22,150 research outputs found
Experimental evidence of thermal fluctuations on the X-ray absorption near-edge structure at the aluminum K-edge
After a review of temperature-dependent experimental x-ray absorption
near-edge structure (XANES) and related theoretical developments, we present
the Al K-edge XANES spectra of corundum and beryl for temperature ranging from
300K to 930K. These experimental results provide a first evidence of the role
of thermal fluctuation in XANES at the Al K-edge especially in the pre-edge
region. The study is carried out by polarized XANES measurements of single
crystals. For any orientation of the sample with respect to the x-ray beam, the
pre-edge peak grows and shifts to lower energy with temperature. In addition
temperature induces modifications in the position and intensities of the main
XANES features. First-principles DFT calculations are performed for both
compounds. They show that the pre-edge peak originates from forbidden 1s to 3s
transitions induced by vibrations. Three existing theoretical models are used
to take vibrations into account in the absorption cross section calculations:
i) an average of the XANES spectra over the thermal displacements of the
absorbing atom around its equilibrium position, ii) a method based on the crude
Born-Oppenheimer approximation where only the initial state is averaged over
thermal displacements, iii) a convolution of the spectra obtained for the atoms
at the equilibrium positions with an approximate phonon spectral function. The
theoretical spectra so obtained permit to qualitatively understand the origin
of the spectral modifications induced by temperature. However the correct
treatment of thermal fluctuation in XANES spectroscopy requires more
sophisticated theoretical tools
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Quantifying Uncertainties in Sequential Chemical Extraction of Soil Phosphorus Using XANES Spectroscopy.
Sequential chemical extraction has been widely used to study soil phosphorus (P) dynamics and inform nutrient management, but its efficacy for assigning P into biologically meaningful pools remains unknown. Here, we evaluated the accuracy of the modified Hedley extraction scheme using P K-edge X-ray absorption near-edge structure (XANES) spectroscopy for nine carbonate-free soil samples with diverse chemical and mineralogical properties resulting from different degrees of soil development. For most samples, the extraction markedly overestimated the pool size of calcium-bound P (Ca-P, extracted by 1 M HCl) due to (1) P redistribution during the alkaline extractions (0.5 M NaHCO3 and then 0.1 M NaOH), creating new Ca-P via formation of Ca phosphates between NaOH-desorbed phosphate and exchangeable Ca2+ and/or (2) dissolution of poorly crystalline Fe and Al oxides by 1 M HCl, releasing P occluded by these oxides into solution. The first mechanism may occur in soils rich in well-crystallized minerals and exchangeable Ca2+ regardless of the presence or absence of CaCO3, whereas the second mechanism likely operates in soils rich in poorly crystalline Fe and Al minerals. The overestimation of Ca-P simultaneously caused underestimation of the pools extracted by the alkaline solutions. Our findings identify key edaphic parameters that remarkably influenced the extractions, which will strengthen our understanding of soil P dynamics using this widely accepted procedure
Multi-scale theoretical approach to X-ray absorption spectra in disordered systems: an application to the study of Zn(II) in water
We develop a multi-scale theoretical approach aimed at calculating from first
principles X-ray absorption spectra of liquid solutions and disordered systems.
We test the method by considering the paradigmatic case of Zn(II) in water
which, besides being relevant in itself, is also of interest for biology. With
the help of classical molecular dynamics simulations we start by producing
bunches of configurations differing for the Zn(II)-water coordination mode.
Different coordination modes are obtained by making use of the so-called dummy
atoms method. From the collected molecular dynamics trajectories, snapshots of
a more manageable subsystem encompassing the metal site and two solvation
layers are cut out. Density functional theory is used to optimize and relax
these reduced system configurations employing a uniform dielectric to mimic the
surrounding bulk liquid water. On the resulting structures, fully quantum
mechanical X-ray absorption spectra calculations are performed by including
core-hole effects and core-level shifts. The proposed approach does not rely on
any guessing or fitting of the force field or of the atomic positions of the
system. The comparison of the theoretically computed spectrum with the
experimental Zn K-edge XANES data unambiguously demonstrates that among the
different a priori possible geometries, Zn(II) in water lives in an octahedral
coordination mode.Comment: 8 pages, 3 figure
Temperature Dependent Polarized XANES Spectra for Zn-doped LSCO system
The cuprates seem to exhibit statistics, dimensionality and phase transitions
in novel ways. The nature of excitations [i.e. quasiparticle or collective],
spin-charge separation, stripes [static and dynamics], inhomogeneities,
psuedogap, effect of impurity dopings [e.g. Zn, Ni] and any other phenomenon in
these materials must be consistently understood. Zn-doped LSCO single crystal
were grown by TSFZ technique. Temperature dependent Polarized XANES [near edge
local structure] spectra were measured at the BL13-B1 [Photon Factory] in the
Flourescence mode from 10 K to 300 K. Since both stripes and nonmagnetic Zn
impurities substituted for Cu give rise to inhomogeneous charge and spin
distribution it is interesting to understand the interplay of Zn impurities and
stripes. To understand these points we have used Zn-doping and some of the
results obtained are as follows: The spectra show a strong dependence with
respect to the polarization angle, , as is evident at any temperature
by comparing the spectra where the electric field vector is parallel with
ab-plane to the one where it is parallel to the c-axis. By using the XANES
[temperature] difference spectra we have determined T* [experimentally we find,
T* 160-170 K] for this sample. The XANES difference spectra shows
that the changes in XANES features are larger in the ab-plane than the c-axis,
this trend is expected since zinc is doped in the ab-plane at the copper site.
Our study also complements the results in literature namely that zinc doping
does not affect the c-axis transport.Comment: To appear in Physica C [ISS2001 Special Issue], related talk
presented at ISS2001 as PC-16, 10 pages revtex and 7 pages of figures (pdf
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Interpreting the Operando XANES of Surface-Supported Subnanometer Clusters: When Fluxionality, Oxidation State, and Size Effect Fight
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