LI Edges Research Articles

Electronic structure studies of bismuth compounds using high energy resolution X-ray spectroscopy and ab initio calculations

Bismuth-based compounds are widely used as superconductors, catalysts and materials for optical devices. Their properties, and the possibility of controlling these properties, are determined by their electronic structure and the local environment of the Bi-centres. Although X-ray spectroscopy is a powerful method for revealing crystal and electronic structure, the results obtained so far have been limited by the energy resolution of the experimental data. Here, for the first time, we report X-ray absorption near edge structure (XANES) data, recorded in high energy resolution fluorescence detection (HERFD) mode at the Bi LIII and LI edges for a number of bismuth compounds. Experimental data are analysed using ab initio calculations based on a finite difference method (FDMNES) code, for metallic Bi, Bi2O3, BiPO4, Bi4(GeO4)3 and NaBiO3 compounds. It was observed that both the oxidation state and the length of the Bi-ligand bonds determine the exact position of the absorption edge, and this is in good agreement with other studies. Additionally, due to the high energy resolution, spectral features corresponding to strong Bi p-d orbital mixing were resolved. The results obtained here can be used as input for further investigations of the electronic structure of bismuth compounds in different chemical states.

High energy-resolution measurements of x-ray into ultraviolet parametric down-conversion with an x-ray tube source

We describe high energy-resolution measurements of parametric down-conversion of x-rays into ultraviolet radiation using a standard laboratory x-ray tube source with very minor modifications. We measure the effect in diamond and in lithium fluoride crystals in the ultraviolet range from 30 eV to 65 eV. We show that the effect depends strongly on the fluorine LI edge at 37 eV and on the K edge of the lithium at 55 eV. The comparison with theory reveals that the classical model that was previously used for the description of the effect agrees with the experimental results in diamond. However, the discrepancies of the model with the experimental results in lithium fluoride are prominent.

Measurements of mass attenuation coefficients and determination of photoionization cross sections at energies across the Li (i=1–3) edges of 66Dy

The absolute values of the mass attenuation coefficients have been measured at sixty two photon energies across the Li (i=1–3) sub-shell absorption edges of 66Dy covering the region 7.6–14.0keV in order to investigate the influence of near-edge processes on the attenuation coefficients. The present measured attenuation coefficients are found to be higher by up to 10% than the theoretical values evaluated from the computer code XCOM (Berger et al., 2010) and the self-consistent Dirac-Hartree-Slater (DHS) model based values tabulated by Chantler (1995) over the energy region 7.6–14.0keV, except at energies in vicinity (few eV) of the Li (i=1–3) sub-shell absorption edge energies where the measured values are significantly higher (up to 37%) than both the sets of theoretical values. Further, the Li (i=1–3) sub-shell photoionization cross sections, (σLiP)exp, deduced from the present measured mass attenuation coefficients are compared with the non-relativistic Hartree-Fock-Slater (HFS) model based values tabulated by Scofield (1973) and those evaluated from the theoretical total photoionization attenuation coefficients tabulated by Chantler (1995). The deduced (σLiP)exp(i=1–3) values are found to be in better agreement with those evaluated from the tabulations given by Chantler (1995) than the values given by Scofield (1973) over the energy region 7.8 – 14.0keV included in this study. However, at photon energies up to few eV above the Li edges, the deduced (σLiP)exp(i=1–3) values are found to be significantly higher (up to 32%) than both the sets of theoretical values.

Optical and X-ray absorption spectroscopy in lead doped lithium fluoride crystals

LiF:Pb doped crystals were successfully grown by Kyropoulos method, starting with drying powders. The presence of Pb2+ ions in the LiF crystals were evidenced by the absorption band at 278 nm and by 375 nm photoluminescence. The presence of some other Pb structures with oxygen compounds in the as made samples was evidenced, decreasing after some annealing procedures. The local environment and valence state of Pb in LiF were studied by X-ray Absorption Spectroscopy at the Pb LIII and LI edges. XANES data reveal that Pb is present as Pb2+ whereas EXAFS data show that it is incorporated in the crystal and not forming PbF2 precipitates. Identical spectra are obtained for samples as prepared and after thermal annealing up to 650 °C demonstrating the stability of the incorporation site. Also the concentration of Pb in the crystal has no effect on the location site of the metal as the same spectrum is obtained for specimens with different dopant concentrations.

Measurement of the x-ray mass-attenuation coefficients of gold, derived quantities between 14 keV and 21 keV and determination of the bond lengths of gold

The x-ray mass-attenuation coefficients of gold are measured at 91 energies between 14 keV and 21 keV using synchrotron radiation. The measurements are accurate to between 0.08% and 0.1%. The photoelectric mass-absorption coefficients and the imaginary component of the form factors of gold are also determined. The results include the LI edge and are the most accurate and extensive gold dataset available in this energy range. An analysis of the LI edge XAFS showed excellent agreement between the measured and simulated XAFS and yielded highly accurate values of the bond lengths of gold. When our results are compared with earlier measurements and with predictions of major theoretical tabulations, significant discrepancies are noted. The comparison raises questions about the nature of discrepancies between experimental and theoretical values of mass-attenuation coefficients.

XANES studies of thallium compounds and valence states and local environment of thallium in some of its superconducting cuprates

The position and shape of x-ray absorption discontinuities have been used to deduce structural and chemical bonding information on model compounds of thallium and its superconducting cuprates. The L x-ray absorption spectra were recorded on a Rigaku x-ray Spectrometer and monochromatized using a flat crystal of Si (1,1,1). The background subtraction was done using Victoreen function. The normalized absorption coefficient is plotted as a function of x-ray photon energy. It is observed that the XANES of LI edge has fewer features as compared to those of LIII edge. The LII edge spectra of thallium are similar to LIII edge spectra. The LIII absorption discontinuity of thallium splits into three components whereas no splitting is observed in LI edges. Thallium LIII edge spectra show a single absorption peak for monovalent compounds whereas in trivalent compounds a weak peak followed by a shoulder on rising absorption which culminates into a strong absorption peak. The important difference between the spectra of monovalent and trivalent compounds is the shift in the inflexion point energy of LIII edges and a shift of the main absorption maximum. The x-ray absorption spectra of superconducting cuprates show similarities with the spectra of Tl2O3 suggesting the presence of Tl3+ ions in octahedral configuration in these cuprates.

Structure of Redispersible SnO2 Nanoparticles

Suspensions of undoped SnO2 nanoparticles and containing Eu3+ ions were prepared by a sol-gel procedure. Using the classical synthesis method (precipitation), the particles tend to grow by a coarsening process in order to minimize the surface free energy. This effect can strongly be reduced by the addition of an amide and surfactant during the synthesis, which decreases the surface free energy of the colloidal particles. These additives promote the formation of powders composed of very small primary particles formed by a crystallite of 10 A, and exhibit good redispersion properties. The local and long order structures of the redispersible powder were studied by X-rays absorption spectroscopy at Sn LI edge and X-rays diffraction, respectively. The structure of the colloidal aggregates in suspension was investigated by small angle X-rays scattering (SAXS). SAXS results indicate the sol are composed by a polidisperse system of hard spheres resulting of agglomeration of the primary particles and their size increasing by agglomeration for progressively higher Eu3+ content.

Structure of lobster apocrustacyanin A1 using softer X-rays.

The molecular basis of the camouflage colouration of marine crustacea is often provided by carotenoproteins. The blue colour of the lobster carapace, for example, is intricately associated with a multimacromolecular 16-mer complex of protein subunits each with a bound astaxanthin molecule. The protein subunits of crustacyanin fall into two distinct subfamilies, CRTC and CRTA. Here, the crystal structure solution of the A(1) protein of the CRTC subfamily is reported. The problematic nature of the structure solution of the CRTC proteins (both C(1) and A(1)) warranted consideration and the development of new approaches. Three putative disulfides per protein subunit were likely to exist based on molecular-homology modelling against known lipocalin protein structures. With two such subunits per crystallographic asymmetric unit, this direct approach was still difficult as it involved detecting a weak signal from these sulfurs and suggested the use of softer X-rays, combined with high data multiplicity, as reported previously [Chayen et al. (2000), Acta Cryst. D56, 1064-1066]. This paper now describes the structure solution of CRTC in the form of the A(1) dimer based on use of softer X-rays (2 A wavelength). The structure solution involved a xenon derivative with an optimized xenon L(I) edge f" signal and a native data set. The hand of the xenon SIROAS phases was determined by using the sulfur anomalous signal from a high-multiplicity native data set also recorded at 2 A wavelength. For refinement, a high-resolution data set was measured at short wavelength. All four data sets were collected at 100 K. The refined structure to 1.4 A resolution based on 60 276 reflections has an R factor of 17.7% and an R(free) of 22.9% (3137 reflections). The structure is that of a typical lipocalin, being closely related to insecticyanin, to bilin-binding protein and to retinol-binding protein. This A(1) monomer or dimer can now be used as a search motif in the structural studies of the oligomeric forms alpha- and beta-crustacyanins, which contain bound astaxanthin molecules.

SnO reduction in lithium cells: study by X-ray absorption, 119Sn Mössbauer spectroscopy and X-ray diffraction

We studied the reduction mechanism of SnO in lithium cells by X-ray absorption near OK and SnLI edge spectroscopy, 119Sn Mössbauer spectroscopy and X-ray diffraction. The reduction mechanism is complex, involving mixed valence intermediate compounds. In the interval 0≤Li/Sn≤2 the main reaction corresponds to a partial reduction of SnII, which is partially reversible, regenerating the tin oxide during the charge. For Li/Sn greater than two, LiSn bonds are formed, but SnO interactions are still present. The charge within this interval also partially regenerates tin oxide, but the reversibility does not extend to Li/Sn lower than two. At low voltage and very large depth of discharge, the formation of LiSn alloys and Li2O clearly takes place, with negligible SnO interactions.

X-ray natural circular dichroism in a uniaxial gyrotropic single crystal of LiIO3

We produce the first experimental evidence of x-ray natural circular dichroism (XNCD) in a uniaxial gyrotropic crystal of α-LiIO3 that is known to crystallize with space group P63 and to exhibit a very strong nonlinear susceptibility. For the sake of simplicity, the optical axis was set colinear to the direction of the exciting x-ray beam in order to get rid of undesirable birefringence and linear dichroism effects. The nicely structured XNCD spectra recorded at the LI, LII, and LIII edges of iodine are assigned to the electric dipole–electric quadrupole (E1.E2) interference terms which do not vanish in gyrotropic crystals but contribute to a second order polarizability. Our interpretation is consistent with a band structure calculation and is fully supported by ab initio multiple scattering simulations. The signatures recorded at the LII and LIII edges are similar and have the same sign. This is in contrast to x-ray magnetic circular dichroism (XMCD) spectra which usually exhibit opposite signs at the LII and LIII edges: while the exchange and spin-orbit interactions are the driving terms in XMCD, this is not the case in XNCD. The XNCD signal detected at the LI edge is found to be relatively more intense and has the opposite sign. These results leave very little space for a contribution of the electric dipole–magnetic dipole (E1.M1) interference terms which are usually dominant at optical wavelengths but should be barely detectable in the x-ray range.

Lithium Detection in Aluminium Lithium Alloys Using PEELS

Aluminium lithium alloys are important alloys for the aerospace industry on account of their good strength to weight properties. However, these alloys suffer from poor short-transverse fracture toughness. Segregation of Li onto the grain boundaries is one mechanism proposed to account for this. Different AlLi alloys have been studied in a 200keV TEM equipped with a Parallel Electron Energy Loss Spectrometer.Typical Li concentrations in these alloys range from 10 to 14 atomic percent (2 to 3 weight percent). This, combined with the proximity of both the Al plasmon and Al L edge and the strong deviation of the Li K edge from the ideal sharp Fermi step makes the detection of Li difficult. Negative “jump ratios”, as shown in fig. 1, make the edge very difficult to see in an unprocessed spectrum and a cursory inspection of such a spectrum might miss the Li the edge. The presence of the Li edge, and also the edge threshold energy, is more visible if the second difference of the spectrum (obtained by convolution with a top-hat filter) is displayed as shown in fig. 2.

VALENCE STATES AND LOCAL ENVIRONMENT OF THALLIUM AND COPPER IONS IN SOME NEW HIGH Tc SUPERCONDUCTORS BY XAS

The valence states of cations inside four high T c thallium superconductors have been investigated by X-ray absorption spectroscopy at thallium LIII , LI and copper K-edges. The thallium ions are observed on LIII and LI edges in the Tl(III) valence state (empty 6s levels) in spite of large Tl-O distances in the (a, b) plane of the structure as deduced from X-ray diffraction. The presence of two types of Tl-O distances have been well established on XANES spectra and the F.T. of EXAFS have shown the presence of two very short Tl-O bonds (2.03 Å). The copper K-edges are characterized by the absence of Cu(I) state (3d10 line).

A time-resolved x-ray absorption study of amorphous Si during pulsed laser irradiation

We report the first time-resolved x-ray absorption measurements on pulsed laser irradiated Si in the photon energy range from 90 to 300 eV, at irradiation energy densities up to 4 J/cm2, and at several delay times between irradiation pulse and x-ray probe pulse. The absorption spectra recorded at a delay time of 12 ns can be classified in three categories. Below ≊0.17 J/cm2 only thermal damping of extended x-ray absorption fine-structure (EXAFS) oscillations is observed. Between ≊0.17 J/cm2 and ≊1 J/cm2 annealing takes place. Several changes are observed, among which a ≊7-eV LII,III edge shifts towards higher energy, while the LI edge remains approximately at the same position. This edge shift behavior is explained by a largely reduced 3s density of states of the conduction band. This supports the idea that pulsed laser molten Si has a metalliclike structure. Thermal damping in this region is so strong that EXAFS oscillations are no longer visible. This is consistent with a lattice temperature above or equal to the melting temperature of Si. Above 1 J/cm2, the damage region, further edge shifts and the development of several peaks are observed. Clear cooling effects are visible at long delay times (60 ns) between irradiation and x-ray probe pulse. Cooling on this time scale can only be explained by evaporation of Si.

A XANES investigation of the bonding of divalent lead in solids

XANES (X-ray absorption near-edge structure) spectra of divalent lead compounds have been studied at both the Pb LIII and LI edges using synchrotron x rays from CHESS (Cornell High Energy Synchrotron Source). The combined LIII and LI near-edge spectral features strongly support a sp3d2 hybridization scheme in covalently bonded yellow PbO. The absorption features associated with the spectra of ionic halides (PbF2, PbCl2, and PbI2) and the oxysalts [Pb(NO3)2, PbSO4, and PbCrO4] have been shown to be consistent with crystal field splittings of the final states using group theoretical arguments. The XANES spectra of a series of PbO–PbCl2 glasses have also been studied to elucidate the nature of bonding and confirm the local structure derived from x-ray diffraction experiments. Shape resonance features associated with LIII and LI edge XANES have also been discussed. This investigation illustrates the novel use of XANES spectroscopy in studying the bonding of nontransition metal ions.