Near-edge x-ray-absorption fine structure of Pb: A comparison of theory and experiment

Newville, Matthew; Livins, Peteris; Yacoby, Y.; Rehr, J. J.; Stern, E. A.

doi:10.1103/physrevb.47.14126

Cited by 880 publications

(577 citation statements)

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“…Characterization using a JEOL 4000FX transmission electron microscope with a point resolution of 0.2 nm at an acceleration voltage of 400 kV confirmed that Pt was present in very high dispersion with a cluster size on the order of 1 nm, but the clusters were labile and rearranged under the influence of the energetic electron beam, preventing a more precise size or structure determination. In such cases extended x-ray absorption fine structure (EXAFS) spectroscopy provides detailed information by analysis of the first nearest neighbor shell [10]. Measurements were conducted at the Pt L 3 edge at the beam line A1 at HASYLAB/Desy (Hamburg) at liquid nitrogen temperature to reduce information loss due to thermal distortion by cutoff of the low k range [11].…”

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confidence: 99%

Structure and Magnetization of Small Monodisperse Platinum Clusters

et al. 2006

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Magnetization measurements of well-characterized monodisperse Pt clusters consisting of 13 2 atoms in a zeolite confirm the predicted extraordinary magnetic polarization with up to 8 unpaired electrons on a cluster, corresponding to a magnetic moment of 0:65 5 B per atom. The effect is partly quenched by hydrogen chemisorption. The study provides insight into the electronic structure of the cluster and is fundamental for an understanding of how magnetism develops in small clusters. DOI: 10.1103/PhysRevLett.97.253401 PACS numbers: 36.40.Cg, 36.40.Mr, 61.10.Ht, 61.46.Bc While enhanced magnetism in clusters of elements that are ferromagnetic as bulk solids is well known and has been demonstrated in Stern-Gerlach deflection experiments [1], theoretical studies predicted high-spin ground states for clusters of up to 13 atoms also of Pd and Pt, i.e., of elements which do not show magnetic ordering in the bulk [2 -4]. Nanoparticles comprising several hundred atoms of Au, Pd, and Pt embedded in a polymer revealed magnetic moments corresponding to several unpaired electron spins per entire particle [5,6], but no experimental studies exist for smaller clusters. The electronic structure of small metal clusters is a strong function of size [7]. This is because the average energy spacing of electronic states at the Fermi level, the Kubo gap N, scales as =N, where N is the number of atoms in the cluster. is the width of the valence band which is only a weak function of size and assumes typically values on the order of a few eV. For a given value of N the cluster undergoes a thermal transition from an insulator to a semiconductor at the temperature where the valence electrons can overcome the Kubo gap, providing the band is incompletely filled with electrons. Alternatively, for a given temperature this transition can occur as a function of N. Furthermore, when N is sufficiently small, electrons will occupy the levels with parallel spins, following Hund's rule [3], which leads to a high spin state that is accompanied with a high magnetic moment. But when N exceeds the spin pairing energy the system switches to a low spin state. On this basis, a small cluster could be expected to be in a low spin state, because of its relatively large value of N. However, the picture is complicated further by symmetry. A high symmetry may lead to a highly degenerate HOMO (highest occupied molecular orbital), favoring high spin states. When the symmetry is broken, degeneracy is lifted and the ground state may be one of lower spin, depending on the extent to which the symmetry is broken. Additional complications can arise in applied magnetic fields when the Zeeman or spin-orbit energies compete with level splittings at the Fermi level, leading to magnetic field and also temperature dependent spin states. Furthermore, when the surface is capped, each chemisorbed species engages one of the potential conduction electrons of the cluster and pins them in a localized chemical bond.It is vis-à-vis this complex background that the present results with...

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Structure and Magnetization of Small Monodisperse Platinum Clusters

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“…The background contribution to the EXAFS spectra was removed using AUTOBK, a program developed by NEWVILLE et al (1993) that minimizes R-space values in low k-space.…”

Section: Exafs Data Analysesmentioning

confidence: 99%

Uranium co-precipitation with iron oxide minerals

Duff

Coughlin

Hunter

2002

Geochimica et Cosmochimica Acta

425

266

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ABSTRACT-In oxidizing environments, the toxic and radioactive element uranium (U) is most soluble and mobile in the hexavalent oxidation state. Sorption of U(VI) on Fe-oxides minerals [such as hematite (α-Fe 2 O 3 ) and goethite (α-FeOOH)] and occlusion of U(VI) by Feoxide coatings are processes that can retard U transport in environments. In aged Ucontaminated geologic materials, the transport and the biological availability of U toward reduction may be limited by co-precipitation with Fe-oxide minerals. These processes also affect the biological availability of U(VI) species toward reduction and precipitation as the less soluble U(IV) species by metal-reducing bacteria.To examine the dynamics of interactions between U(VI) and Fe oxides during crystallization, indicated that almost all of the Fe in these solids was crystalline and that most of the U was associated with these crystalline Fe oxides. X-ray diffraction and Fourier-transform infrared (FT-IR) spectroscopic studies indicate that hematite formation is preferred over that of goethite when the amount of U in the Fe-oxides exceeds 1 mol % U (~4 wt % U). FT-IR and room temperature continuous wave luminescence spectroscopic studies with unleached U/Fe solids indicate a relationship between the mol % U in the Fe oxide and the intensity or existence of the spectra features that can be assigned to UO 2 2+ species (such as the IR asymmetric υ 3 stretch for O=U=O for uranyl). These spectral features were undetectable in carbonate-or oxalate-leached Molecular modeling studies reveal that U 6+ species could bond with O atoms from distorted Fe octahedra in the hematite structure with an environment that is consistent with the results of the XAFS. The results provide compelling evidence of U incorporation within the hematite structure.

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“…The analysis of the observed absorption spectra was done following the standard FEFF procedure [6,7] including the background treatment with AUTOBK [8].…”

Section: Methodsmentioning

confidence: 99%

Local structure around Mn in Mn containing ZnO nanocrystals

Koteski¹,

Zhou

Farangis

et al. 2007

Physica Status Solidi (b)

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Published online zzz PACS 61.72. Vv, 61.46.Hk, 61.10.Ht Among the diluted magnetic semiconductors of current interest, especially ZnO is experiencing a renaissance with the incorporation of Mn because of the recent predictions from theory. We have obtained local structure information from X-ray Absorption Spectroscopy (XAS) complementing previous electron paramagnetic resonance (EPR) studies on Mn in ZnO nanocrystals and Zn(OH) 2 . Mn is found to be incorporated substitutionally into the ZnO nanocrystals with a sizeable distortion as compared to the regular ZnO lattice structure. Depending on the preparation of the nanocrystals Mn is found in the ZnO core as well as in a Zn(OH) 2 surface shell with a fraction which decreases upon annealing as the shell disintegrates forming additional ZnO. No indications for Mn clusters were found.

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Near-edge x-ray-absorption fine structure of Pb: A comparison of theory and experiment

Cited by 880 publications

References 10 publications

Structure and Magnetization of Small Monodisperse Platinum Clusters

Structure and Magnetization of Small Monodisperse Platinum Clusters

Uranium co-precipitation with iron oxide minerals

Local structure around Mn in Mn containing ZnO nanocrystals

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