High Temperature X-ray Absorption Research Articles

Unravelling the amorphous structure and crystallization mechanism of GeTe phase change memory materials

The reversible phase transitions in phase-change memory devices can switch on the order of nanoseconds, suggesting a close structural resemblance between the amorphous and crystalline phases. Despite this, the link between crystalline and amorphous tellurides is not fully understood nor quantified. Here we use in-situ high-temperature x-ray absorption spectroscopy (XAS) and theoretical calculations to quantify the amorphous structure of bulk and nanoscale GeTe. Based on XAS experiments, we develop a theoretical model of the amorphous GeTe structure, consisting of a disordered fcc-type Te sublattice and randomly arranged chains of Ge atoms in a tetrahedral coordination. Strikingly, our intuitive and scalable model provides an accurate description of the structural dynamics in phase-change memory materials, observed experimentally. Specifically, we present a detailed crystallization mechanism through the formation of an intermediate, partially stable ‘ideal glass’ state and demonstrate differences between bulk and nanoscale GeTe leading to size-dependent crystallization temperature.

Operando High-Temperature X-Ray Absorption Spectroscopy of LiNiO2 Cathode Material during Synthesis

The increasing demand for higher energy density in batteries for automotive applications has led to a trend toward increasing the Ni content in layered oxide cathode materials to increase their reversible capacity. However, LiNiO2, which has the highest Ni content, shows a faster capacity drop during charge-discharge cycling, which is attributed to its inherent structural instability in both the pristine and delithiated states. Therefore, to address the former issue and to better understand the appropriate synthesis conditions for LiNiO2, we have studied the changes in the local structure of LiNiO2 during the synthesis process using operando analysis techniques. Recently, high temperature in situ X-ray diffraction during LiNiO2 synthesis was reported by Bianchini [1] and Weber [2], which confirmed the loss of lithium and oxygen and the migration of Ni to Li layers above 700 °C. To further understand the mechanism of Li/Ni mixing and oxygen loss, operando high-temperature X-ray absorption spectroscopy (XAS) was performed during the synthesis of LiNiO2 to clarify the electrical structure of Ni at high temperatures. Pre-heated Li x NiO2 (x = 0.76, 1.01, 1.26, and 1.51) precursors were prepared by heating LiOH·H2O and Ni(OH)2 at 480 °C for 24 h in air. The precursor was pelletized and set in a quartz sample holder. Operando high-temperature Ni K-edge XAS spectra of the precursor were obtained at BL9A and BL12C in Photon Factory (PF), High Energy Accelerator Research Organization (KEK) by a transmission mode. The operando spectra of all samples except Li/Ni = 0.76 revealed a clear edge shift to the lower energy side corresponding to nickel reduction above 700 °C. The Li/Ni = 0.76 sample has a lithium-deficient composition and Ni is not oxidized to Ni3+ below 700 °C, so the Li/Ni = 0.76 sample has almost no Ni K-edge energy shift above 700 °C. These results suggest that Ni in LiNiO2 is reduced above 700 °C even in an oxygen atmosphere, regardless of whether the Li/Ni ratio is greater than 1. The Ni reduction above 700 °C is probably the cause of the Li/Ni mixture.Reference[1] M. Bianchini, J. Janek, et al., Angew. Chem., Int. Ed., 58, 10434 (2019).[2] R. Weber, J. R. Dahn, et al., J. Electrochem. Soc., 167, 100501 (2020).

Development of extremely high-temperature X-ray absorption fine structure measurement method for oxide samples

ABSTRACT Since the severe accident at Fukushima Daiichi Nuclear Power Plant, there has been an interest in the properties of oxide fuels in their near-melting temperature range. X-ray absorption fine structure (XAFS) is an effective method for understanding the fine structure and chemical bonding state of a sample; however, high-temperature XAFS measurements performed to date have been limited to 2000 K. We have developed a method for XAFS measurements of high-melting oxides above 2000 K. This method utilizes resistance heating of tungsten and confines the sample within a 0.06 mm slit to enable transmission-type XAFS measurements. We successfully performed an XAFS test with yttria-stabilized zirconia loaded at the tip of the slit and heated to 3000 K.

Low thermal-expansion and high proton uptake for protonic ceramic fuel cell cathode

Nowadays, the excessive thermal expansion behavior of Co-based electrode always leads to the cell degradation or delamination. Especially for BaCoO3-δ-type perovskite oxides, as the result of the large ionic radius of Ba2+ (1.61 Å), the phase structures of these materials are not stable. Herein, we developed a novel single-phase electrode Ba2Sc0.1Nb0.1Co1.5Fe0.3O6-δ (BSNCF) with a stable cubic perovskite structure and suitable thermal expansion coefficient (TEC, 11.9 × 10−6 K−1), which showed a great stability in symmetrical cell area specific resistances (ASRs) subjecting to the harsh thermal cycling procedure with 30 cycles between 300 °C and 600 °C (increased from 0.197 Ω cm2 to 0.222 Ω cm2,13% increase). The high-temperature hard X-ray absorption spectroscopy measurement directly monitored a small change of Co valence in BSNCF as the temperatures rising. Also, BSNCF exhibits well proton uptake for its appropriate oxygen-site basicity and excellent surface reaction activity. The single cell based on BSNCF achieved an outstanding peak power density of 977 mW cm−2 at 600 °C.

Elucidating the Formation Mechanisms of Fe-N-C Electrocatalysts Using High-Temperature in Situ Characterisation Techniques

Pt-free cathode catalysts are attracting increased interest to replace platinum in polymer electrolyte membrane fuel cells (PEFCs). Heat-treated nitrogen-doped carbons decorated with iron have been the most widely studied alternative. However, lack of understanding of the formation of active sites and degradation mechanisms in such catalysts is hindering their progress. Here, we use a nitrogen-doped carbon foam support with optimised porosity, surface area, and conductivity which is then infiltrated with three different iron precursors: iron (II) acetate (FeAc); iron (III) chloride (FeCl3); and iron phthalocyanine (FePc). These are then pyrolyzed to form Fe-N-C-based ORR electrocatalysts. This approach takes advantage of an already optimized conductive, porous, high surface area and temperature-stable support. The stability of the support helps to separate the role of the different iron precursors from other factors in the generation of catalytic active sites. High temperature X-ray absorption fine structure (HT-XAFS) and high temperature X-ray photoelectron spectroscopy (HT-XPS) were employed to study changes in the iron precursors during the synthesis of Fe-N-C electrocatalysts. Furthermore, near ambient pressure XPS (NAP-XPS) and in-situ electrochemical XAFS were performed to study the interaction between the synthesized catalysts and oxygen. These results are then related to the electrochemical activity.

In-Situ High-Temperature X-Ray Absorption Spectroscopy of Fe-N-C Electrocatalysts during Pyrolysis

Deeper understanding of the formation of active sites during the pyrolysis of iron-nitrogen-carbon (Fe-N-C) electrocatalysts for the oxygen reduction reaction (ORR) is required. Here, we present a nitrogen-doped carbon foam support treated at high temperature (1600 °C) to stabilise the material properties. Iron-containing precursors are then adsorbed onto the surface, followed by heat treatment. This changes the chemical state of the adsorbed iron, whilst the material properties of the support are effectively unchanged. The two-step synthesis method used to create these model electrocatalysts decouples porosity, microstructure, and conductivity from the electrochemistry, which can instead be attributed mainly to changes in the chemical environment of iron.Advanced characterisation techniques such as in-situ high-temperature X-ray absorption spectroscopy (HT-XAS) and high-temperature near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) are employed to gain insights into the state of the adsorbed iron as it decomposes during pyrolysis to form active sites. Transmission electron microscopy (TEM) is used to track the microstructure for different types of precursor heated at different temperatures. Meanwhile, in-situ electrochemical XAS is used to understand how the chemical environment of the catalysts change during the ORR.The results are discussed by relating different chemical environment with the electrochemical ORR activity. Despite the two-step synthesis technique, these model Fe-N-C electrocatalysts are highly active. The load-cycling durability and start-stop durability are measured, and the degradation mechanisms are discussed.

Investigation of ionic local structure in molten salt fast reactor LiF-ThF4-UF4 fuel by EXAFS experiments and molecular dynamics simulations

Abstract This article focuses on the speciation of molten fluoride mixtures based on ThF4 and UF4 actinides used in molten salt reactors. The local structure of molten AF-MF4 systems (A = Li+, Na+, K+; M = Th4+, U4+) was studied in situ by combining measurements by high temperature X-ray absorption spectroscopy and molecular dynamics simulations. In the molten state, 20 higher than the melting temperature, these mixtures are composed of free fluorine and anionic species [MF7]3−, [MF8]4− and [MF9]5− whose distribution varies with the amount of MF4 (M = Th4+, U4+). Regardless of the cation, these complexes consist of an average of 8 F− neighbors for MF4 content is lower than 35 mol%. This value decreases to 7 as the size of the alkaline ion A (A = Li+, Na+, K+) increases. The [MFx]4−x species are linked together by bridging fluorine ions to form long chains [MxFY]4x−y. The addition of a few percent UF4 to the eutectic LiF-ThF4 composition (77.5 mol% - 22.5 mol%) at leads to a slight increase in the population of free fluorine ions due to the breakdown of the Th-F links to form complexes [UFx]4−x isolated or connected to the (ThxFy)4x−y chains. This change in the structure of the liquid results in a slight decrease in viscosity when a few mol. % of UF4 is added.

Investigation of the local structure of molten ThF4-LiF and ThF4-LiF-BeF2 mixtures by high-temperature X-ray absorption spectroscopy and molecular-dynamics simulation.

The microscopic structures of ThF4-LiF and ThF4-LiF-BeF2 molten salts have been systematically investigated by in situ high-temperature X-ray absorption fine-structure (XAFS) spectroscopy combined with molecular-dynamics (MD) simulations. The results reveal that the local structure of thorium ions was much more disordered in the molten state of the ThF4-LiF-BeF2 salt than that in ThF4-LiF, implying that the Th and F ions were exchanged more frequently in the presence of Be ions. The structures of medium-range-ordered coordination shells (such as Th-F2nd and Th-Th) have been emphasized by experimental and theoretical XAFS analysis, and they play a significant role in transport properties. Using MD simulations, the bonding properties in the molten ThF4-LiF and ThF4-LiF-BeF2 mixtures were evaluated, confirming the above conclusion. This research is, to the best of our knowledge, the first systematic study on the ThF4-LiF-BeF2 molten salt via quantitative in situ XAFS analysis and MD simulations.

Kr environment in feldspathic glass and melt: A high pressure, high temperature X-ray absorption study

Noble gases are used to trace the evolution of the Earth's atmosphere and magmatic processes. However little is known about their solubility mechanisms in silicate melts or glasses, whereas it could be fundamental to interpret these data. We report here the first in situ X-ray absorption spectroscopy data at the Kr K-edge on a feldspathic Kr-bearing glass and melt (0.5 wt% Kr) up to 2.7 GPa and 1086 °C. Kr fitting of the EXAFS signal shows that Kr is surrounded by oxygen atoms. Two Kr-O distances are identified: 2.49 ± 0.1 Å and 3.32 ± 0.1 Å. Results can be interpreted as two Kr populations. Kr surrounded by O atoms at 3.32 ± 0.1 Å suggests a filling of the large cavities available in the silicate network, in agreement with previous studies. On the contrary, Kr surrounded by O atoms at 2.49 ± 0.1 Å, and the observed +0.6 eV shift of the edge position at high pressure, suggest Kr bonding to O atoms and Kr oxidation inside cages formed by the largest aluminosilicate rings (i.e. ~12-membered-rings). Present results show that heavy noble gases incorporation in silicate melts can no longer be considered as a passive filling of the voids.

Chromium Reaction Mechanisms for Speciation using Synchrotron in-Situ High-Temperature X-ray Diffraction.

We use in situ high-temperature X-ray diffraction (HT-XRD), ex-situ XRD and synchrotron X-ray absorption near edge structure spectroscopy (XANES) to derive fundamental insights into mechanisms of chromium oxidation during combustion of solid fuels. To mimic the real combustion environment, mixtures of pure eskolaite (Cr(3+)2O3), lime (CaO) and/or kaolinite [Al2Si2O5(OH)4] have been annealed at 600-1200 °C in air versus 1% O2 diluted by N2. Our results confirm for the first time that (1) the optimum temperature for Cr(6+) formation is 800 °C for the coexistence of lime and eskolaite; (2) upon addition of kaolinite into oxide mixture, the temperature required to produce chromatite shifts to 1000 °C with a remarkable reduction in the fraction of Cr(6+). Beyond 1000 °C, transient phases are formed that bear Cr in intermediate valence states, which convert to different species other than Cr(6+) in the cooling stage; (3) of significance to Cr mobility from the waste products generated by combustion, chromatite formed at >1000 °C has a glassy disposition that prevents its water-based leaching; and (4) Increasing temperature facilitates the migration of eskolaite particles into bulk lime and enhances the extent to which Cr(3+) is oxidized, thereby completing the oxidation of Cr(3+) to Cr(6+) within 10 min.

Polyamorphism in cerium based bulk metallic glasses: Electronic and structural properties under pressure and temperature by x-ray absorption techniques

High pressure and high temperature x-ray absorption near edge spectroscopy experiments have been carried out on Ce60Al20Cu20 bulk metallic glass showing an electronic delocalization of the 4f-electron of cerium under pressure. In parallel, high pressure extended x-ray absorption fine structure spectroscopy reveals large structural modifications of the cerium local environment. This study provides experimental evidence that an electronic driven structural transformation occurs in cerium based bulk metallic glasses (Ce-BMGs). The effect of temperature on the hysteresis of this amorphous-amorphous phase transition is also discussed, suggesting the existence of a critical point in the phase diagram of Ce-BMGs. This work will encourage further investigations on Ce-based metallic glasses phase diagrams in order to support, or refute, the actual theoretical understanding of polyamorphism.

High temperature emissivity, reflectivity, and x-ray absorption of BiFeO3

We report on the lattice evolution of BiFeO3 as function of temperature using far infrared emissivity, reflectivity, and x-ray absorption local structure. A power law fit to the lowest frequency soft phonon in the magnetic ordered phase yields an exponent β=0.25 as for a tricritical point. At about 200 K below TN∼640 K it ceases softening as consequence of BiFeO3 metastability. We identified this temperature as corresponding to a crossover transition to an order-disorder regime. Above ∼700 K strong band overlapping, merging, and smearing of modes are consequence of thermal fluctuations and chemical disorder. Vibrational modes show band splits in the ferroelectric phase as emerging from triple degenerated species as from a paraelectric cubic phase above TC∼1090 K. Temperature dependent x-ray absorption near edge structure (XANES) at the Fe K edge shows that lower temperature Fe3+ turns into Fe2+. While this matches the FeO wüstite XANES profile, the Bi LIII-edge downshift suggests a high temperature very complex bond configuration at the distorted A perovskite site. Overall, our local structural measurements reveal high temperature defect-induced irreversible lattice changes, below, and above the ferroelectric transition, in an environment lacking of long-range coherence. We did not find an insulator to metal transition prior to melting.

Spin-state crossover in multiferroicCa3Co2−xMnxO6

Ca_3Co_{2-x}Mn_xO_6 (x ~ 0.96) is a multiferroic with spin-chains of alternating Co(2+) and Mn(4+) ions. The spin state of Co(2+) remains unresolved, due to a discrepancy between high temperature X-ray absorption (S=3/2) and low temperature neutron (S=1/2) measurements. Using a combination of magnetic modeling and crystal-field analysis, we show that the existing low temperature data cannot be reconciled within a high spin scenario by invoking spin-orbit or Jahn-Teller distortions. To unify the experimental results, we propose a spin-state crossover with specific experimental predictions.

Silica polymorphs, glass and melt: An in situ high temperature XAS study at the Si K-edge

Abstract High temperature X-ray absorption spectra at the Si K-edge were obtained for SiO 2 quartz from room temperature up to 2030 K. Important modifications are observed for the XANES spectra. These change are related to rearrangements of the SiO 4 tetrahedra beyond the short-range correlations. To interpret these spectral evolutions, SiO 2 polymorph samples were observed at room temperature and XANES calculations using FDMNES were performed. Very strong differences are shown between the different polymorphs even between α and β phases for which only small displacive angle rotations of the SiO 4 tetrahadra occurs. Therefore the quartz α to β transition could be identified at its expected temperature, 842 K. A badly defined transition toward β-cristobalite is observed between 1670 and 1940 K. The dynamics of this totally reconstructive transition was further investigated on heat treated cherts. Finally the liquid is reached around 2000 K. Many similarities were observed on SiO 2 between its glass at room temperature, β-cristobalite and liquid at high temperature.

A diamond anvil cell with resistive heating for high pressure and high temperature x-ray diffraction and absorption studies

In this paper we describe a prototype of a diamond anvil cell (DAC) for high pressure/high temperature studies. This DAC combines the use of a resistive oven of 250 W power in a very small volume, associated with special conical seats for Boehler-type diamond anvils in order to have a large angular acceptance. To protect the diamond anvils from burning and to avoid the oven oxidation, the heated DAC is enclosed in a vacuum chamber. The assemblage was used to study the melting curve of germanium at high pressure (up to 20 GPa) and high temperature (up to 1200 K) using x-ray diffraction and x-ray absorption spectroscopy.

Kinetics of iron redox reactions in silicate liquids: A high-temperature X-ray absorption and Raman spectroscopy study

The oxidation kinetics of a Fe-bearing supercooled liquid of the system SiO2–CaO–MgO–Na2O–FeO has been determined near the glass transition range by X-ray absorption near edge structure (XANES) and Raman spectroscopies. Both techniques yield room-temperature iron redox ratios in accord with wet chemical, Mössbauer and electron microprobe analyses. Similar oxidation kinetics have also been observed with both methods. At constant temperature, the kinetics obey an exponential law with a characteristic time that follows an Arrhenian temperature dependence. As redox changes are too fast to be accounted for in terms of diffusion of either ionic or molecular oxygen, these results lend further support to the idea that the rate-limiting factor for oxidation near the glass transition is diffusion of network-modifying cations along with a flux of electron holes.

The Local Structure of Molten CdBr2

The local structure of molten CdBr2 was investigated by high temperature X-ray absorption fine structure (XAFS) analysis. The quartz cell designed for hygroscopic high temperature molten salts was successfully used in the measurement. At room temperature the nearest neighbor Cd2+-Br− distance decreased from 2.71 Å in solid state to 2.60 Å in the molten state. The coordination number decreased from 6 to 4 on melting. The obtained structural parameters showed that (CdBr4)2− is predominant in molten CdBr2.

X-ray structural analysis of molten PbCl 2

The structure of molten PbCl 2 was investigated by using high temperature X-ray diffraction(XRD) and X-ray absorption fine structure (XAFS) analysis techniques. It is concluded from the Fourier transformation and Debye scattering analyses of the XRD data and the curve fitting analysis of the XAFS function that the nearest Pb 2+–Cl − pair consists of relatively rigid 4-fold coordination and additional loose 2&3 coordination structures.

High-pressure, high-temperature x-ray absorption fine structure transmission cell for the study of aqueous ions with low absorption-edge energies

We describe a method to acquire x-ray absorption fine structure (XAFS) spectra of low-Z atoms dissolved in high-pressure liquids and supercritical fluids. The method is applicable to energies at and below the Ca K edge (4038.5 eV). The cell design incorporates 25-μm-thick×700 μm diameter diamond x-ray windows in a Poulter-type seal geometry. X-ray focusing mirrors were used to reduce both the horizontal and vertical beam size to about 200 μm so that the incident beam would pass cleanly through the 300 μm aperture of the cell. Pathlengths in the range from 50 to 1000 μm can be selected with appropriate spacers. The maximum operating conditions of the flow-through cell design are 500 °C and 1 kbar. We show that only a relatively small number of XAFS scans are required to generate high signal-to-noise ratio spectra for a supercritical water solution (400 °C) containing 1 m CaCl2 using a bending-magnet beamline (20-BM, PNC-CAT, Advanced Photon Source, Argonne National Laboratory).

X-ray absorption study of molten uranium chloride system

The local structure of molten UCl3 in LiCl-KCl eutectic was studied by using a high-temperature X-ray absorption fine structure (XAFS) technique. The coordination behavior of Cl- ion around U3+ ion was investigated by XAFS of the U LIII-absorption edge. The distance and the coordination number of the 1st U3+-Cl- correlation were 2.82±0.1 Å and 7.3±0.6, respectively. It is concluded that the uranium ion has trivalent state in the melt by comparing with the XANES spectra of tetravalent uranium compounds and solid UCl3.