XANES and EXAFS in Cu-Ti and Ni-Zr glasses

X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements have been done at the K-edge of Cu in Cu-Ti glasses and on the K-edges of Ni and Zr in Ni-Zr glasses using a synchrotron radiation source. The results are discussed in terms of the shape shift and intensity of the absorption edge as well as the principal absorption maximum. The values of bondlength calculated by the one-electron multiple scattering XANES theory as well as the graphical analysis EXAFS technique show good agreement.     

EXAFS measurements on FeB metallic glasses: Asymmetry of the radial distribution function

The EXAFS structure measured above the K-edge of Fe in the disordered Fe₈₀B₂₀ alloy is reported. Complete agreement is obtained with X-ray diffraction results in the position of the first neighbours coordination shell when the asymmetry of the Radial Distribution Function (R.D.F.) into the EXAFS formula is included. A Finney-like R.D.F. is used to describe the pair distribution of these metallic glasses. The weak temperature dependence of the spectra is discussed in terms of the structural properties of these amorphous alloys.     

Correlation between I-Ag distance and ionic conductivity in AgI fast-ion-conducting glasses

A large number of AgI-based fast-ion-conducting glasses have been investigated by K-iodine extended x-ray absorption fine structure spectroscopy (EXAFS) measurements at liquid nitrogen temperature. A general correlation between the I-Ag distance measured by EXAFS and the glass activation energy for dc ionic conductivity has been found out: glasses with longer I-Ag distances display higher ionic conductivity, independently from the chemical composition of their host glassy matrix. This behavior can be related to the progressive increase of the "pathway volume" for ionic conduction.     

Extended X-ray-absorption fine structure of mixed-coordination systems

An analysis method of the extended x-ray absorption fine structure (EXAFS) for mixed-coordination systems (MCS) is provided to determine structure parameters of the same type of atoms located at different coordination sites and proportion of them. It's demonstrated that if the effects of different sites on EXAFS amplitudes are ignored, the coordination numbers gotten from EXAFS analyzing must be wrong mostly. The method has been well tested by analyzing a known structure Gd₃Ga₅O₁₂ (GGG) crystal in which the Ga atoms have two different coordination sites. Li₂O-ZnO-GeO₂ glass system is also analyzed by means of the EXAFS method for MCS. Two different coordination sites for Ge atoms in the glasses. are found and the occupancies of them and the structure parameters are determined.     

SiO2-PbF2-CdF2 glasses and glass ceramics

Lead-Cadmium fluorosilicate stable glasses were prepared and the vitreous domain region determined in the composition diagram. Characteristic temperatures were obtained from thermal analysis and the structural studies performed illustrate clearly the role played by lead atoms in the glasses crystallization behavior and the glass-forming ability of cadmium atoms. The occurrence of either a cubic lead fluoride or a lead-cadmium fluoride solid solution in crystallizing samples was found to be dependent on Er³⁺ doping. The optically active ions were found to concentrate in the crystalline phase and in fact play the role of nucleating agent as suggested from X-ray diffraction and EXAFS measurements.     

XANES and EXAFS study of some metal-metalloid glasses

XANES and EXAFS techniques are proving very popular in the study of local environment in disordered systems. Results of such studies in a large number of metal (Fe, Co, Ni, etc)-metalloid (B, Si, C, etc) glasses are reported. Experiments were done with synchrotron radiation as well as an x-ray tube. The values of bond lengths and co-ordination numbers computed from one-electron single scattering Fourier transform method turn out substantially smaller. The values of bondlength determined from the other EXAFS calculation method and the multiple-scattering computation scheme show good agreement. Importance of choice of suitable reference materials for analysis of data is emphasized. The experimental work was done at EXAFS 5.1 Station at Daresbury Laboratory, U.K.     

Icosahedral order in Cu-Zr amorphous alloys studied by means of X-ray absorption fine structure and molecular dynamics simulations

We have used the X-ray absorption fine structure method and molecular dynamics (MD) simulations to characterize atomic order in Cu-Zr metallic glasses (MGs). The microstructure of these MGs is described in terms of interconnected icosahedral-like clusters (superclusters) which are basic building units reproducing the stoichiometry of the system. The equilibrium MD configurations are used as an input for ab initio calculations of the extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectra. The theoretical EXAFS and XANES spectra are compared with those measured for rapidly quenched glassy Cu-Zr alloys. We demonstrate that the experimental results are well reproduced by EXAFS modeling of the population of the superclusters derived from the MD configuration. The average local structural motif can be approximated by Cu-centered icosahedral-like cluster satisfying the condition of maximal local packing efficiency and approximating the system stoichiometry. The simulated XANES exhibits good agreement with experiment, indicating that the atomic order of the MD configuration is consistent with that of the real alloy structure over distances of about 1?nm.     

Local atomic structures in Zr-Ni metallic glasses

Local atomic structures of Zr100−xNix (x=33.3, 36, 50 at%) binary metallic glasses were investigated by means of extended X-ray absorption fine structure (EXAFS) probe. Structural parameters show that the Zr-Ni bond length, RZr-Ni, keeps a constant value of 2.62 Å, regardless of alloy compositions. This result implies that there is a strongly chemical interaction between Zr and Ni atoms, leading to significant chemical short-range orders (CSROs) in the present metallic glasses. Further analysis indicates that the SRO structures in these metallic glasses are extremely similar with those in their crystalline counterparts. It is interesting to note that the CSROs in the eutectic Zr₆₄Ni₃₆ metallic glass consist of Zr₂Ni-like and ZrNi-like CSROs.     

Investigation of gamma radiation induced changes in local structure of borosilicate glass by TDPAC and EXAFS

Gamma radiation induced changes in local structure around the probe atom (Hafnium) were investigated in sodium barium borosilicate (NBS) glass, used for immobilization of high level liquid waste generated from the reprocessing plant at Trombay, Mumbai. The (NBS) glass was doped with ¹⁸¹Hf as a probe for time differential perturbed angular correlation (TDPAC) spectroscopy studies, while for studies using extended X-ray absorption fine structure (EXAFS) spectroscopy, the same was doped with 0.5 and 2 % (mole %) hafnium oxide. The irradiated as well as un-irradiated glass samples were studied by TDPAC and EXAFS techniques to obtain information about the changes (if any) around the probe atom due to gamma irradiation. TDPAC spectra of unirradiated and irradiated glasses were similar and reminescent of amorphous materials, indicating negligible effect of gamma radiation on the microstructure around Hafnium probe atom, though the quaqdrupole interaction frequency (ω _Q) and asymmetry parameter (η) did show a marginal decrease in the irradiated glass compared to that in the unirradiated glass. EXAFS measurements showed a slight decrease in the Hf-O bond distance upon gamma irradiation of Hf doped NBS glass indicating densification of the glass matrix, while the cordination number around hafnium remains unchanged.     

Characterization of yellow and colorless decorative glasses from the Temple of the Emerald Buddha, Bangkok, Thailand

Yellow and colorless ancient glasses, which were once used to decorate the Temple of the Emerald Buddha, Bangkok, Thailand, around 150 years ago, are studied to unravel the long-lost glass-making recipes and manufacturing techniques. Analyses of chemical compositions, using synchrotron x-ray fluorescence (SRXRF), indicate that the Thai ancient glasses are soda lime silica glasses (60 % SiO₂; 10 % Na₂O; 10 % CaO) bearing lead oxide between 2–16 %. Iron (1.5–9.4 % Fe₂O₃) and manganese (1.7 % MnO) are present in larger abundance than the other 3d transition metals detected (0.04–0.2 %). K-edge x-ray absorption near edge spectroscopy (XANES) and extended x-ray absorption fine structure spectroscopy (EXAFS) provide conclusive evidence on the oxidation states of Fe being 3+ and Mn being 2+ and on short-length tetrahedral structures around the cations. This suggests that iron is used as a yellow colorant with manganese as a decolorant. L ₃-edge XANES results reveal the oxidation states of lead as 2+. The results from this work provide information crucial for replicating these decorative glasses for the future restoration of the Temple of the Emerald Buddha.     

Synthesis and structural investigations on TeO2-PbF2-CdF2 glasses and transparent glass-ceramics

New glasses have been prepared in the oxifluoride mixed system TeO₂-PbF₂-CdF₂. Starting from pure TeO₂ the addition of the fluorides leads to a decrease in the glasses characteristic temperatures. Also from Raman scattering results a structural evolution was observed where the number of structural units described as [TeO₃] trigonal pyramids and [TeO₃₊₁] polyhedra increases at the expense of the [TeO₄] trigonal bipyramids supposed to exist in the TeO₂ rich samples. Transparent glass ceramics were obtained from the glass with composition 80TeO₂-10PbF₂-10CdF₂ (mol%) with the PbTe₃O₇ crystalline phase being identified by X-ray diffraction and EXAFS measurements performed at the Te K, Cd K and Pb L_III edges. Also from Exafs measurements it is proposed that cadmium ions are preferentially surrounded by oxygen atoms although they were in a fluoride anion environment in the starting material.     

Structure and electrical properties of SrO-borovanadate (V2O5)0.5(SrO)0.5−y(B2O3)y glassses

SrO-borovanadate glasses with nominal composition (V₂O₅)_0.5(SrO)_0.5−y(B₂O₃)_y, 0.0≤y≤0.4 were prepared by a normal quench technique and investigated by direct current (DC) electrical conductivity, inductively coupled plasma (ICP) spectroscopy, infrared (IR) spectroscopy and X-ray powder diffraction (XRD) studies in an attempt to understand the nature of mechanism governing the DC electrical conductivity and the effect of addition of B₂O₃ on the structure and electrical properties of these glasses. XRD patterns confirm the amorphous nature of the present glasses and actual compositions of the glasses were determined by ICP spectroscopy. The temperature dependence of DC electrical conductivity of these glasses has been studied in terms of different hopping models. The IR results agree with previous investigations on similar glasses and it has been concluded that similar to SrO-vanadate glasses, metavandate chain-like structures of SrV₂O₆ and individual VO₄ units also occur in SrO-borovanadate glasses. The SrV₂O₆ and VO_n polyhedra predominate in the low B₂O₃-containing SrO-borovanadate glasses as B substitutes into the V sites of the various VO_n polyhedra and only when the concentration of B₂O₃ exceeds the SrO content do BO_n structures appear. This qualitative picture of three distinct structural groupings for Sr-vanadate and Sr-borovanadate glasses is consistent with the proposed glass structure on previous IR and extended X-ray absorption fine structure (EXAFS) studies on these types of glasses. The conductivity results were analyzed with reference to theoretical models existing in the literature and the analysis shows that the conductivity data are consistent with Mott's nearest neighbor hopping model. Analysis of the conductivity data shows that they are consistent with Mott's nearest neighbor hopping model. However, both Mott VRH and Greaves models are suitable to explain the data. Schnakenberg's generalized polaron hopping model is also consistent with temperature dependence of activation energy. However, various model parameters such as density of states, hopping energy, etc. obtained from the best fits were not found to be in accordance with the prediction of the Mott model.     

Structure and charge transfer dynamics of uranyl ions in boron oxide and borosilicate glasses

Laser spectroscopic experiments, molecular dynamics simulation, and charge transfer-lattice interaction modeling have been conducted for studying the electronic and structural properties of the uranyl ion UO ₂ ²⁺ in boron oxide and borosilicate glasses. The charge transfer electronic and vibrational energy levels for uranyl ions in the glass matrices were obtained from laser excitation and fluorescence spectra of UO ₂ ²⁺ . A model structure for uranyl ions in the glass matrices was established using the method of molecular dynamics (MD) simulation in comparison with the results of extended x-ray absorption fine structure (EXAFS) for U⁶⁺ ions in the glasses we studied. The formation and stabilization of uranyl clusters in glass matrices are interpreted in terms of charge transfer-lattice interactions on the basis of self-consistent charge transfer accompanied by lattice distortion. The latter is in the framework of the simultaneous action of pseudo-Jahn-Teller and pseudo-Jahn-Teller analog effects on charge transfers between oxygen and uranium ions.     

Electron magnetic resonance and magnetooptical studies of nanoparticle-containing borate glasses

We report electron magnetic resonance (EMR) and magnetooptical studies of borate glasses of molar composition 22.5K₂O-22.5Al₂O₃-55B₂O₃ co-doped with low concentrations of Fe₂O₃ and MnO. In as-prepared samples the paramagnetic ions, as a rule, are in diluted state. However, in the case where the ratio of the iron and manganese oxides in the charge is 3/2, magnetic nanoparticles with characteristics close to those of manganese ferrite are formed already at the first stage of the glass preparation, as evidenced by both magnetic circular dichroism (MCD) and EMR. After thermal treatment all glasses show characteristic MCD and EMR spectra, attesting to the presence of magnetic nanoparticles, predominantly including iron ions. Preliminary EXAFS measurements at the Fe K-absorption edge show an emergence of nanoparticles with a structure close to MnFe₂O₄ after annealing the glasses at 560 °C.By computer simulating the EMR spectra at variable temperatures, a superparamagnetic nature of relatively broad size and shape distribution with the average diameter of ca. 3-4 nm. The characteristic temperature-dependent shift of the apparent resonance field is explained by a strong temperature dependence of the magnetic anisotropy in the nanoparticles.The formation of magnetic nanoparticles confers to the potassium-alumina-borate glasses magnetic and magneto-optical properties typical of magnetically ordered substances. At the same time, they remain transparent in a part of the visible and near infrared spectral range and display a high Faraday rotation value.     

The effect of self‐consistent potentials on EXAFS analysis

A theory program intended for use with extended X‐ray‐absorption fine structure (EXAFS) spectroscopy and based on the popular FEFF8 is presented. It provides an application programming interface designed to make it easy to integrate high‐quality theory into EXAFS analysis software. This new code is then used to examine the impact of self‐consistent scattering potentials on EXAFS data analysis by methodical testing of theoretical fitting standards against a curated suite of measured EXAFS data. For each data set, the results of a fit are compared using a well characterized structural model and theoretical fitting standards computed both with and without self‐consistent potentials. It is demonstrated that the use of self‐consistent potentials has scant impact on the results of the EXAFS analysis.     

Copper blue in an ancient glass bead: a XANES study

The blue colour in ancient soda-lime glasses has been attributed to the presence of copper and/or cobalt but the origin of different shades is not yet fully interpreted. As a contribution to this question, a non-destructive X-ray absorption study at [ _Cu]K-edge was undertaken on the blue (turquoise) layer from a “Nueva Cadiz” type tubular glass bead dated pre-XVII century where copper is the unique colouring agent. Minerals configuring two distinct blue tonalities due to Cu (2+) in similar square coordination were selected as basic model compounds: azurite, which is a classical navy-blue pigment used in ancient wall paintings over plaster, and chalcanthite, displaying exactly the same turquoise-blue tonality of tubular glass beads manufactured since the Egyptian Antiquity. Theoretical modelling of the XAFS spectra was undertaken using the FEFF code. The IFEFFIT software package was used for fitting the calculated spectra to experimental data. EXAFS results are discussed in view of the crystal structures of copper minerals chosen to model the speciation state and structural situation of that element prevailing in the turquoise-blue archaeological glass. Special attention is focused on the difficulties in theoretical modelling [ _Cu]K-XANES spectra of ancient glasses with different colourings. PACS 61.43.Fs; 61.46.+w; 41.60.Ap; 61.10.Ht.     

The effect of statistical noise on structural parameters in EXAFS data analysis

Summary We analyse the effect of the statistical noise on the physical parameters derived from an EXAFS spectrum. Two separate classes of experimental cases are discussed, namely heavy and light backscatterers. The weight of the noise in the two cases is, in fact, quite different, depending on thek range useful for the analysis of the data. We find that for heavy backscatterers the statistical noise can usually be neglected, while for light backscatterers it can be one of the main limitations in obtaining reliable results from EXAFS. Paper presented at the Workshop on ?EXAFS Data Analysis in Disordered Systems?, held in Parma, October 5–7, 1981.     

Local order in light emitting porous silicon studied by XEOL and TEY

The local structure of porous silicon has been studied exciting its optical luminescence by X-rays (XEOL). The photoluminescence yield and the total electron yield (TEY), recorded simultaneously as a function of the X-ray energy at the Si K edge, give rise to the extended X-ray absorption fine structures (EXAFS). Analysis of EXAFS data confirms that the optical luminescence of porous Si originates from the nanocrystalline cores and shows that XEOL–EXAFS and TEY–EXAFS are sensitive to different Si local environment. It can be assumed that XEOL–EXAFS is related only to the light emitting sites while TEY–EXAFS is sampling both luminescent and non-luminescent Si sites.     

A comparative study of the spectra recorded at RRCAT synchrotron BL-8 dispersive EXAFS beamline with other beamlines

The aim of the present work is to make a comparative study of the EXAFS spectra recorded at the BL-8 dispersive EXAFS beamline at 2 GeV Indus-2 synchrotron source at RRCAT, Indore (India) with those recorded at other synchrotron EXAFS beamlines, viz., X-19A at NSLS, BNL (USA), EXAFS wiggler beamline 4-1 at the SSRL (USA) and beamline 11.1 at ELETTRA (Italy). For this purpose, EXAFS spectra at Cu K-edge in copper metal have been recorded at these four beamlines. Further, EXAFS spectra at Cu K-edge in a copper complex have also been recorded at BL-8 beamline and beamline 11.1 at ELETTRA (Italy). The obtained experimental μ(E) data have been background-subtracted and then normalized. The normalized data have been then converted to χ(k) data, which have been Fourier-transformed and then fitted with the theoretical model, thereby yielding different structural parameters. It has been shown that the results obtained from the EXAFS spectra recorded at the BL-8 beamline are comparable with those obtained from other synchrotron EXAFS beamlines and also with the crystallographic results reported by earlier workers. The reliability, usefulness and data quality of the BL-8 beamline have been discussed.