Extended X-ray Absorption Fine Structure Data Research Articles

Vacancy-Rich SnO2 Quantum Dot Stabilized by Polyoxomolybdate as Electrocatalyst for Selective NH3 Production.

The pronounced conductivity of tin dioxide (SnO2) nanoparticles makes it an ideal multifunctional electrode material, while the challenge is to stabilize the quantum dot (QD) SnO2 nanocore in water. An Anderson-type polyoxomolybdate, (NH4)6[Mo7O24], is employed as an inorganic ligand to stabilize a ca. 6 nm SnO2 QD (Mox@SnO2). X-ray scattering and diffraction studies confirm the tetragonal SnO2 nanocore in Mox@SnO2. Elemental analyses are in good agreement with the mass spectrometric detection of the [Mo7O24]6- cluster present in Mox@SnO2. The ionic POMs attached to the SnO2 surface through [Mo-O-Sn] covalent linkages have been established by surface zeta potential, shift of the [Mo = O]t Raman vibration, and extended X-ray absorption fine structure (EXAFS) analyses. The presence of the [Mo7O24]6- cluster in the Mox@SnO2 is responsible for the remarkable aqueous stability of Mox@SnO2 in the pH range of 3-9. Dominant oxygen vacancy in the SnO2 core, identified by EXAFS data and the anisotropic electron paramagnetic resonance (EPR) signals (g ∼ 2.4 and 1.9), results in facile electronic conduction in Mox@SnO2 while being deposited on the electrode surface. Mox@SnO2 acts as an active catalyst for the electrocatalytic nitrate reduction (eNOR) to ammonia with 94% faradaic efficiency (FE) at -0.2 V vs RHE and a yield rate of 28.9 mg h-1 cm-2. The stability of Mox@SnO2 in acidic pH provides scope to reuse the Mox@SnO2 electrode at least four times with notable NH3 selectivity and a superior production rate (239.06 mmol g-1(cat) h-1). This study demonstrates the essential role of POM in stabilizing SnO2 QD, harnessing its electrochemical activity toward electrocatalytic ammonia production.

Unraveling the interlayer and intralayer coupling in two-dimensional layered MoS[formula omitted] by X-ray absorption spectroscopy and ab initio molecular dynamics simulations

Understanding interlayer and intralayer coupling in two-dimensional layered materials (2DLMs) has fundamental and technological importance for their large-scale production, engineering heterostructures, and development of flexible and transparent electronics. At the same time, the quantification of weak interlayer interactions in 2DMLs is a challenging task, especially, from the experimental point of view. Herein, we demonstrate that the use of X-ray absorption spectroscopy in combination with reverse Monte Carlo (RMC) and ab initio molecular dynamics (AIMD) simulations can provide useful information on both interlayer and intralayer coupling in 2DLM 2Hc-MoS2. The analysis of the low-temperature (10–300 K) Mo K-edge extended X-ray absorption fine structure (EXAFS) using RMC simulations allows for obtaining information on the means-squared relative displacements σ2 for nearest and distant Mo–S and Mo–Mo atom pairs. This information allowed us further to determine the strength of the interlayer and intralayer interactions in terms of the characteristic Einstein frequencies ωE and the effective force constants κ for the nearest ten coordination shells around molybdenum. The studied temperature range was extended up to 1200 K employing AIMD simulations which were validated at 300 K using the EXAFS data. Both RMC and AIMD results provide evidence of the reduction of correlation in thermal motion between distant atoms and suggest strong anisotropy of atom thermal vibrations within the plane of the layers and in the orthogonal direction.

Bridging the Gap between the X-ray Absorption Spectroscopy and the Computational Catalysis Communities in Heterogeneous Catalysis: A Perspective on the Current and Future Research Directions

X-ray absorption spectroscopy (XAS) [extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES)] is a key technique within the heterogeneous catalysis community to probe the structure and properties of the active site(s) for a diverse range of catalytic materials. However, the interpretation of the raw experimental data to derive an atomistic picture of the catalyst requires modeling and analysis; the EXAFS data are compared to a model, and a goodness of fit parameter is used to judge the best fit. This EXAFS modeling can often be nontrivial and time-consuming; overcoming or improving these limitations remains a central challenge for the community. Considering these limitations, this Perspective highlights how recent developments in analysis software, increased availability of reliable computational models, and application of data science tools can be used to improve the speed, accuracy, and reliability of EXAFS interpretation. In particular, we emphasize the advantages of combining theory and EXAFS as a unified technique that should be treated as a standard (when applicable) to identify catalytic sites and not two separate complementary methods. Building on the recent trends in the computational catalysis community, we also present a community-driven approach to adopt FAIR Guiding Principles for the collection, analysis, dissemination, and storage of XAS data. Written with both the experimental and theory audience in mind, we provide a unified roadmap to foster collaborations between the two communities.

Iron speciation in soil size fractions under different land uses

Iron (Fe) (oxyhydr)oxides represent a significant phase for the organic carbon (OC) stabilization. Due to their high surface areas, short-range-ordered Fe minerals, like ferrihydrite, show a higher ability to stabilize OC than crystalline secondary minerals, like lepidocrocite, goethite, and magnetite. However, how Fe phases and their crystallinity relate to soil organic matter (SOM) composition is still not completely known. We investigated Fe solid phase speciation in two soil particle size fractions (i.e., fine sand — FSa — and fine silt and clay — FSi + Cl —), isolated from coniferous (CF) and broadleaved forest soils (BF), grassland soils (GL), and technosols (TS). All samples were characterized by Fe K-edge extended X-ray absorption fine structure (EXAFS) and X-ray diffraction, and a subset by 57Fe Mössbauer spectroscopy. Further, ramped combustion thermal analyses (coupled differential scanning calorimetry (DSC) and CO2 evolved gas analysis (CO2-EGA)) were used to evaluated SOM stability. With the only exception of TS, goethite was the main Fe phase in FSa fractions, whereas less crystalline phases (i.e., ferrihydrite, based on EXAFS) dominated the FSi + Cl fractions. The proportion of goethite and ferrihydrite in both fractions decreased with increasing OC content, while that of Fe(III)-SOM in FSa increased with increasing OC content. Mössbauer and EXAFS data clearly indicated a presence of hematite in GL soils. Our data suggest that more crystalline Fe forms, like goethite and hematite, may be important for OC abundance in the FSa fraction and in soils with high OC contents, like GL. Thermal analysis showed the dominance of mineral associated organic matter in low-OC soils, and of plant residues in high-OC soils. As a whole, we posit that the FSi + Cl fractions contain Fe phases of less crystallinity because of presumed association with SOM, and that SOM in the FSi + Cl fraction is also more thermodynamically stable than in FSa, although differences are observed across land uses. Our observations suggest that the nature of Fe-SOM interactions can vary substantially with soil particle size and land use, which has important implication for SOM persistence.

Combining the Best of Two Chelating Titans: A Hydroxypyridinone-Decorated Macrocyclic Ligand for Efficient and Concomitant Complexation and Sensitized Luminescence of f-Elements.

An ideal chelator for f-elements features rapid kinetics of complexation, high thermodynamic stability, and slow kinetics of dissociation. Here we present the facile synthesis of a macrocyclic ligand bearing four 1-hydroxy-2-pyridinone units linked to a cyclen scaffold that rapidly forms thermodynamically stable complexes with lanthanides (Sm3+ , Eu3+ , Tb3+ , Dy3+ ) and a representative late actinide (Cm3+ ) in aqueous media and concurrently sensitizes them. Extended X-ray absorption fine structure (EXAFS) spectroscopy revealed an increase in the Ln/An-O bond lengths following the trend Cm>Eu>Tb and EXAFS data were compatible with time-resolved luminescence studies, which indicated one to two water molecules in the inner metal coordination sphere of Eu(III) and two water molecules for the Cm(III) complex. Spectrofluorimetric ligand competition titrations against DTPA confirmed the high thermodynamic stability of DOTHOPO complexes, with pM values between 19.9(1) and 21.9(2).

Fluorescence-detected quick-scanning X-ray absorption spectroscopy.

Time-resolved X-ray absorption spectroscopy (XAS) offers the possibility to monitor the state of materials during chemical reactions. While this technique has been established for transmission measurements for a number of years, XAS measurements in fluorescence mode are challenging because of limitations in signal collection as well as detectors. Nevertheless, measurements in fluorescence mode are often the only option to study complex materials containing heavy matrices or in samples where the element of interest is in low concentration. Here, it has been demonstrated that high-quality quick-scanning full extended X-ray absorption fine-structure data can be readily obtained with sub-second time resolution in fluorescence mode, even for highly diluted samples. It has also been demonstrated that in challenging samples, where transmission measurements are not feasible, quick fluorescence can yield significant insight in reaction kinetics. By studying the fast high-temperature oxidation of a reduced LaFe0.8Ni0.8O3 perovskite type, an example where the perovskite matrix elements prevent measurements in fluorescence, it is shown that it is now possible to follow the state of Ni in situ at a 3 s time resolution.

Deciphering the Solvent Effect for the Solvation Structure of Ca2+ in Polar Molecular Liquids

Although the crystal structures for many inorganic compounds are readily available, researchers are still working hard to understand the relations between the structures and chemical properties of solutions because most of the chemical reactions take place in solutions. A huge amount of effort has been put toward modeling the ion solvation structure from the perspectives of both experiments and theories. In this study, the solvation structures of Ca2+ ions in aqueous and alcoholic solutions at different concentrations were carefully evaluated by Ca K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses. Density functional theory (DFT) calculations were also performed to correlate the results with the experimental data and then further extended to other similar systems. It was found that the number of coordinating solvent molecules decreases with increasing Ca2+ concentration and increasing solvent molecule sizes. From the EXAFS data, it was observed that the first solvation shell of Ca2+ splits into two Ca-O distances in a methanol solution and the counter ion Cl- might also be within the first shell at high concentrations. For the first time, the effects of solvents with different polarities and sizes on the ion solvation environment were systematically evaluated.

Analysis of Anharmonic EXAFS Spectra of Crystalline Nickel Using High-order Debye-Waller Factors

The extended X-ray absorption fine structure (EXAFS) has been developed into a powerful technique and is widely applied to determine many structural parameters and dynamic properties of materials. The EXAFS technique is now the technique of choice in many materials science investigations, and the EXAFS data analysis is being performed in many laboratories spread around the world. In this work, the anharmonic EXAFS spectra of crystalline nickel (Ni) has been analyzed based on the quantum anharmonic correlated Einstein model. The anharmonic EXAFS oscillation presented in terms of the Debye-Waller factors using the cumulant expansion approach up to the fourth-order. This calculation model has been developed from the high-order anharmonic effective potential that described the contribution of their nearest-neighbor atoms to the pair interaction potential. The analytical expressions of the anharmonic EXAFS cumulants are not only explicit forms but also satisfy all of their fundamental properties in temperature dependence. The analysis of the anharmonic EXAFS spectra was performed by evaluating the contributions of the cumulants to the amplitude reduction and the phase shift of the anharmonic EXAFS oscillation. The numerical results for Ni were in good agreement with those obtained using the other theoretical methods and experiment at various temperatures, which are useful for analyzing the experimental EXAFS data of the metal crystals.

Structure–Charge Transport Relationships in Fluoride-Doped Amorphous Semiconducting Indium Oxide: Combined Experimental and Theoretical Analysis

Anion doping of transparent amorphous metal oxide (a-MO) semiconductors is virtually unexplored but offers the possibility of creating unique optoelectronic materials owing to the chemical tuning, modified crystal structures, and unusual charge-transport properties that added anions may impart. We report here the effects of fluoride (F–) doping by combustion synthesis, in an archetypical metal oxide semiconductor, indium oxide (In–O). Optimized fluoride-doped In–O (F:In–O) thin films are characterized in depth by grazing incidence X-ray diffraction, X-ray reflectivity, atomic force microscopy, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure (EXAFS). Charge-transport properties are investigated in thin-film transistors (TFTs), revealing that increasing fluoride content (0.0 → 1.57 atom %) slightly lowers the on-current (Ion) and electron mobility due to scattering from loosely bound F– centers but enhances important TFT performance parameters such as the Ion/Ioff ratio, subthreshold swing, and bias stress stability, yielding superior TFT switching versus undoped In–O. These results are convincingly explained by ab initio molecular dynamics simulations and density functional theory electronic structure calculations. Combined with the EXAFS data, the experimental and theoretical results show that F– hinders crystallization by enhancing the local and medium-range disorder, promotes a uniform film morphology, and favors the formation of deeper, more localized trap states as compared to F–-free In–O. These data also show that the local organization and electronic structure of amorphous F–-doped oxide semiconductors are significantly different from those of F–-doped crystalline oxide semiconductors and suggest new avenues to further modify a-MOs for enhanced optoelectronic properties.

X-ray absorption spectroscopy study of cobalt mononitride thin films

We present a comprehensive study to resolve the debate about the structure of cobalt mononitride (CoN). In contradiction with theoretical predictions, experimentally CoN has been claimed to synthesize in a rock-salt (RS) and in a zinc-blende (ZB)-type structure under ambient pressure and temperature. Utilizing X-ray absorption near-edge spectroscopy (XANES) at Co and N K-edges and extended X-ray absorption fine structure (EXAFS) at Co K-edge, we investigated the structure of CoN in detail. The presence of a strong pre-edge feature at the Co K-edge and the absence of t $$_2$$ feature at the N K-edge indicate towards the tetrahedral coordination that is expected in the ZB-type structure of CoN. Theoretical simulations of XANES spectra unambiguously confirm the ZB-type structure in CoN. Also from EXAFS data, we found that Co–N and Co–Co bond distances match well with ZB-CoN. Magnetic properties of CoN were examined using polarized neutron reflectivity and bulk magnetization measurements. It was found that CoN sample exhibits a paramagnetic behaviour as expected in ZB-CoN. Obtained results clearly demonstrate that CoN indeed crystalizes in the ZB-type structure, and the possibility of the RS-type CoN can be ruled out under ambient pressure and temperature. It is anticipated that the information about the structure of CoN can be utilized for its usage in emerging areas such as low-cost catalyst in oxygen and hydrogen evaluation reactions, high capacity anode in ion batteries and in photovoltaic applications.

The effect of orbital-lattice coupling on the electrical resistivity of YBaCuFeO5 investigated by X-ray absorption

Temperature-dependent X-ray absorption near-edge structures, X-ray linear dichroism (XLD) and extended X-ray absorption fine structure (EXAFS) spectroscopic techniques were used to investigate the valence state, preferred orbital and local atomic structure that significantly affect the electrical and magnetic properties of a single crystal of YBaCuFeO5 (YBCFO). An onset of increase of resistivity at ~180 K, followed by a rapid increase at/below 125 K, is observed. An antiferromagnetic (AFM)-like transition is close to the temperature at which the resistivity starts to increase in the ab-plane and is also observed with strong anisotropy between the ab-plane and the c-axis. The XLD spectra at the Fe L3,2-edge revealed a change in Fe 3d eg holes from the preferential {bf{3}}{{boldsymbol{d}}}_{{{bf{x}}}^{{bf{2}}}{boldsymbol{-}}{{bf{y}}}^{{bf{2}}}} orbital at high temperature (300–150 K) to the {bf{3}}{{boldsymbol{d}}}_{{{bf{3}}{bf{z}}}^{{bf{2}}}{boldsymbol{-}}{{bf{r}}}^{{bf{2}}}} orbital at/below 125 K. The analysis of the Fe K-edge EXAFS data of YBCFO further revealed an unusual increase in the Debye-Waller factor of the nearest-neighbor Fe-O bond length at/below 125 K, suggesting phonon-softening behavior, resulting in the breaking of lattice symmetry, particularly in the ab-plane of Fe-related square pyramids. These findings demonstrate a close correlation between electrical resistivity and coupling of the preferred Fe 3d orbital with lattice distortion of a single crystal of YBCFO.

Au25 clusters as precursors for the synthesis of AuPd bimetallic nanoparticles with isolated atomic Pd-surface sites

This study documents the synthesis of Al2O3-supported AuPd bimetallic nanoparticles with isolated atomic Pd-surface sites using Au25(SC8H9)18 clusters as starting precursors. Here, previously synthesized thiolate protected AuPd bimetallic clusters were supported on Al2O3 and subjected to thermal and LiBH4 treatments. Extended X-ray absorption fine structure (EXAFS) analysis shows that thermal and LiBH4 treatments of Al2O3 supported AuPd bimetallic clusters result in the formation of AuPd/Al2O3 bimetallic nanoparticles with isolated atomic Pd-surface sites. EXAFS data also shows that Pd atoms present on the surface were still bonded with thiolate stabilizers. Finally, AuPd/Al2O3 catalysts were tested for the hydrogenation of allyl alcohol. Data show that while Au25/Al2O3 materials were inactive for the hydrogenation reaction, Al2O3 supported AuPd bimetallic clusters were catalytically active. The catalytic activity was further enhanced after thermal and LiBH4 treatment and this was attributed to the enhanced access of substrates to surface Pd sites.

Local structure investigation of Ga and Yb dopants in Co4Sb12 skutterudites

We report comprehensive x-ray absorption spectroscopy studies at both the Ga $K$ edge and Yb ${L}_{2}$ edge to elucidate the local structure of Ga and Yb dopants in ${\mathrm{Yb}}_{x}{\mathrm{Ga}}_{y}{\mathrm{Co}}_{4}{\mathrm{Sb}}_{12}$. Our extended x-ray absorption fine structure (EXAFS) data confirm that Ga atoms occupy two crystallographic sites: one is the $24g$ site replacing Sb, and the other is the $2a$ site in the off-center void position. We find that the occupancy ratio of these two sites varies significantly as a function of the filling fraction of additional Yb, which exclusively occupies the $2a$ on-center site. At low concentrations of Yb, ${\mathrm{Ga}}_{24g}$ and ${\mathrm{Ga}}_{2a}$ dopants coexist and they form a charge-compensated compound defect proposed by Qiu et al. [Adv. Funct. Mater. 23, 3194 (2013)]. The ${\mathrm{Ga}}_{24g}$ occupancy increases gradually with increasing Yb concentration, and almost all Ga occupies the $24g$ site for the highest Yb concentration studied ($x=0.4$). In addition to the local structural evidence provided by our EXAFS data, we also present x-ray absorption near-edge structure (XANES) spectra, which show a small Ga $K$-edge energy shift as a function of Yb concentration consistent with the change from predominantly ${\mathrm{Ga}}_{2a}$ to ${\mathrm{Ga}}_{24g}$ states. Our result suggests that the increased solubility of Yb in Yb-Ga co-doped ${\mathrm{Co}}_{4}{\mathrm{Sb}}_{12}$ skutterudites is due to the increased ${\mathrm{Ga}}_{24g}$ electron acceptor, and thus provides an important strategy to optimize the carrier concentration in partially filled skutterudites.

Crystallographic properties and electronic structure of V-doped AlN films that absorb near ultraviolet-visible-infrared light

For highly efficient photoconversion devices, 3d-transition-metal-doped AlN is a candidate intermediate-band material. Here, we synthesized and investigated V-doped AlN (AlVN; V ≤ 11%) films. The optical absorption spectra of the films showed characteristic features including a peak in the infrared region and shoulders in the visible light region. These features remained essentially unchanged for the various V concentrations. X-ray diffraction (XRD), transmission electron microscopy (TEM), and V K-edge X-ray absorption fine structure (XAFS) measurements were carried out to clarify the crystallographic origin of the optical absorption features. The XRD profiles revealed that all films had a c-axis-oriented wurtzite structure. The TEM analyses supported the XRD results. The V K-edge X-ray absorption near-edge structure indicated that the V atoms in the AlN lattice were surrounded by N atoms with non-centrosymmetric conditions and had an oxidation state close to 3+. Extended XAFS (EXAFS) analyses implied that the V atoms had C3v symmetry. The results of ab initio lattice relaxation calculations for a model wurtzite structure of an Al35V1N36 supercell were consistent with the EXAFS data. Electronic structure calculations using this model showed that additional energy bands, mainly consisting of V d states, were formed in the band gap of AlN, and the Fermi level was between the additional bands. Hence, in the optical absorption spectra, the peak was explained by d-d transitions partially allowed thorough hybridization with the p component, and the shoulders were attributed to transitions from the valence band to the new bands in the band gap of AlN.

Formation of nickel magnetic nanoparticles and modification of nickel phthalocyanine matrix by sodium doping

Data for the vapor-phase doping (300°C) of nickel phthalocyanine (NiPc) by sodium taken in different concentrations (x), as well as structural analysis data for Na x = 0.2NiPc, Na x = 1NiPc, and Na x = 3NiPc samples, have been reported. The structure of the samples and their atomic configuration versus the doping level have been studied by transmission electron microscopy, Raman scattering, X-ray diffraction, X-ray absorption spectroscopy, and extended X-ray absorption fine structure (EXAFS) spectroscopy. The structural parameters of Ni–N, Ni–C, and Ni–Ni bonds have been determined, and it has been found that, at a low level of doping by sodium, local structural distortions are observed in some molecules of the NiPc matrix near nickel atoms. The fraction of these molecules grows as the doping level rises from x = 0.2 to x = 1.0. It has been shown that doping changes the oscillation mode of light atoms, which indicates a rise in the electron concentration on five- and six-membered rings. At a high level of sodium doping (x = 3.0), nickel nanoparticles with a mean size of 20 nm and molecule decomposition products have been observed in the NiPc matrix. It has been found that the fraction of nickel atoms in the Na x = 3NiPc nanoparticles as estimated from EXAFS data is sufficient for the room-temperature magnetic properties of the samples to persist for a long time.

Insights into the Coordination and Extraction of Yttrium(III) Ions with a Phenoxyacetic Acid Ionic‐Liquid Extractant

Phenoxyacetic acid type extractants such as sec‐octylphenoxyacetic acid (CA‐12), sec‐nonylphenoxyacetic acid (CA‐100), and the resulting bifunctional ionic‐liquid extractants were developed to separate yttrium from heavy rare‐earth elements (REEs). To obtain information on the coordination chemistry of this type of bifunctional ionic‐liquid extractant, the CA‐12 analogue n‐octylphenoxyacetic acid (HOCTPOA) was synthesized, and its structure was characterized by single‐crystal X‐ray diffraction analysis. Then, a bifunctional ionic‐liquid extractant based on HOCTPOA, namely, [N1888][OCTPOA] (N1888 is methyltrioctylammonium) was synthesized. The study of the extraction of yttrium with [N1888][OCTPOA] in chloride media was conducted by IR spectroscopy, log D versus log C slope analysis, ESI‐MS, extended X‐ray absorption fine structure (EXAFS), and DFT studies. The EXAFS data indicate that a YIII extraction complex with phenoxyacetic acid formed. Three possible YIII coordination cations were calculated by the DFT method, and the obtained Y–O bond lengths were comparable to the results from EXAFS fitting. A possible structure for the overall extracted complex was proposed, and this knowledge should contribute to a better understanding of the extraction process with phenoxyacetate‐type bifunctional ionic liquids.

Anomalous behavior of displacement correlation function and strain in lanthanum cobalt oxide analyzed both from X-ray powder diffraction and EXAFS data

A combined X-ray powder diffraction (XPD) and high-resolution extended X-ray absorption fine structure (EXAFS) at the Co and Ga K-edges study has been performed for LaCoO3 and LaGaO3 ceramics, the latter sample was used as a reference without spin transitions. Based on the X-ray diffraction data, we have found that isotropic atomic displacement parameters (ADP) or mean-squared displacement of the Co–O bond exhibit gradual growth below ~50 K, wherein the strain dependencies testify rapid increase below 150 K for the LaCoO3 having rhombohedral structure. No similar features could be observed for LaGaO3 sample. Above ~100 K the isotropic ADP of the Co–O bond indicate a gradual growth, whereas strain curves show distinct bend near the spin-state transition temperature at about 150 K. According to the EXAFS data, the correlated parallel mean squared relative displacement (MSRD||) of Co–O and Ga–O bonds exhibit a gradual growth above 150 K; however, in the LaCoO3 this parameter is notably bigger. It is supposed that at low temperature the cobalt ions are dominantly in low-spin (LS) state, while certain amount of Co3+ ions located within the surface layer of the crystallines have high-spin state (HS). Temperature growth leads to a gradual transformation of the HS state of the cobalt ions into the highly-hybridized intermediate-spin (IS) state, while the cobalt ions located in the inner part of the crystallines remain LS configuration up to 150 K. Further temperature increase leads to a spin transition of the Co3+ ions located within the crystallines from the LS state into the IS one.

Structural, Optical and EXAFS Studies of Nickel Substituted Copper Ferrites Nano-Particle by Sol-Gel Auto Combustion Method

The Cu-Ni ferrites with general formula Cu1-xNixFe2O4 (where x=0.0, 0.1, 0.2, 0.3) were prepared by sol gel method. The Extended X-ray absorption fine structure (EXAFS), X- ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) measurements were carried out. EXAFS spectra have been recorded at the k edge of Iron using the dispersive EXAFS (DEXAFS) beam line at 2.5 GeV at Indus -2 synchrotron radiation source RRCAT, Indore, India. The EXAFS data have been analyzed using the computer software Athena. These have been used to determine the bond lengths in these ferrites with the help of three different methods, namely, Levy's, Lytle's and Lytle, Sayers and Stern's (LSS) methods. The phase uncorrected bond lengths have also been obtained from Fourier transformation method and the results obtained have been compared with the results obtained from LSS method, which also gives phase uncorrected bond lengths. XRD shows the structure is the tetragonal, and FTIR was used to determine the nature of the vibrational modes present in the system.

Characterization and EXAFS Studies of Cobalt Synthesized by sol-gel auto Combustion Method

The Co-Ni ferrites with general formula NiXCo1-XFe2O4 (where X=0.0, 0.1, 0.15, 2, 0.4) were prepared by Sol-gel auto combustion method. X-ray, FTIR and EXAFS measurement were carried out. Extended X ray Absorption Fine Structure (EXAFS) spectra have been recorded at the K-edge of Fe using the Dispersive EXAFS (DEXAFS) beam line-8 Indus-2 synchrotron radiation source RRCAT, Indore, India. The EXAFS data have been analysed using the computer softer Athena. These have been used to determine the Bond length in these ferrites with help of four different methods, namely, Levy's, Lytle's and Lytle, Sayers and stern's (LSS).

Local structure of Ge quantum dots determined by combined numerical analysis of EXAFS and XANES data

The sensitivity of X-ray absorption near-edge structure (XANES) to the local symmetry has been investigated in small (∼4 nm) matrix-free Ge quantum dots. The FDMNES package was used to calculate the theoretical XANES spectra that were compared with the experimental data of as-prepared and annealed nanoparticles. It was found that XANES data for an as-prepared sample can only be adequately described if the second coordination shell of the diamond-type structural model is included in the FDMNES calculations. This is in contrast to the extended X-ray absorption fine-structure data that show only the first-shell signal. These results suggest that, despite the high degree of disorder and a large surface-to-volume ratio, as-prepared small Ge quantum dots retain the diamond-type symmetry beyond the first shell. Furthermore, we utilized this sensitivity of XANES to the local symmetry to study annealed Ge quantum dots and found evidence for significant structural distortion which we attribute to the existence of surface disorder in the annealed oxygen-free Ge quantum dots.