The Fe K x-ray absorption near edge structure of BaFe(2-x)Co(x)As(2) superconductors was investigated. No appreciable alteration in shape or energy position of this edge was observed with Co substitution. This result provides experimental support to previous ab initio calculations in which the extra Co electron is concentrated at the substitute site and do not change the electronic occupation of the Fe ions. Superconductivity may emerge due to bonding modifications induced by the substitute atom that weakens the spin-density-wave ground state by reducing the Fe local moments and/or increasing the elastic energy penalty of the accompanying orthorhombic distortion.

The vibrational properties of Ge nanocrystals (NCs) produced by ion implantation in SiO 2 followed by thermal annealing were determined from temperature dependent Extended X-Ray Absorption Fine Structure (EXAFS) spectroscopy measurements. Using a correlated anharmonic Einstein model and thermodynamic perturbation theory it was possible to extract information about thermal and static disorder, thermal expansion and anharmonicity effects for the Ge NCs. Comparison with results for bulk crystalline and amorphous Ge indicates that the Ge NCs bonds are stiffer than those of both bulk phases of Ge. Also, the values of the anharmonic linear thermal expansion and the thermal expansion coefficient obtained for the Ge NCs were considerably smaller those for bulk crystalline Ge. Similar trends are reported in the literature for other semiconductor NC systems. They suggest that the increased surface to volume ratio of nanocrystals and the presence of the surrounding SiO 2 matrix might be responsible for the different vibrational properties of the nanocrystal phase.

We investigate here the internal structure of zinc ferrite nanoparticles designed and prepared by a soft chemistry method to elaborate magnetic nanocolloids. The strategy used to avoid acid dissolution modifies the chemical composition of the surface of the nanoparticles, which are described as a core of stoichiometric zinc ferrite surrounded by a maghemite shell. Measurements of X-ray absorption nearedge spectroscopy, extended X-ray absorption fine structure, and X-ray diffraction are undertaken to investigate the local structure of nontreated nanocrystals and of surface-treated ones as a function of their sizes. The qualitative analysis of X-ray absorption results indicates a nonequilibrium cation distribution among the interstitial sites of the zinc ferrite nanocrystals core. Ab-initio calculations of theoretical photoelectron backscattering phases and amplitudes give, by fitting Fourier transformed EXAFS data at both Zn and Fe K-edges, an average inversion degree of 0.34. This value well matches the result of Rietveld refinement of X-ray diffraction data. Magnetization measurements performed on dilute aqueous nanocrystal dispersions, liquid at room temperature and frozen at low temperatures, are carried out in order to test the obtained results.

The effects of K and Co substitutions and quasi-hydrostatic applied pressure (P < 9 GPa) in the local atomic structure of BaFe2As2, Ba(Fe0.937Co0.063)2As2 and Ba0.85K0.15Fe2As2 superconductors were investigated by extended x-ray absorption fine structure (EXAFS) measurements in the As K absorption edge. The As-Fe bond length is found to be slightly reduced ( 0.01Å) by both Co and K substitutions, without any observable increment in the corresponding Debye Waller factor. Also, this bond is shown to be compressible (κ = 3.3(3) × 10 −3 GPa −1 ). The observed contractions of As-Fe bond under pressure and chemical substitutions are likely related with a reduction of the local Fe magnetic moments, and should be an important tuning parameter in the phase diagrams of the Fe-based superconductors.

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We report on the lattice evolution of BiFeO 3 as function of temperature using far infrared emissivity, reflectivity, and X-ray absorption local structure.A power law fit to the lowest frequency soft phonon in the magnetic ordered phase yields an exponent β=0.25 as for a tricritical point. At about 200 K below T N ~640 K it ceases softening as consequence of BiFeO 3 metastability. We identified this temperature as corresponding to a crossover transition to an order-disorder regime. Above ~700 K strong band overlapping, merging, and smearing of modes are consequence of thermal fluctuations and chemical disorder. Vibrational modes show band splits in the ferroelectric phase as emerging from triple degenerated species as from a paraelectric cubic phase above T C ~1090 K. Temperature dependent X-ray absorption near edge structure (XANES) at the Fe K-edge shows that lower temperature Fe 3+ turns into Fe 2+ . While this matches the FeO wüstite XANES profile, the Bi L III -edge downshift suggests a high temperature very complex bond configuration at the distorted A perovskite site. Overall, our local structural measurements reveal high temperature defect-induced irreversible lattice changes, below, and above the ferroelectric transition, in an environment lacking of long-range coherence. We did not find an insulator to metal transition prior to melting.

Extended x-ray absorption fine structure (EXAFS) spectroscopy was used to identify structural perturbations in Ge nanocrystals produced in silica by ion implantation and annealing. Although the nanocrystals retained tetrahedral coordination, both the short- and medium-range orders were perturbed relative to bulk crystalline material. Equivalently, the nanocrystal interatomic distance distribution deviated from that of bulk crystalline Ge, exhibiting enhanced structural disorder of both Gaussian and non-Gaussian forms in the first, second, and third nearest-neighbor shells. The relative influences of nanocrystal size, bonding distortions, multiple phases, and a matrix-induced compression were considered.

Cu nanocrystals ͑NCs͒ were produced by multiple high-energy ion implantations into 5-m-thick amorphous silica ͑SiO 2 ͒ at liquid-nitrogen temperature. The Cu-rich SiO 2 films were subsequently annealed to reduce irradiation-induced damage and promote NC formation. The NC size distribution and structure were studied utilizing a combination of Rutherford backscattering spectroscopy, x-ray diffraction, cross-sectional transmission electron microscopy, and extended x-ray-absorption fine-structure ͑EXAFS͒ spectroscopy. We present results derived from all four techniques, focussing on EXAFS measurements to study the local atomic structure surrounding Cu atoms, and comparing NC samples with bulk standards. Using a unique sample preparation method, we drastically improve the signal-to-noise ratio to extract high-quality EXAFS data to enable the determination of a non-Gaussian bond length distribution via the third-order cumulant. We quantify subtle concentration-and annealing-temperature-dependent changes in the Cu NC short-range order and relate such changes to NC size. Relative to a bulk Cu standard, enhanced structural disorder is observed in addition to both a suppressed coordination number and bond length contraction. Deviations from bulklike structure increase as the NC size decreases. Samples of low Cu concentration and/or low annealing temperature contain a significant fraction of Cu oxides, as either oxidized NCs or Cu bonding to O in the SiO 2 matrix. EXAFS and x-ray-absorption near-edge structure analyses demonstrate Cu in an oxidized form exhibits an oxidation state and local coordination similar to crystalline Cu 2 O albeit in a disordered form.