroscopy when only valence and conduction states are involved in the final state excitation) has developed into a major tool for understanding the electronic properties of complex materials. Presently it provides access to electron excitations in the few hundred meV range with element and bulk selectivity. Recent progress in X-ray optics and synchrotron radiation engineering have opened up new perspectives for this powerful technique to improve resolving power and efficiency. We briefly present the basics of the method and illustrate its potential with examples chosen from the literature. To cite this article: J. Lüning, C.F. Hague, C. R. Physique 9 (2008).     

Chromium doped manganites: evidence for electronically phase-separated systems

X-ray absorption spectroscopy (XAS) and soft-X-ray magnetic circular dichroism (SXMCD) at the Mn L_2,3-, Cr L_2,3- and O-K edges of Sm_0.5Ca_0.5Mn_1-xCr_xO₃ () bulk polycrystalline samples have been performed at T=20 K below the ferromagnetic Curie temperature. We show the existence of a magnetic sublattice on each of the probed cations. Considering an electronically phase-separated system, results are compared with magnetization and resistivity measurements and a tentative correlation with magnetoresistance properties on such doped compounds is discussed. Received 7 January 2000     

Remarks on the diffraction by the diamonds of the high-pressure diamond-anvil cell

Abstract

The diffraction by diamonds of a diamond-anvil cell has been observed to cause sharp dips in the intensity transmitted by the cell. As a result, the intensities of sample reflections measured in X-ray single crystal studies at high-pressure are significantly reduced when the diamond diffraction attenuates either the beam incident on the sample, or the one diffracted from it. This effect should be taken into account in the data collection, by avoiding, for each reflection, the diffractometer settings at which diffraction from the diamonds occurs.     

On‐line optical and X‐ray spectroscopies with crystallography: an integrated approach for determining metalloprotein structures in functionally well defined states

X‐ray‐induced redox changes can lead to incorrect assignments of the functional states of metals in metalloprotein crystals. The need for on‐line monitoring of the status of metal ions (and other chromophores) during protein crystallography experiments is of growing importance with the use of intense synchrotron X‐ray beams. Significant efforts are therefore being made worldwide to combine different spectroscopies in parallel with X‐ray crystallographic data collection. Here the implementation and utilization of optical and X‐ray absorption spectroscopies on the modern macromolecular crystallography (MX) beamline 10, at the SRS, Daresbury Laboratory, is described. This beamline is equipped with a dedicated monolithic energy‐dispersive X‐ray fluorescence detector, allowing X‐ray absorption spectroscopy (XAS) measurements to be made in situ on the same crystal used to record the diffraction data. In addition, an optical microspectrophotometer has been incorporated on the beamline, thus facilitating combined MX, XAS and optical spectroscopic measurements. By uniting these techniques it is also possible to monitor the status of optically active and optically silent metal centres present in a crystal at the same time. This unique capability has been applied to observe the results of crystallographic data collection on crystals of nitrite reductase from Alcaligenes xylosoxidans, which contains both type‐1 and type‐2 Cu centres. It is found that the type‐1 Cu centre photoreduces quickly, resulting in the loss of the 595 nm peak in the optical spectrum, while the type‐2 Cu centre remains in the oxidized state over a much longer time period, for which independent confirmation is provided by XAS data as this centre has an optical spectrum which is barely detectable using microspectrophotometry. This example clearly demonstrates the importance of using two on‐line methods, spectroscopy and XAS, for identifying well defined redox states of metalloproteins during crystallographic data collection.     

Evaluation of structural and electrochemical properties of the MnSb–Li system as anode for Li-ion batteries

The electrochemical reaction mechanisms between lithium and cystalline MnSb are investigated by X-ray diffraction, ¹²¹Sb Mössbauer spectroscopy, and X-ray absorption spectroscopy (XAS). The analysis of the experimental data at different depths of the electrochemical discharge process reveals a complex reaction mechanism comprising two steps. The main reaction of the first step corresponds to the dispersion of lithium in the MnSb matrix with formation of the intermediate compound LiMnSb. The second step corresponds to a Li–Sb alloying process with formation of Li₃Sb.