X-ray Absorption Measurements Research Articles

Pressure-induced quantum phase transitions in aYbB6single crystal

Topological insulators (TIs) containing 4f electrons have recently attracted intensive interests due to the possible interplay of their non-trivial topological properties and strong electronic correlations. YbB6 and SmB6 are the prototypical systems with such unusual properties, which may be tuned by external pressure to give rise to new emergent phenomena. Here, we report the first observation, through in-situ high pressure resistance, Hall, X-ray diffraction and X-ray absorption measurements, of two pressure-induced quantum phase transitions (QPTs) in YbB6. Our data revealthat the two insulating phases are separated by a metallic phase due to the pressure-driven valence change of Yb f-orbitals. In combination with previous studies, our results suggest that the two insulating states may be topologically different in nature and originate from the d-p and d-f hybridization, respectively. The tunable topological properties of YbB6 revealed in this study may shed light on the intriguing correlation between the topology and the 4f electrons from the perspective of pressure dependent studies.

Decacyclene Trianhydride at Functional Interfaces: An Ideal Electron Acceptor Material for Organic Electronics.

We report the interface energetics of decacyclene trianhydride (DTA) monolayers on top of two distinct model surfaces, namely, Au(111) and Ag(111). On the latter, combined valence band photoemission and X-ray absorption measurements that access the occupied and unoccupied molecular orbitals, respectively, reveal that electron transfer from substrate to surface sets in. Density functional theory calculations confirm our experimental findings and provide an understanding not only of the photoemission and X-ray absorption spectral features of this promising organic semiconductor but also of the fingerprints associated with the interface charge transfer.

Ultrathin CoOOH Oxides Nanosheets Realizing Efficient Photocatalytic Hydrogen Evolution

Producing H2 by means of direct photocatalytic water splitting using suspension system is highly desired in efficiently transforming solar energy to chemical energy. Here, using an ultrathin β-CoOOH nanosheet as photocatalyst suspended in aqueous Na2SO3 solution, we realize highly efficient and stable photocatalytic hydrogen evolution under visible light irradiation. The as-prepared ultrathin β-CoOOH nanosheets suspension can directly photocatalyze hydrogen evolution in a high rate of 1200 μmol/g/h and large quantum efficiencies of 10.7% at 380 nm, 6.9% at 420 nm, and 2.3% at 450 nm. X-ray photoelectron and X-ray absorption measurements reveal that the excellent hydrogen evolution activity of the β-CoOOH nanosheet is attributed to the surface low-coordinated Co catalytic site, which results in a strong hybridization of Co 3d states with the S p states of SO32+, and facilitates the electron transfer to reduce H+. The results may be helpful for designing and understanding two-dimensional oxyhydroxide-based ...

Hydrothermal controls on the genesis of REE deposits: Insights from an in situ XAS study of Yb solubility and speciation in high temperature fluids (T < 400 °C)

High pressure-high temperature experiments are used to investigate the effect of temperature, pH and fluid composition on the transport and deposition of REE and provide constraints on the role of hydrothermal fluids in the formation of rare metal deposits. In situ X-ray absorption (XAS) measurements were conducted at 50MPa to determine simultaneously the solubility of Yb compounds (Yb2O3, Yb2Si2O7 and YbPO4) and Yb speciation from 200 to 350–400°C in fluids involving Cl, F and P ligands. Yb concentrations were monitored down to tens of ppm from the height of the absorption edge while XANES and EXAFS analyses were used to refine the aqueous speciation of Yb in the high temperature fluids. The XAS analysis suggests that Yb solubility is retrograde from 200°C and that 10 to 100ppm Yb can be dissolved in acidic fluids (pH<2) at T<400°C, most probably as Yb[(H2O)7,8]3+ species. Increasing pH and adding phosphorus trigger the precipitation of Yb compounds from 200 to 400°C. Hence buffering of acid orthomagmatic fluids and leaching of P from host rocks could be major mechanisms in the development of (H)REE enrichments in magmatic-hydrothermal systems.

Correlation between intercalated magnetic layers and superconductivity in pressurized EuFe2(As0.81P0.19)2

We report comprehensive high-pressure studies on the correlation between intercalated magnetic layers and superconductivity in a EuFe2(As0.81P0.19)2 single crystal through in situ high-pressure resistance, specific-heat, X-ray diffraction and X-ray absorption measurements. We find that an unconfirmed magnetic order of the intercalated layers coexists with superconductivity in a narrow pressure range 0–0.5 GPa, and then it converts to a ferromagnetic (FM) order at pressure above 0.5 GPa, where its superconductivity is absent. The obtained temperature-pressure phase diagram clearly demonstrates that the unconfirmed magnetic order can emerge from the superconducting state. In stark contrast, the superconductivity cannot develop from the FM state that is evolved from the unconfirmed magnetic state. High-pressure X-ray absorption (XAS) measurements reveal that the pressure-induced enhancement of Eu's mean valence plays an important role in suppressing the superconductivity and tuning the transition from the unconfirmed magnetic state to a FM state. The unusual interplay among valence state of Eu ions, magnetism and superconductivity under pressure may shed new light on understanding the role of the intercalated magnetic layers in Fe-based superconductors.

Combined X-ray and Raman Studies on the Effect of Cobalt Additives on the Decomposition of Magnesium Borohydride

Magnesium borohydride (Mg(BH4)2) is one of the most promising hydrogen storage materials. Its kinetics of hydrogen desorption, reversibility, and complex reaction pathways during decomposition and rehydrogenation, however, present a challenge, which has been often addressed by using transition metal compounds as additives. In this work the decomposition of Mg(BH4)2 ball-milled with CoCl2 and CoF2 additives, was studied by means of a combination of several in-situ techniques. Synchrotron X-ray diffraction and Raman spectroscopy were used to follow the phase transitions and decomposition of Mg(BH4)2. By comparison with pure milled Mg(BH4)2, the temperature for the γ → ε phase transition in the samples with CoF2 or CoCl2 additives was reduced by 10–45 °C. In-situ Raman measurements showed the formation of a decomposition phase with vibrations at 2513, 2411 and 766 cm−1 in the sample with CoF2. Simultaneous X-ray absorption measurements at the Co K-edge revealed that the additives chemically transformed to other species. CoF2 slowly reacted upon heating till ~290 °C, whereas CoCl2 transformed drastically at ~180 °C.

Sensitivity to actinide doping of uranium compounds by resonant inelastic X-ray scattering at uranium L3 edge.

Valence-to-core resonant inelastic X-ray scattering (RIXS) and high energy resolution fluorescence detection (HERFD) X-ray absorption measurements were performed at the U L3 edges of UO2 and UO2(NO3)2(H2O)6. The results are compared with model calculations based on the local-density-approximation formalism, taking into account Coulomb interaction U (LDA + U). We show that despite strong 5f-5f electronic correlations in the studied systems and the use of core-level excitations in the intermediate stage of the spectroscopic process, the RIXS technique probes a convolution of the single-particle densities of states in the valence and conduction bands. For UO2, the detected crystal-field splitting between the U 6d eg and t2g orbitals from the RIXS spectra (∼3.5 eV) is larger than that previously derived from optical spectroscopy. Furthermore, by using an example of the U0.75Pu0.25O2 mixed oxide, we show that the RIXS technique at the U L3 edges is sensitive to the substitution of U with other actinide, in contrast to conventional X-ray absorption methods. That is, due to changes in the occupied part rather than in the unoccupied part of the U 6d states caused by the substitution.

Characterization and magnetic properties of BaSn1−x Fe x O3 nanoparticles prepared by a modified sol–gel method

BaSn1−x Fe x O3 (x = 0, 0.05, 0.075 and 0.1) nanoparticles were prepared by a modified sol–gel method. Phase, morphology and oxidation state of samples were characterized using X-ray diffraction (XRD), transmission electron microscopy, scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy and X-ray absorption near-edge spectroscopy. XRD results show the main reflections of the perovskite-type cubic structure for all powders calcined at 1050 °C for 3 h in air. X-ray absorption measurements indicate the substitution of Fe3+ ions on the Sn site of the BaSnO3 host. The field dependence of magnetization M–H curve measured at room temperature exhibits diamagnetic behavior for undoped sample, whereas Fe-doped samples show the clear M–H loops with saturation magnetization and coercive field in the order of 0.34–1.25 emu/g and 260 Oe, respectively. Ferromagnetic (FM) ordering is interpreted in terms of overlapping of polarons mediated through oxygen vacancy defects based on the bound magnetic polaron (BMP) model. Observed FM data are fitted well with the BMP model involving localized carriers and magnetic cations.

Characteristic X-ray absorptiometry applied to the assessment of tissue-engineered cartilage development.

Transmission and tomographic X-ray measurements are useful in assessing bone structures, but only a few studies have examined cartilage growth because of the poor contrast in conventional X-ray imaging. In this study, we attempted to use the linear attenuation coefficient (LAC) as a metric of tissue-engineered cartilage development, which would be useful in high-throughput screening of cartilage products. Assuming that the LAC is related to the amount of extracellular matrix (ECM) in terms of the density and its atomic components, we measured X-ray absorption through tissue-engineered cartilage constructs. Characteristic X-ray beams from a molybdenum microfocus X-ray tube were employed to avoid beam hardening. The correlation of the LAC with mechanical properties was analyzed for verification. The LAC was higher for chondrocyte constructs and lower for fibroblast-dominant constructs and was consistent with the quantification of toluidine blue staining, which is a proof of ECM production. The LAC was positively correlated with the bending modulus but negatively correlated with the dynamic elastic modulus and stiffness, possibly because of the remaining scaffold. The LAC has the potential to be used as a metric of development of tissue-engineered cartilage. However, the calcified regions should be excluded from analysis to avoid decreasing the correlation between the LAC and the amount of ECM.

Electronic structure, local magnetism, and spin-orbit effects of Ir(IV)-, Ir(V)-, and Ir(VI)-based compounds

Element- and orbital-selective x-ray absorption and magnetic circular dichroism measurements are carried out to probe the electronic structure and magnetism of Ir $5d$ electronic states in double perovskite ${\mathrm{Sr}}_{2}{M\text{IrO}}_{6}$ ($M=\mathrm{Mg}$, Ca, Sc, Ti, Ni, Fe, Zn, In) and ${\mathrm{La}}_{2}{\mathrm{NiIrO}}_{6}$ compounds. All the studied systems present a significant influence of spin-orbit interactions in the electronic ground state. In addition, we find that the Ir $5d$ local magnetic moment shows different character depending on the oxidation state despite the net magnetization being similar for all the compounds. Ir carries an orbital contribution comparable to the spin contribution for ${\mathrm{Ir}}^{4+} (5{d}^{5})$ and ${\mathrm{Ir}}^{5+} (5{d}^{4})$ oxides, whereas the orbital contribution is quenched for ${\mathrm{Ir}}^{6+} (5{d}^{3})$ samples. Incorporation of a magnetic $3d$ atom allows getting insight into the magnetic coupling between $5d$ and $3d$ transition metals. Together with previous susceptibility and neutron diffraction measurements, the results indicate that Ir carries a significant local magnetic moment even in samples without a $3d$ metal. The size of the (small) net magnetization of these compounds is a result of predominant antiferromagnetic interactions between local moments coupled with structural details of each perovskite structure.

Observing heme doming in myoglobin with femtosecond X-ray absorption spectroscopy.

We report time-resolved X-ray absorption measurements after photolysis of carbonmonoxy myoglobin performed at the LCLS X-ray free electron laser with nearly 100 fs (FWHM) time resolution. Data at the Fe K-edge reveal that the photoinduced structural changes at the heme occur in two steps, with a faster (∼70 fs) relaxation preceding a slower (∼400 fs) one. We tentatively attribute the first relaxation to a structural rearrangement induced by photolysis involving essentially only the heme chromophore and the second relaxation to a residual Fe motion out of the heme plane that is coupled to the displacement of myoglobin F-helix.

Structural, optical and electronic structure studies of Al doped ZnO thin films

Structural, optical and electronic structure of Al doped ZnO thin films grown using pulsed laser deposition on glass substrate are investigated. X-ray diffraction measurements reveal that all the films are textured along the c-axis and have wurtzite structure. Al doping in ZnO films leads to increase in grain size due to relaxation in compressive stress. Enhancement in band gap of ZnO films with the Al doping is also noticed which can be ascribed to the Brustein–Moss shift. The changes in the electronic structure caused by Al in the doped thin film samples are understood through X-ray absorption measurements.

Synthesis and magnetic properties of double-perovskite oxideLa2MnFeO6thin films

We have investigated epitaxial structures and physical properties of double-perovskite ${\mathrm{La}}_{2}{\mathrm{MnFeO}}_{6}$ films grown by pulsed-laser deposition. The films showed magnetic hysteresis at low temperatures regardless of the degree of Mn/Fe order and the saturation magnetization became smaller for the higher Mn/Fe-order films. The x-ray absorption and x-ray magnetic circular dichroism measurements revealed antiferromagnetic coupling between ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Fe}}^{3+}$ ions, resulting in ferrimagnetism of double-perovskite ${\mathrm{La}}_{2}{\mathrm{MnFeO}}_{6}$. A band structure was established based on the synchrotron radiation photoemission and optical spectra. The optical gap ranged 1.1 eV above the valence state dominated by the Mn $3d {e}_{g}$ orbitals, which is similar to the charge-transfer insulating state in the end member compound of ${\mathrm{LaMnO}}_{3}$.

The nanoscale structure and unoccupied valence electronic states in FeSe1-xTex chalcogenides probed by X-ray absorption measurements.

We have studied the nanoscale structure and unoccupied electronic states in FeSe1-xTex by a combined analysis of Se K, Te L1 and Fe K-edges X-ray absorption measurements. Extended X-ray absorption fine structure (EXAFS) results show that iron-chalcogen (Fe-Se and Fe-Te) distances in ternary FeSe1-xTex are similar to those measured for binary FeSe and FeTe. The local Fe-Se/Te distances determined by different absorption edges fit well in the characteristic Z-plot of random alloys, providing unambiguous support to the inhomogeneous nanoscale structure of the ternary FeSe1-xTex system. X-ray absorption near-edge structure (XANES) spectra reveal a gradual evolution of the unoccupied valence electronic states as a function of Te-substitution in FeSe1-xTex. The Fe 3d-Se 4p/Te 5p hybridization is found to decrease with Te-substitution, accompanied by an increase in unoccupied Se 4p states and a decrease in unoccupied Te 5p states. The results are discussed in the frame of local inhomogeneity in the FeSe1-xTex system driven by random alloying of Se/Te atoms.

Structure and effects of annealing in colloidal matrix-free Ge quantum dots.

The structure of small (2-5 nm) Ge quantum dots prepared by the colloidal synthesis route is examined. Samples were synthesized using either GeO2 or GeCl4 as precursor. As-prepared samples were further annealed under Ar or H2/Ar atmosphere at different temperatures in order to understand the effect of annealing on their structure. It was found that as-prepared samples possess distinctly different structures depending on their synthesis route as indicated by their long-range ordering. An appreciable amount of oxygen was found to be bound to Ge in samples prepared with GeO2 as a precursor; however, not for GeCl4. Based on combined transmission electron microscope, Raman, X-ray diffraction and X-ray absorption measurements, it is suggested that as-prepared samples are best described by the core-shell model with a small nano-crystalline core and an amorphous outer layer terminated either with oxygen or hydrogen depending on the synthesis route. Annealing in an H2Ar atmosphere leads to sample crystallization and further nanoparticle growth, while at the same time reducing the Ge-O bonding. X-ray diffraction measurements for as-prepared and annealed samples indicate that diamond-type and metastable phases are present.

Electrochemical Intercalation of Sodium Ions into Thermally Reduced Graphite Oxide

The structure of carbon obtained from the thermal reduction of graphite oxide at 900°C after electrochemical storage sodium ions was investigated. X-ray diffraction measurement of the carbon sample after charged to various potentials indicated that sodium ions are stored between the layers of carbon below 0.3 V vs. Na/Na+ though the interlayer spacing of it was similar to that of graphite. The lower LUMO energy as revealed from the broad π* peak in the region of CK observed in soft X-ray absorption measurement was responsible for the intercalation of a large amount of sodium ions into the present carbon material.

Surface engineering of a Zr-based bulk metallic glass with low energy Ar- or Ca-ion implantation.

In the present study, low energy ion implantation was employed to engineer the surface of a Zr-based bulk metallic glass (BMG), aiming at improving the biocompatibility and imparting bioactivity to the surface. Ca- or Ar-ions were implanted at 10 or 50 keV at a fluence of 8 × 10(15)ions/cm(2) to (Zr0.55Al0.10Ni0.05Cu0.30)99Y1 (at.%) BMG. The effects of ion implantation on material properties and subsequent cellular responses were investigated. Both Ar- and Ca-ion implantations were suggested to induce atom displacements on the surfaces according to the Monte-Carlo simulation. The change of atomic environment of Zr in the surface regions as implied by the alteration in X-ray absorption measurements at Zr K-edge. X-ray photoelectron spectroscopy revealed that the ion implantation process has modified the surface chemical compositions and indicated the presence of Ca after Ca-ion implantation. The surface nanohardness has been enhanced by implantation of either ion species, with Ca-ion implantation showing more prominent effect. The BMG surfaces were altered to be more hydrophobic after ion implantation, which can be attributed to the reduced amount of hydroxyl groups on the implanted surfaces. Higher numbers of adherent cells were found on Ar- and Ca-ion implanted samples, while more pronounced cell adhesion was observed on Ca-ion implanted substrates. The low energy ion implantation resulted in concurrent modifications in atomic structure, nanohardness, surface chemistry, hydrophobicity, and cell behavior on the surface of the Zr-based BMG, which were proposed to be mutually correlated with each other.

Local structure and structural rigidity of the green phosphor β-SiAlON:Eu2+

Eu2+ inserted in β-Si3−xAlxOxN4−x is a material that shows exceptional promise as a green-emitting phosphor. Synchrotron X-ray and neutron scattering, in conjunction with first-principles calculations and Eu L3 X-ray absorption measurements, yield a consistent picture of the composition, and the favorable position for Eu2+ substitution in the crystal structure. The Debye temperature Θ D, which is a proxy for structural rigidity relating to effectiveness as a phosphor, is very high for the starting β-Si3N4 framework and is determined to decrease only slightly for the small amounts of Al3+ and O2− co-substitution that are required for charge balance associated with Eu2+ insertion.

Local structure of the SnTe topological crystalline insulator: Rhombohedral distortions emerging from the rocksalt phase

${\mathrm{A}}^{\mathrm{IV}}{\mathrm{B}}^{\mathrm{VI}}$ crystals are believed to possess a rhombohedral (ferroelectric) structure at low temperature that changes to the rocksalt (paraelectric) structure above the Curie temperature. For GeTe it has been recently demonstrated that locally the structure retains the subsets of the shorter and longer bonds across the ferroelectric-to-paraelectric transition despite acquiring the cubic structure on average. Nothing is known about the existence of local distortions in SnTe, a prototypical topological crystalline insulator, where the crystal symmetry plays a crucial role. In this work we report the results of x-ray absorption measurements. We find that the structure is locally rhombohedrally distorted, and the distortions increase at $Tg100\phantom{\rule{4.pt}{0ex}}\text{K}$, breaking the rocksalt average symmetry. Our density functional theory simulations performed at 0 K indicate that the role of spin-orbit coupling in the formation of the local structure of SnTe at low temperature is negligibly small. The small stochastic distortions do not affect the intrinsic band inversion of SnTe.

Determination of temperature-dependent atomic displacements in theCa10Ir4As8(Fe2As2)5superconductor with a metallic spacer layer

We have used the combination of Fe $K$-edge and Ir ${L}_{3}$-edge x-ray absorption measurements as a function of temperature to investigate local atomic displacements in the newly discovered ${\mathrm{Ca}}_{10}{\mathrm{Ir}}_{4}{\mathrm{As}}_{8}{({\mathrm{Fe}}_{2}{\mathrm{As}}_{2})}_{5}$ superconducting system. We find relatively relaxed Fe-Fe atomic pair correlations with large displacements in the ${\mathrm{FeAs}}_{4}$ tetrahedra, revealed by Fe $K$-edge extended x-ray absorption fine structure (EXAFS) analysis. Similarly, the temperature dependence of Ir ${L}_{3}$-edge EXAFS shows nanoscale disorder in the IrAs layer that should have a significant effect on the active FeAs-layer characteristics. Furthermore, x-ray absorption near edge structure data are presented to discuss the evolution of the unoccupied electronic states revealing the marginal role of spin-orbit coupling, while the interlayer interactions and disorder should be important for describing the physics of the ${\mathrm{Ca}}_{10}{\mathrm{Ir}}_{4}{\mathrm{As}}_{8}{({\mathrm{Fe}}_{2}{\mathrm{As}}_{2})}_{5}$ system.