F Rocca

EXAFS studies of silver ion coordination in silver borate glasses

EXAFS measurements at the K and L 3 edges of silver have been performed on the binary ion conducting glasses (Ag 2 O · n B 2 O 3 ), with n = 2, 3, 4, 6. The first coordination shell of silver is characterized by a mean Ag-O distance r = 2.27 A and a very low coordination number ( N 2). This result indicates the presence of a local structure in the environment of the silver cation, almost independent of the Ag 2 O content. This structure is different from those found in crystalline silver-borate compounds and in sodium-borate glasses. For the vitreous Ag 2 O · 2B 2 O 3 a further coordination Ag-O with r = 1.99 A is singled out, confirming the possible bond between silver ions and non-bridging oxygens at high Ag 2 O content.

X-ray absorption spectroscopy study of ReO3 lattice dynamics

The lattice dynamics of ReO3 is studied for the first time by X-ray absorption spectroscopy (XAS) in the temperature range from 77 to 350 K at the rhenium L3 edge, The Debye-Waller factors are extracted for the first, fourth and sixth coordination shells of rhenium, and their temperature dependences are discussed on the basis of the Debye and Einstein vibrational models. The obtained characteristic temperatures Theta D and Theta E are compared with the ones found by other experimental techniques. The application of the Debye and Einstein models to the analysis of the XAS and X-ray diffraction data for compounds with different degrees of correlation in atomic motion is also discussed.

EXAFS and XANES study of GaAs on Ga and As K edges

The extended X-ray absorption fine structure (EXAFS) at the K edges of Ga and As has been calculated in the multiple-scattering approach using the fast spherical approximation. It is shown that, for both edges, multiple-scattering contributions are negligible for wavevector values greater than 3 AA-1, and the single-scattering analysis can be used without significant loss of accuracy in that region. The calculated spectra are in good agreement with measurements at 77 K. The X-ray absorption near-edge structure (XANES) at both edges have been calculated by the continued-fraction expansion method. The origin of the main XANES features and the differences between Ga and As edges have been interpreted. The first main peak is attributed to the bound-to-bound 1s to 4p transition. The second feature is a scattering resonance within the second coordination shell: its position is defined by the different phase shifts of the photoelectron wave under scattering from the Ga or As atoms.

An EXAFS study of thermal disorder in GaAs

This paper presents the experimental results of a cumulant analysis of the extended X-ray absorption fine structure amplitude of GaAs at the K edges of Ga and As. The second and fourth cumulants of the distance distributions of the near neighbours of Ga and As have been obtained as a function of temperature in the range 77-450 K. The fourth cumulant of Ga-As nearest-neighbour distance distributions is zero in the examined temperature range; a non-Gaussian distribution of the second and third coordination shell distances of Ga and As has been revealed for the first time. Temperature dependences of mean square relative displacements (MSRDS) of the first three shells of Ga and As have been compared with Einstein and correlated Debye models. The MSRD of the first-shell Ga-As distance is connected to the optical modes at 7.6-8 THz of the phonon spectrum of GaAs.

EXAFS analysis for anharmonic systems

Abstract Recent developments of the cumulant technique for EXAFS analysis are presented. They concern the model-independent reconstruction of the Ague5f8I distance in AgI by the splice method, evaluation of anharmonic contributions to MSRD and characterization of short range static disorder.

Temperature dependence of EXAFS Debye-Waller factors in beta - and gamma -AgI

EXAFS at the iodine L3 edge in beta - and gamma -AgI has been studied. The EXAFS Debye-Waller factors of the first and second coordination shells of iodine have been extracted by a standard analysis based on the harmonic approximation. No difference has been found between the temperature dependence of mean square relative displacements (MSRD) of the two beta and gamma phases. The harmonic approximation has been checked against more complete anharmonic EXAFS and X-ray diffraction analyses; it has been found adequate to interpret the MSRDs of AgI for temperatures lower than 300 K. The experimental MSRDs for the two distances I-Ag and I-I have been compared with MSRDs calculated on the grounds of the available lattice dynamics models. The sensitivity of EXAFS to phonon polarization properties has been made evident.