EXAFS spectra are often modeled using a small number of virtual coordination shells which can vary in coordination number, bond length, and disorder. While this bottom-up approach is generally successful for single-phase samples, it becomes intractable when the element of interest occupies multiple complex coordination states. Such cases may require a top-down approach, in which the measured spectrum is fitted using models with fixed atom positions and disorder. Given reasonable starting assumptions, such as ab initio molecular dynamics (AIMD) simulations of each possible component of the sample, the actual coordination states can be identified and quantified. I will present two examples of top-down solutions to difficult EXAFS problems: (1) the mechanisms of uranium incorporation into iron (oxyhydr)oxide minerals, and (2) the distribution of crystal phases in ferroelectric HfO2/ZrO2 nanostructures
As part of the Advanced Photon Source (APS) Multibend Achromat lattice upgrade two new beamlines for spectroscopy will be constructed on a canted undulator source at Sector 25. The programs at the 20-ID beamline at the APS need to move to sector 25 to make room for a planned long beamline. These will be combined with some other APS spectroscopy programs at sector 25 to use two new beamlines on a canted undulator. These two beamlines will service existing and upgraded endstations covering a variety of spectroscopy applications. There will be a microprobe branch that will provide sub-micron beams for fluorescence imaging, and micro-XAFS. These can be combined with confocal optics for micron level depth sensitivity. This branch will also have a station for XAFS experiments requiring a high-brilliance high-flux beam such as doped thin films or ultra-dilute samples. The second Advanced Spectroscopy branch will provide beam to two inline hutches. The first will have stations for both an enhanced LERIX spectrometer for non-resonant inelastic scattering (x-ray Raman), and spectrometers for high resolution emission spectroscopy. The second hutch will provide space for experiments requiring extensive setup, such as time-resolved pump-probe experiments. Both hutches will have a variety of focusing options providing beam sizes down to a few microns. To provide greater beam separation, both lines will have side deflecting mirrors for harmonic rejection, and focusing/collimation. The planned energy ranges are 4-32 keV for the microprobe branch, and 4-40 keV for the Advanced Spectroscopy branch. The horizontal deflection mirrors allow use of small offset monochromators equipped with liquid nitrogen cooled Si (111) crystals for monochromatic beam, and wide-bandpass multilayers providing higher flux for experiments that do not need high resolution such as imaging and non-resonant emission spectroscopy. The Advanced Spectroscopy branch will also have a secondary monochromator for experiments needing better resolution than provided by Si (111).
The talk will show how multimodal microfocus X-ray spectroscopy has been applied to characterize depleted uranium particles from munitions testing to predict and understand their environmental behavior. A combination of spatially resolved XRD, XRF and XAS techniques, combined with laboratory SEM observations, is shown to effectively differentiate DU particulate contamination in near surface and burial sites, and provide evidence of particulate alteration. This knowledge was used to design and understanding more effective remediation measures for depleted uranium contamination, with multimodal characterisation supporting process optimisation.
As a scientist have been always fascinated with how chemical systems react and interact with the natural world. Spectroscopy is a great tool to look at systems in real-time and operation conditions. Hard X-ray photon-in photon-out spectroscopy offers great possibilities due to its chemical sensitivity and speciation as well as high penetration. In this webinar, I will talk about two chemical processes and the developments in X-ray spectroscopy that made the studies possible. My studies have been made possible with the development of the dispersive von Hamos-type spectrometer (1), which enabled us to perform high-resolution studies on real systems due to its simple and versatile geometry. The first case study is called atomic telemetry, a method that enables us to follow chemotherapy drugs action mechanism under physiological conditions. The methodology was validated with a known drug (cisplatin) but in this webinar, I will present the findings from a drug (Pt-103) with an unknown action mechanism (2). The second case relates to the understanding of plasmonic materials hot carrier formation, and their importance to solar applications. Using synchrotron radiation and HR-XAS we were able to demonstrate for the first time the formation of hot electrons (3). However, to address their dynamics one needed to develop a way to perform RIXS measurements at the XFEL with the shortest and most intense X-ray pulses. I will show how we can do that (4) and what we plan to do with this methodology.
Modern free electron lasers provide intense X-ray pulses with 10^12 photons within ~10-100 femtoseconds. Such pulses enable new experimental techniques and provide unique opportunities for investigation of electronic and nuclear dynamics on their intrinsic time-scales. Interaction of ultra-bright, ultra-short X-ray pulses with matter can induce a multitude of nonlinear excitation processes which must be carefully considered when planning spectroscopic measurements and interpreting data recorded at XFELs. In most cases correct interpretation of the spectroscopic response and analysis of the electronic structure hinges on the assumption of single photon excitations. Here we attempted to answer the fundamental question on the limits to probing the ground (or native) electronic structure of a 3d transition metal ion at XEFL sources. Ions of the 3d transition metal, e.g., Mn(II), in a lighter element (O, C, H) environment were used as a model system. X-ray emission spectroscopy recorded from Mn2+ at different pulse conditions demonstrate spectral changes as a function of increased pulse intensity and pulse duration. To explain these changes, we develop a rate equation based on sequential ionization and relaxation events forming multiply ionized states during a single pulse which agree with observed spectroscopic trends. The percentage of Mn Kbeta emission recorded after the 1st, 2nd and 3rd 1s ionization events is calculated from the developed rate equation model and validated by experimental measurements. A method for estimating shifts in atomic X-ray emission lines from sequential ionization during a single XFEL pulse is given. From our data we infer that, in addition to multiple ionization, the impact of electron cascades is more significant for longer pulses. We note that while use of shorter X-ray pulses will help to counteract additional effects of electron cascades it will not help to overcome the spectral shifts due to sequential ionization. Presented data and associated analysis will help with experimental designs at current and upcoming XFELs where even higher intensities and shorter pulses are expected. 3d elements have a variety of important applications such as in bio-inorganic catalysis, chemical catalysis and energy storage / conversion making robust protocols for their XFEL analysis of general importance.
The safe use and disposal of UO2 based nuclear fuels relies on the stability of their material properties, under extreme conditions of temperature and irradiation, and with constantly evolving chemical composition. Among them, the uranium valence state’s behavior is at the heart of safety assessment during the entire fuel lifecycle. First, the oxidation from UO2 to UO3 induces for example considerable reorganization of the crystal structure and results in a volume expansion of about 36% when reaching the U3O8 intermediate state, eventually leading to fuel cladding failure. Secondly, the uranium valence state U6+ and to a lesser extent U5+ are known to have solubility several orders of magnitude higher than U4+, which is a critical parameter during fuel final disposal, the UO2 matrix very slow dissolution acting as the ultimate barrier of radioactivity release into the environment. X-ray absorption spectroscopy (XAS) is a key technique to assess U valence states in nuclear fuels thanks to its elemental sensitivity. However, XAS applied to nuclear fuels suffers from the presence of almost all the periodic table in spent fuel, from the high radioactive background limiting the number of available beamlines, from the fact that transmission mode is often impossible to perform, and from the large core-hole broadening effects. I will discuss how High Energy Resolution Fluorescence Detected XAS (HERFD-XAS) can help overcoming those drawbacks through a selection of a few recent examples, and I will describe the main benefits of HERFD for U valence state evaluation.
Databases of x-ray fluorescence line energies such as those of Deslattes (2003) and Bearden (1967) are critical to the calibration of any analytical tools that identify elemental compositions by their x-ray “fingerprints.” To be useful, such tables have to favor completeness over accuracy. Unfortunately, a full 75% of the lines in the current NIST database (SRD-128) rely on measurements at least 50 years old, coming from an age when the SI meter and the x-ray wavelength had never been tied together. Worse, SRD-128 lacks all information about line shapes or any M lines whatsoever. At NIST, we have begun a program to measure as many lines as possible with transition-edge sensor (TES) microcalorimeters, starting with the hard x-ray L lines of certain lanthanide metals. As Kelsey Morgan described on March 30, a TES combines advantages of solid-state detectors and diffractometers: it measures an enormous spectral region at once with resolving power of 1000 or higher. In tandem, we have also rejuvenated the SI-traceable diffractometer of Deslattes’ team, in order to expand the limited set of lines available for calibration of the TES spectrometer.
The conservation of marine archaeological wood is complicated by the presence of iron and sulfur. The sulfur originates from sulfate ions in seawater being transformed by sulfur reducing bacteria, and iron from dissolved fixture and artefacts. Incorporated in to the wood as reduced sulfur/iron compounds, such as pyrite, they can become problematic during the drying of the wood as rapid oxidation can result in acid formation which can promote degradation. The Mary Rose was a 16th century Tudor warship, commissioned by Henry VIII. After 34 years sailing, the ship sank off the coast of Portsmouth in 1545. Rediscovered in the late 1960s, the remaining hull emerged from the Solent in 1982 and now resides in a purpose built museum in Portsmouth Historic. In 2013 the consolidation treatment of the wood, to compensate for degradation, was completed and an air drying process commenced. During this phase samples have been periodically taken to monitor the evolution of iron and sulfur as a function of drying time using X-ray Absorption Spectroscopy. Alongside this Fourier-Transform Infrared Spectroscopy has been used to correlate any observed changes to the degradation levels within the wood. This information is crucial in understanding the chemical state of the wood and designing future conservation strategies.
References:
1. Schofield, E. J., “Illuminating the past: X-ray analysis of our cultural heritage” Nat. Rev. Mater. 3, 285-287, 2018
2. Schofield, E. J., Sarangi, R., Mehta, A., Jones, A. M. and Chadwick, A. V. “Nanotechnology and Synchrotron light in the service of Henry VIII: preserving the Mary Rose” Materials Today, Vol 14 (7-8) Pages: 354-358 2011
3. M. Sandström, F. Jalilehvand, I Persson, U. Gelius, P. Frank, I. Hall-Roth, Nature 2002, 415, 893–897.
Using High-Energy-Resolution-Fluorescence-Detection (HERFD), one can sharpen the pre-edge structures revealing their multiplet nature. In ionic systems, they can be calculated from the transition from the 3dN ground state to the 1s13dN+1 final state, where covalent systems need the inclusion of charge transfer effects.[1] The main edge is usually interpreted from the calculation of the transitions to empty states, using DFT theory, for example multiple scattering (FEFF). The 1s XPS spectra of transition metal oxides show multiple peaks, implying that one photon energy gives rise to electrons with multiple kinetic energies. This implies that the 1s XAS spectral shape must be described as the convolution of the empty states with the 1s XPS spectral shape [2]. Isolated transition metal ions only show a pre-edge and an edge, but bulk oxides show additional intensity between the pre-edge and edge. One can show that this intensity is related to dipole transitions of metal p-character that is part of the 3d-states of a near-neighbour metal ion [3]. X-MCD measurements of CrO2 reveal that the pre-edge is invisible in normal XAS [4]. In systems with two different metal ions one can observe the metal-metal charge transfer in the non-local peaks. [5]
References:
[1] J. Phys. Cond. Matt. 21, 104207 (2009).
[2] Ghiasi et al., Phys. Rev. B. 100, 075146 (2019).
[3] Juhin et al., Phys. Rev. B. 81, 115115 (2010).
[4] Zimmermann et al., J. Elec. Spec. 222, 74 (2018).
[5] Juhin et al. Inorg. Chem. 56, 10882 (2017).
We will look at how electrochemically active materials such as batteries can be studied in a concerted way by tuning to relevant resonant x-ray absorption at both the soft and the hard x-ray energy regimes. We will specifically examine the role of oxygen in the charge compensation of LiMO2 delafossite compounds, that serve as cathodes for an important class of rechargeable batteries, and the consequences of such for battery safety and for unprecedented changes to magnetic properties. As a case study of the reactions in LiCoO2, the electronic and atomic structure local to oxygen was first examined indirectly using hard x-rays by operando resonant Co K-level measurements and was later complemented by operando measurements at the O K-level using soft x-rays. A charge compensation roadmap was established for the LiCoO2 cathodes where we find tandem charge compensation reactions, first involving oxygen and later by cobalt. The generation of electron holes at oxygen sites explains the transitions in magnetic moment observed in LiCoO2 as a function of de-lithiation. A similar case was found in in the de-lithiation of Li[Ni1/3Co1/3Mn1/3]O2 cathodes. We hope to make the case that a detailed electrochemical roadmap for the reaction mechanisms in LiMO2 cathodes could only be revealed by a combination of soft and hard x-ray based experiments.
References:
F. M. Alamgir et al., LiCoO2 Thin-Film Batteries: Structural Changes and Charge Compensation, Journal of The Electrochemical Society},152, 5, (2005) A845. doi: 10.1149/1.1872672 ;
C. F. Petersburg, et al, Oxygen and transition metal involvement in the charge compensation mechanism of LiNi1/3Mn1/3Co1/3O2 cathodes, J. Mater. Chem., 22, 37, (2012) 19993-20000;
C. F. Petersburg, et al, Soft X-ray characterization technique for Li batteries under operating conditions, J. Synchrotron Rad., 16, (2009) 610-615.