J. Chaboy

The hydration of Cu2+: Can the Jahn-Teller effect be detected in liquid solution?

The long elusive structure of Cu(II) hydrate in aqueous solutions, classically described as a Jahn-Teller distorted octahedron and recently proposed to be a fivefold coordination structure [Pasquarello et al., Science 291, 856 (2001)], has been probed with x-ray-absorption spectroscopy by performing a combined theoretical and experimental analysis. Two absorption channels were needed to obtain a proper reproduction of the x-ray-absorption near-edge structure (XANES) region spectrum, as already observed in other Cu(II) complexes [Chaboy et al., Phys. Rev. B 71, 134208 (2005)]. The extended x-ray-absorption fine-structure (EXAFS) spectrum was analyzed as well within this approach. Quite good reproductions of both XANES and EXAFS spectra were attained for several distorted and undistorted structures previously proposed. Nevertheless, there is not a clearly preferred structure among those including four-, five-, and sixfold coordinated Cu(II) ions. Taking into account our results, as well as many more from several other authors using different techniques, the picture of a distorted octahedron for the Cu(II) hexahydrate in aqueous solution, paradigm of the Jahn-Teller effect, is no longer supported. In solution a dynamical view where the different structures exchange among themselves is the picture that better suits the results presented here.

EXAFS and XANES analysis of oxides at the nanoscale.

This work presents a discussion of the possibilities offered by X-ray absorption spectroscopy (XAS) to study the local structure of nanomaterials. The current state of the art for the interpretation of extended X-ray absorption fine structure (EXAFS), including an advanced approach based on the use of classical molecular dynamics, is described and exemplified in the case of NiO nanoparticles. In addition, the limits and possibilities of X-ray absorption near-edge spectroscopy (XANES) in determining several effects associated with the nanocrystalline nature of materials are also discussed in connection with the development of ZnO-based dilute magnetic semiconductors and iron oxide nanoparticles.

Double-channel excitation in the X-ray absorption spectrum of Fe3+ water solutions

Abstract We present a detailed analysis of the X-ray absorption near edge structure (XANES) spectra of [Fe(H2O)6]2+ and [Fe(H2O)6]3+ complexes in solution. While the Fe2+ spectra are well accounted for by a one-electron multiple-scattering theory, Fe3+ spectra present additional features that can only be accounted by two excitation channels using the sudden limit of the multichannel multiple-scattering theory. Other possible explanations based on possible structural changes of the local environment are ruled out on the basis of the new MXAN fitting procedure and the experimental finding for which the pH value does not influence the XANES spectra.

Relationship between the structural distortion and the Mn electronic state in La1−xCaxMnO3: a Mn K-edge XANES study

A theoretical study of the X-ray absorption near-edge structure (XANES) spectra at the Mn K-edge in the La(1-x)Ca(x)MnO(3) series is reported. The relationship between the edge shift, the Ca-La substitution and the distortion of the MnO(6) octahedra in these systems has been studied. It is shown that, by correctly considering these effects simultaneously, the experimental XANES data are consistent with the presence of two different Mn local environments in the intermediate La(1-x)Ca(x)MnO(3) compounds. By taking into account the energy shift associated with the modification of the MnO(6) distortion as Ca substitutes for La, it is possible to reproduce the XANES spectra of the intermediate-doped compounds starting from the experimental spectra of the end-members LaMnO(3) and CaMnO(3). These results point out the need to re-examine the conclusions derived in the past from the simple analysis of the Mn K-edge XANES edge-shift in these materials. In particular, it is shown that the modification of the Mn K-edge absorption through the La(1-x)Ca(x)MnO(3) series is well reproduced by considering the simultaneous presence of both distorted and undistorted octahedra and, consequently, that the existence of charge-ordering phenomena cannot be ruled out from the XANES data.

EXAFS and XANES Investigation of the Structure of TiCl4 and TiCl4-Filodiene Reaction Intermediates in Solution

The local structure around TiCl4 and TiCl4- -Filodiene compexes in solution has been studied by means of x-ray absorption spectroscopy. Analysis of the EXAFS spectra shows that TiCl4 coordinate to two oxygen atoms belonging to the organic counterpart Theoretical XANES spectra have been calculated and compared with the experimental data and the features present in the spectra interpreted in terms of multiple scattering contributions.

Detection of hydrogen-induced effects in Ce2Fe14BHx and Ce2Fe17Hx permanent magnets by LIII absorption edge of cerium

Abstract We have applied X-ray absorption spectroscopy to the study of RE 2 Fe 14 BH x (RE La, Ce) and Ce 2 Fe 17 H x systems at variable hydrogen concentration. The L III XANES spectra at the selected rare earth site have been measured to investigate the influence of hydrogen absorption on the electronic properties and to study the local structure around the rare earth atoms. The magnetic properties of the hydrides are analyzed in the framework of 4f electron localization. Valence change of cerium after the introduction of the hydrogen in the metal host is also discussed.

On the Amplitudes of EXAFS Spectra at the L Edges

In this work we present a detailed analysis of the EXAFS amplitudes at the L edges for different elements, running from Z=46 to 56. Our analyses show the existence of an anomalous ratio between the EXAFS signals at the L2,3 and L1 edges. The observed experimental behaviour is just the opposite than that expected as due only to core hole relaxation terms.

X-ray absorption study of the local order around Mn in Mn:ZnO thin films: the role of vacancies and structural distortions

This work reports an x-ray absorption near-edge structure spectroscopy (XANES) study at both Mn and Zn K edges in Zn(0.95)Mn(0.05)O thin films prepared with different sputtering gases (pure Ar, Ar/N(2) and Ar/O(2)). We have studied the local order around Mn in the films, with special emphasis on the role played by both oxygen and zinc vacancies and by the structural distortion associated with the substitution of Zn by Mn in the ZnO host. Our results indicate that the dependence of the magnetic properties of these Mn:ZnO thin films on the different sputtering gases used in the preparation is related to the structural distortion around Mn sites.

Ab initio X-ray absorption spectroscopy study of the solvation structure of Th(IV), U(IV), and Np(IV) in aqueous solution.

The coordination structures of U(IV), Np(IV), and Th(IV) in aqueous solution have been determined by studying the X-ray absorption near edge structure (XANES) of the actinide (An) L(3)-edge absorption spectra. The high sensitivity of XANES to the bonding geometry provides an unambiguous determination of the coordination polyhedron. On the basis of the comparison of ab initio computations with the experimental data we conclude that the hydration sphere of the three An(IV) aqua-ions studied is best modeled by 9 water molecules forming a tricapped trigonal prism.

An experimental and theoretical study of multi-electron excitations at the L3 absorption edge in some rare earth alloys and their hydrides

Abstract In this work, an extensive X-ray absorption study at the L edges of the cubic Laves phase rare earth compounds: LaRu 2 , CeRu 2 and CeFe 2 , and of some related hydrides is presented. The EXAFS spectra of these materials were measured as a function of the hydrogen concentration. The structural disorder induced by the hydridation process smears the EXAFS oscillatory signal, and in both L 2 and L 3 spectra a new structure at about 130 eV above the Ce and La edges is observed. This structure, absent in the L 1 EXAFS spectra, is interpreted as an intra-atomic two-electron transition involving 2p and 4d electrons.