Unravelling redox processes of Li7MnN4 upon electrochemical Li extraction-insertion using operando XAS.

A large data set of XAS (X-ray Absorption Spectroscopy) Manganese K-edge spectra has been collected operando and studied upon the electrochemical oxidation of the promising Li-ion battery anode material Li7MnN4. Using chemometric tools such as PCA (Principal Component Analysis) and MCR-ALS (Multivariate Curve resolution - Alternating Least Squares), three independent environment spectra were insulated. Based on the faradaic yield and well-chosen comparison of absorption spectrum energies within the frame of the coordination charge model, these environments were ascribed to unusual oxidation states allowed by nitride chemistry at a low potential (∼1.2 V vs. Li+/Li), i.e. Mn5+ (3d2), Mn6+ (3d1) and Mn7+ (3d0). Also, their relative amounts are discussed with regard to the long-range structural variation which can be simply described by two successive biphasic domains followed by a solid-solution behaviour. Gathering this long-range and local structure information provides a complete picture of the redox mechanisms occurring in Li7MnN4.

EXAFS Debye-Waller factors issued from Car-Parrinello molecular dynamics: application to the fit of oxaliplatin and derivatives.

One of the main pitfalls in EXAFS fitting is correlation among parameters, which can lead to unreliable fits. The use of theoretical Debye-Waller factors (DWs) is a promising way to reduce the number of fitted parameters. When working with molecular dynamics, it is not only possible to evaluate DWs from the statistical distributions issued from the trajectory but also to estimate the distribution anharmonicity, and to compute simulated average EXAFS spectra that can be fitted as experimental ones, in order to assess the ability of EXAFS fitting to recover information on DWs, as well as other structural and spectroscopical parameters. The case studied is oxaliplatin, a third generation anticancer drug. The structural information and the simulated average spectra were derived from a Car-Parrinello molecular dynamics (CP-MD) trajectory of a compound closely related to oxaliplatin. We present the DWs issued from this simulation and their use, by taking their theoretical absolute values (no DW fitted) or their ratios (one DW fitted). In this second approach, the fit of oxaliplatin experimental spectra leads to DWs values very close to the theoretical ones. This shows that the CP-MD trajectory provides a good representation of the distance distributions for oxaliplatin. Transferability of oxaliplatin DWs, for all relevant single and multiple scattering paths, to closely related compounds is proven for the case of bis(oxalato)platinum(II) and bis(ethylene diamine)platinum(II).

Apparent mismatch between extended x-ray absorption fine structure and diffraction structures of crystalline metastable WO3 phases.

The local structure of monoclinic, monohydrate, hexagonal, and pyrochlore WO3 phases was investigated by the extended x-ray absorption fine structure spectroscopy as preliminary studies of model compounds of amorphous and thin film WO3 based electrochromic species. In the four cases, we found a large W-O distribution of distances ranging from 1.70 to 2.35 A. The apparent discrepancy of these results and previously published crystal structures are discussed and interpreted as the detection of vacancies and local distortion disorder.

Multiple solutions in data fitting: a trap in EXAFS structural analysis and some ideas to avoid it.

A possible source of error on interatomic distance determination in EXAFS multishell data analysis is described on the basis of fitting a simulated signal for a cluster of Rh atoms with an interaction of O atoms; a fit of an experimental signal is also presented. The origin of this type of mistake is briefly discussed.

CRYSTALFF--from crystallography to EXAFS multiple-scattering calculations with FEFF.

CRYSTALFF is an alternative utility to ATOMS containing most of the features of this standard program for converting crystallographic data to FEFF input. In addition, it offers an interface with molecular modelling programs via the PDB format and new coordination sphere analysis options.

X-FINE--a refinement program for FEFF input parameters.

X-FINE is an interactive and user-friendly tool to optimize input parameters (S20, sigma2, DeltaE) of the FEFF program, a package for EXAFS analysis.

Ligand binding processes in hemoglobin. Chemical reactivity of iron studied by XANES spectroscopy.

K-absorption edge of coordinated ions exhibits a fine structure (through the use of XANES, or x-ray absorption near edge structures) that reflects the electronic repartition and the chemical reactivity of these ions. Comparative analysis of iron K-absorption-edge shape for hemoglobin derivatives with different ligand affinity suggests strongly that in hemoglobin, iron-forms with high and low affinity are highly improbable.

EXAFS and thermodynamics of Fe(II) spin transition polymeric compounds.

We have studied the temperature variations of the EXAFS spectra of three Fe(II)/triazole-based spin transition polymeric compounds compared to a monomer belonging to the same family. These compounds have various temperatures of transition and hysteresis loop widths. In the three polymers, the Fe-Fe-Fe alignment, detected by a multiple scattering signal at the double Fe-Fe distance, is preserved through the spin transition. For the four compounds, we have studied the variations versus temperature of the Debye-Waller factors for both FeN6 (first shell) and Fe-Fe-Fe (multiple scattering path, only for the polymers). We report a strong increase of (sigma2(FeN6) through the spin transition for the monomer and two of the polymeric species. For the polymers an increase of sigma2(Fe-Fe-Fe) is also observed. These observations confirm the results available in the literature, and we confirm the interpretation as an increase of the vibrational part of the DW. For one particular polymer, we observe an unexpected behaviour: no significant increase of the Debye-Waller factor from low spin to high spin states. The EXAFS study of these non mono-crystalline species is the only way to discuss the local structure-thermodynamics properties relationships, and particularly the so-called cooperative effect in the spin transition process.

Unexpected Fe local order in iron oxide coated nanocrystalline magnesium oxides with exceptional reactivities against environmental toxins.

Mg oxide nanoparticles are very reactive materials used to mitigate atmospheric pollution and to sequester polluting molecules. Using Fe K-edge XAFS, we have studied the structure of iron oxide-coated MgO nanoparticles before and after reaction with CCl4. Before reaction, the local structure around Fe is totally different from that in iron oxide coatings on SrO and CaO nanoparticles, although these coated materials were prepared in the same way. In SrO and CaO, the iron oxide coating has been shown to be well separated from the bulk of the nanoparticle, whereas in MgO, Fe was found to mix with MgO. After reaction with CCl4, Fe-Cl bonds can be detected when the coated nanoparticle is saturated. Such Fe-Cl EXAFS signals have not been observed in previously studied nanoparticles.

Synthesis, crystal structure, EXAFS, and magnetic properties of catena [mu-tris(1,2-bis(tetrazol-1-yl)propane-N1,N1')iron(II)] bis(perchlorate). First crystal structure of an iron(II) spin-crossover chain compound.

[Fe(btzp)3](ClO4)2 (btzp = 1,2-bis(tetrazol-1-yl)propane) represents the first structurally characterized Fe(II) linear chain compound exhibiting thermal spin crossover. It shows a very gradual spin transition (T1/2 = 130 K) which has been followed by magnetic susceptibility measurements and 57Fe Mössbauer spectroscopy. The structure has been solved at 200 and 100 K by single-crystal X-ray analysis. It crystallizes in the trigonal space group P3c1 with Z = 2 Fe(II) units at both temperatures. The molecular structure consists of chains running along the c axis in which the Fe(II) ions are linked by three N4,N4' coordinating bis(tetrazole) ligands. The main difference between the two forms appears to be in the Fe-N bond lengths, which are 2.164(4) A at 200 K and 2.038(4) A at 100 K. The Fe-Fe separations are 7.422(1) A at 200 K and 7.273(1) A at 100 K. The EXAFS results are consistent with the crystal structure. In both spin states, the FeN6 octahedron is almost regular within the EXAFS resolution. The Fe-N distance is found as 2.16(2) A at 300 K and 2.00(2) A at 40 K. The absence of the "7 A peak" in the EXAFS spectra of [Fe(btzp)3](ClO4)2, in contrast with what has been observed for the [Fe(4-R-1,2,4-triazole)3]-(anion)2 chain compounds, confirms that this peak can be used as the signature of a metal alignment only when it involves a strongly enhanced multiple scattering M-M-M path, with M-M spacing less than 4 A. Irradiation with green light at 5 K has led to the population of the metastable high-spin state for the iron(II) ion. The nature of the spin-crossover behavior has been discussed on the basis of the structural features.