1,721,033 research outputs found
On the origin of the differences in the Cu K-edge XANES of isostructural and isoelectronic compounds
No full textCu K-edge x-ray absorption near-edge structure (XANES) spectra of trigonal (3R) CuScO2 and CuLaO2 and of hexagonal (2H) CuScO2 were investigated experimentally and theoretically, in order to study differences between spectra of isostructural and isoelectronic compounds. Significant differences were found in the Cu K-edge XANES of 3R CuScO2 and 3R CuLaO2; these differences can be understood by considering the calculated polarization dependence of the XANES spectra and the differences between the phaseshifts of Sc and La. Spectra of the 3R and 2H polytypes of CuScO2 differ only weakly and the difference originates from the long-range order. The pre-edge peak around 8980 eV is generated by the same mechanism as the pre-edge peak in Cu2O, i.e. involving scattering by the Cu atoms in the plane which is perpendicular to the O-Cu-O axis
New EXAFS measurements by XEOL and TEY on porous silicon
No full textResults of an EXAFS investigation on porous Silicon carried out by X-ray Excited Optical Luminescence (XEOL) and Total Electron Yield (TEY) techniques, at the Si K absorption edge, are reported. For the first time XEOL spectra of porous silicon have been recorded in a wide energy range (1800-2500 eV) and EXAFS signals have been singled out from them. Simultaneous TEY and XEOL measurements yield to different results: in particular TEY-EXAFS is sensitive up to the third coordination shell of Si, while XEOL-EXAFS reveals only the contributions of the first two coordination shells; moreover they show a different dependence on changes of the etching parameters. This evidences the sensitivity of XEOL technique to the local structure of the quantum confined luminescent sites. The dependence of the light emission properties on the main preparation parameters and their influence on the short-range structure of red and yellow porous silicon samples are also investigated
On the cumulant analysis of EXAFS in crystalline solids
Full text linkThe analysis of temperature-dependent EXAFS spectra based on the
cumulant expansion is critically reviewed, seeking for accurate
relations between EXAFS parameters and physical properties of
crystals. The treatment is based on the distinction between the real
and effective distribution of distances, and is divided into three
logical steps. (a) The connection between lattice dynamics and
cumulants Cn* of the real distribution is studied and the extent of the
usual approximations are numerically evaluated. Atomic vibrations
perpendicular to the bond direction make the EXAFS thermal
expansion larger than the crystallographic one; the difference is
connected to a shift of the effective pair potential rather than to its
asymmetry. Peculiar information on lattice dynamics of crystals can
be obtained from accurate EXAFS measurements and their cumulant
analysis. (b) The differences between cumulants of the real and
effective distribution (Cn* and Cn, respectively) are calculated for
various physically realistic distributions. The largest discrepancy
concerns the first cumulant: C1* measures the thermal expansion of
the interatomic bond, while C1 is a better estimate of the
crystallographic thermal expansion. (c) The convergence properties
of the cumulant series are discussed and some phenomenological
procedures are suggested to monitor and possibly work out the
connected failures of the cumulant method. Benefits and risks of the
use of an effective pair potential are at last debated
X-ray absorption spectroscopy of strongly disordered glasses: Local structure around Ag ions in g-Ag2O center dot nB(2)O(3)
No full textThe local structure around Ag ions in silver borate glasses g-Ag2O center dot nB(2)O(3) (n=2,4) was studied by x-ray absorption spectroscopy at the Ag K edge for temperatures from 77 to 450 K. Extended x-ray absorption fine structure (EXAFS) analysis based on cumulant expansion or multishell Gaussian model fails for these systems. Therefore, the radial distribution functions (RDFs) around Ag ions were reconstructed using a method based on the direct inversion of the EXAFS expression. The RDFs consist of about eight atoms (oxygens and borons), exhibit a relatively weak temperature dependence, and indicate the presence of strong static disorder. Two main components can be identified in RDFs, located at about 2.3-2.4 A and 2.5-3.4 A, respectively. The chemical types of atoms contributing to the RDF were determined via a simulation of configurationally averaged x-ray absorption near-edge structure (XANES) and EXAFS signals. The immediate neighborhood of Ag contains mostly oxygens while borons dominate at larger distances. The combination of EXAFS and XANES techniques allowed us to determine a more complete structural model than would be possible by relying solely on either EXAFS or XANES alone
Nearest-neighbour distribution of distances in crystals from extended X-ray absorption fine structure
Full text linkExtended X-ray absorption fine structure (EXAFS) is a powerful probe of the distribution of nearest-neighbour distances around selected atomic species. We consider here the effect of vibrational disorder in crystals. The potential of EXAFS for the accurate evaluation of the coefficient of bond thermal expansion and its temperature dependence is discussed, with the aim of stimulating and facilitating the comparison with the results from total scattering experiments. The meaning of the distribution asymmetry in crystals and its connection with the effective potential anharmonicity and the bond expansion is quantitatively explored by comparing the results for a number of different systems. The extent of the relative atomic vibrations perpendicular to the bond direction and the perpendicular to parallel anisotropy are correlated with the extent of lattice negative thermal expansion as well as with the ionic mobility in superionic crystals
X-ray absorption fine structure: characterization of thermal and structural disorder in non-crystalline solids
No full textA great progress has been made in the last years in the XAFS studies of thermal and structural disorder, due to the increased quality of measurements allowed by the intense third generation synchrotron radiation sources, and the refinement of the analysis techniques. A description of XAFS strengths and limits is given and some paradigmatic investigations on thermal and structural characterization of crystalline and non-crystalline solids are reported. In particular, the environments of the nearest neighbors of selected species in some germanium based compounds and silver borate glasses are described in terms of radial distribution functions and their main parameters, such as mean distance, variance and asymmetry. The determination of local thermal expansion coefficients in crystalline and non-crystalline solids by temperature dependent studies is shown. (C) 2004 Elsevier B.V. All rights reserved
Size and surface effects in porous silicon studied by X-ray absorption spectroscopy
No full textX-ray absorption spectroscopy (XAS) at the Si-K absorption edge has been applied to study the influence of size and chemical-physical surface effects on the optical properties of porous silicon (PS). The comparison between XAS spectra obtained by monitoring both total electron yield (TEY) and photoluminescence yield (PLY) has allowed us to discriminate between light emitting and non-emitting sites in PS. Thanks to partial PLY-XAS measurements and their site selectivity, we have been able to detect the presence of energy levels inside the band gap of the Si nanocrystallites. The origin of these radiative states is related to the surface passivation, whose influence becomes particularly significant at decreasing sizes
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