A. Marcelli

EXAFS and XANES joint analyses for semiconducting vanadium phosphate glasses

Abstract Vanadium EXAFS spectra of 50V2O5-50P2O5 glasses with different C = V+4/Vtot have been measured. The V-O distances increase by ΔR 1 = (0.03 ± 0.02) A to ΔR 2 = (0.07 ± 0.03) A going from a glass with C = 0.64 to C = 0.84 and from C = 0.51 to C = 0.84, respectively. The EXAFS data show a basically similar structure of the vanadium sites for both the V4+ and V5+ ionic states. The density of the glasses increases with C from the value d1 = 2.81 g/cm3 for C = 0.51 to d2 = 2.92 g/cm3 for C = 0.84 indicating a more random packing of glass units.

Determination of mixing of 4f-ligand orbitals in Ce(SO4)2 by Xanes is Ce(SO4)2 a mixed valent insulating system?

Abstract High resolution XANES spectra of insulating lanthanum and cerium 4f 0 and 4f 1 compounds have been measured. We demonstrate that final state effects like charge transfer satellites (configuration interactions in the final state) are quenched in XANES, and that the splitting of core excitations in L III- and L I- XANES of Ce(SO 4 ) 2 like in CeO 2 is due to configuration interaction in the initial state. We show that the ground state electronic structure of formally Ce 4+ compounds is given by mixing of multielectron configurations involving oxygen ligand orbitals. These materials (with NiO) can be classified in a new class of materials “inter-atomic intermediate valent” (IAIV) described by configuration interactions 4f 0 x 4f 1 L -1 .

Correlation between local vibrations and metal mass in AlB2-type transition-metal diborides.

Lattice vibrations have been investigated in TiB2, ZrB2 and HfB2 by temperature-dependent extended X-ray absorption fine structure (EXAFS) experiments. Data clearly show that the EXAFS oscillations are characterized by an anomalous behavior of the Debye-Waller factor of the transition-metal-boron pair, which is suggested to be associated with a superposition of an optical mode corresponding to phonon vibrations induced by the B sublattice and an acoustic mode corresponding to the transition-metal (TM) sublattice. Data can be interpreted as a decoupling of the metal and boron vibrations observed in these transition-metal diborides (TMB2), a mechanism that may be responsible for the significant reduction of the superconducting transition temperature observed in these systems with respect to the parent MgB2 compound. The vibrational behavior of TM-TM bonds has also been investigated to study the occurrence of anisotropy and anomalies in the lattice vibrational behavior of TM-TM bonds.

Localization mixing and / or hybridization in intermetallic compounds RPd3 (R = La, Ce, Pr, Nd, Sm) by xanes

Abstract An expensive investigation of the XANES of palladium 3d, 2p, 2s and of rare earth 4p, 2p, 2s core levels has been carried out at the Frascati synchrotron radiation facility with the X-rays and the soft X-rays “grasshopper” beam lines. We demonstrate that we can experimentally determine, from comparison of XANES spectra of many different core levels, the 4f-5d hybridization and/or the inter-atomic 4f-ligand orbital mixing giving two different types of mixed valent systems. We have found that while in CePd 3 the 4f-5d hybridization is the dominant effect determining valence fluctuations, the inter-atomic mixing of La(4f)-Pd(4d) orbitals is present in LaPd 3 , a formal 4f 0 system.

High energy spectroscopy in Ce and La compounds

Abstract X-ray photoemission, X-ray photoabsorption and bremmstrahlung isochromat spectroscopy in Ce and La compounds are theoretically analyzed by using the single-site Anderson model incorporated with various final state interactions. Discussions are given on the physical information derived from the analysis and also on similarities and differences in spectral features between metallic and insulating systems.

XANES of Palladium Rare Earth Intermetallics (RPd3): Determination of Hybridization and Mixing of 4f-Orbitals

The single site promotional model of heavy rare earth valence fluctuating compounds has been supported by XPS and XANES experiments [1,2]. Light rare earth compounds exibith anomalous electronic properties due to hybridization of the 4f-orbitals of the rare earth with the conduction band.

Local Order at the Manganese Sites in Ionic Solutions by XANES (X-ray absorption near edge structure)

The idea that important geometrical informations about local structure are contained in XANES spectra has stimulated theoretical and experimental studies in this field(1,4). The low energy part of the x-ray absorption spectra, in fact, is sensitive to the geometrical arrangement of the environment around the absorbing atom because of the strong scattering power of these low energy photoelectrons, favoring multiple scattering (MS) processes. Going to the high energy region of the spectra a gradual turn-over occurs from the MS regime to the single scattering (SS) regime where the modulation in the absorbtion coefficient (EXAFS) is substantially due to the interference effect of the outgoing photoelectron wave from the absorbing atom and the backscattered wave from each surrounding atom (5,6). Hence, while this part of the spectrum provides information about the pair correlation function, the first one contains information about higher order distribution functions by MS pathways which begin and end at the absorbing atom.