Fe K-edge Extended X-ray Absorption Fine Structure Research Articles

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.

Resolving the structure of the E1 state of Mo nitrogenase through Mo and Fe K-edge EXAFS and QM/MM calculations.

Biological nitrogen fixation is predominately accomplished through Mo nitrogenase, which utilizes a complex MoFe7S9C catalytic cluster to reduce N2 to NH3. This cluster requires the accumulation of three to four reducing equivalents prior to binding N2; however, despite decades of research, the intermediate states formed prior to N2 binding are still poorly understood. Herein, we use Mo and Fe K-edge X-ray absorption spectroscopy and QM/MM calculations to investigate the nature of the E1 state, which is formed following the addition of the first reducing equivalent to Mo nitrogenase. By analyzing the extended X-ray absorption fine structure (EXAFS) region, we provide structural insight into the changes that occur in the metal clusters of the protein when forming the E1 state, and use these metrics to assess a variety of possible models of the E1 state. The combination of our experimental and theoretical results supports that formation of E1 involves an Fe-centered reduction combined with the protonation of a belt-sulfide of the cluster. Hence, these results provide critical experiment and computational insight into the mechanism of this important enzyme.

Local structure examination of mineral-derived Fe2O3 powder by Fe K-edge EXAFS and XANES

Fe K-edge XANES (X-ray Absorption Near Edge Structure) and EXAFS (Extended X-ray Absorption Fine Structure) spectra have been used to investigate the local structures of hematite (Fe2O3) powders which were synthesized from local iron stone. The formation of hematite with various calcination temperature (500, 650, 800) °C was observed from XRD data of each powder, and Fe K-edge EXAFS and XANES were performed to sample with calcination temperature 800 °C. The XANES spectra confirmed the oxidation state of the powders. The energy position of the first pre-edge peak at (7113.5 ± 0.5) eV confirm Fe3+. The absorption edges of Fe2O3 powders was 7123.41 eV. The first EXAFS data analysis showed that Fe2O3 powder exhibited nearest-neighbor distances RFe-O = 1.56449 Å and RFe-Fe = 3.01449 Å. The XRD lattice parameter values for the sample is in a fair agreement with the nearest-neighbor distances according to the EXAFS data.

Synchrotron X-ray absorption spectroscopy study of disordered cation distribution of nanosized zinc ferrite powders

The zinc ferrite (ZnFe2O4) powders of various nanoparticle sizes were synthesised at different milling times (0 to 24 h) of the as-combusted powders. The non-equilibrium site occupancy of zinc (Zn2+) and ferric (Fe3+) ions was investigated through Zn and Fe K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra. The XRD and SEM strongly confirm the particle size of these ferrites decreasing with the increasing milling time. Compared with the bulk specimen of zinc ferrite, both XANES and EXAFS spectra of zinc ferrite powders clearly exhibit the large translocation of Zn2+ ions from the tetrahedral (A) sites to the octahedral (B) sites and the opposite translocation of some of Fe3+ ions without affecting the long-range structural order. Moreover, the curve-fitting analysis of Zn and Fe K-edge EXAFS spectra indicates that the degree of inversion increases as the particle size decreases resulting in significant differences in the magnetic behaviours.

In situ x-ray absorption spectroscopic study of oxide film on Fe alloy

The nature of passive film grown on the bulk surface of Fe–20Cr alloy in a pH 8.4 boric-borate buffer was investigated with in situ synchrotron x-ray absorption spectroscopy. The main electrochemical test method was potentiostatic polarization and various potentials were applied from −1.0 V to 0.6 V vs saturated calomel electrode (SCE). X-ray absorption data from the passive film formed in the boric-borate buffer were collected from an in situ reflection x-ray absorption spectroscopic experiment in a grazing-incident geometry that is suitable for investigating the nanometer-thick passive film. It was observed in the Fe K-edge XANES (X-ray Absorption Near-Edge Structure) region that the Fe absorption edge moved from the absorption edge of metal Fe to that of Fe oxide as the applied potential increased. It was observed from an analysis of the Radial Distribution Function (RDF) from Fe K-edge EXAFS (Extended X-ray Absorption Fine Structure) data that the Fe–Fe bond intensity of the first shell decreased and Fe–O bond intensity increased as the applied potential increased. The RDF of the Fe oxide is consistent with γ-Fe2O3.

Determination of Fe-substituted sites in the MFI structure by Fe K-edge EXAFS

The local structure around iron in the MFI structured ferrisilicate was investigated by the Fe K-edge extended X-ray absorption fine structure (EXAFS) method to identify the Fe substituted sites in the framework. After removal of the intense first neighbor FeO peak, the second and further peaks in the Fourier transformed EXAFS spectra of the MFI structured ferrisilicate were analyzed by comparing the experimentally-obtained spectra with the simulated ones for twelve Si sites in the MFI framework. The results suggest that a few of the twelve Si sites in the framework are selectively substituted by Fe.

Structural Studies of Feocl Intercalated with Tetrathiafulvalene and Related Materials

Abstract For FeOCl and its intercalates, FeOCl.TTF1/9 (TTF=tetrathiafulvalene), FeOCl.TTN1/9 (TTN=tetrathianaphthalene), and FeOC1.TTT1/7 (TTT= tetrathiatetracene), the combination of X-ray powder diffraction data and Fe K-edge EXAFS (Extended X-ray Absorption Fine Structure) spectroscopy provides a consistent picture of the structural changes of the FeOCl host upon intercalation. A structural model based upon the data obtained from both techniques is discussed, in which the intercalated radical cations are tilted at an angle 6 from the b axis (θ=25±5° for TTF, 42±5° for TTN, TTT). In the TTN and TTT intercalates, the radical cations are also tilted at an additional angle o from the ac plane (o = 14±5°).