Extended X-ray absorption fine structure

About: Extended X-ray absorption fine structure is a research topic. Over the lifetime, 10452 publications have been published within this topic receiving 276744 citations.

Optical Properties of a Magnetic Semiconductor: Chalcopyrite CuFeS 2 . I. Absorption Spectra of CuFeS 2 and Fe-Doped CuAlS 2 and CuGaS 2

Abstract: Optical absorptions have been measured in chalcopyrite, CuFeS 2 , and Fe-doped CuAlS 2 and CuGaS 2 An extra absorption band with two peaks is observed at the low energy region of the absorption edge for Fe-doped CuAlS 2 and CuGaS 2 Energy positions of two absorption peaks are 13 eV and 20 eV for CuAlS 2 and 12 eV and 19 eV for CuGaS 2 , respectively The intensity of this absorption band increases with the increase of doped Fe ions and grows into the absorption edge of CuFeS 2 Its oscillator strength comes up to 7·10 -2 In chalcopyrite photoconductivity is observed and its maximum is just at the same energy region of the absorption edge By comparing our results with those of absorption measurements for Cu- or Fe-doped ZnS it is concluded that this absorption band originates from the charge transfer transitions relating to 3d electron of Fe ions and the absorption edge of CuFeS 2 rises from the band-to-band transition corresponding to this charge transfer transition

Fe site occupancy in magnetite-ulvöspinel solid solutions: A new approach using X-ray magnetic circular dichroism

Abstract: Ordering of Fe{sup 3+} and Fe{sup 2+} between octahedral (Oh) and tetrahedral (Td) sites in synthetic members of the magnetite (Fe{sub 3}O{sub 4}) - ulvoespinel (Fe{sub 2}TiO{sub 4}) solid-solution series was determined using Fe L{sub 2,3}-edge X-ray magnetic circular dichroism (XMCD) coupled with electron microprobe and chemical analysis, Ti L-edge spectroscopy, Fe K-edge EXAFS and XANES, Fe{sub 57} Moessbauer spectroscopy, and unit cell parameters. Microprobe analysis, cell edges and chemical FeO determinations showed that the bulk compositions of the samples were stoichiometric magnetite-ulvoespinel solid-solutions. Surface sensitive XMCD showed that the surfaces of these oxide minerals were more sensitive to redox conditions and some samples required re-equilibration with suitable solid-solid buffers. Detailed site-occupancy analysis of these samples gave XMCD-Fe{sup 2+}/Fe{sup 3+} ratios very close to stoichiometric values. L{sub 2,3}-edge spectroscopy showed that Ti{sup 4+} was restricted to Oh sites. XMCD results showed that significant Fe{sup 2+} only entered Td when the Ti content was > 0.40 apfu while Fe{sup 2+} in Oh increased from 1 a.p.f.u in magnetite to a maximum of {approx}1.4 apfu in USP45. As the Ti content increased from this point, the steady increase in Fe{sup 2+} in Td sites was clearly observable in the XMCD spectra, concurrentmore » with a slow decrease in Fe{sup 2+} in Oh sites. Calculated magnetic moments showed a steady decrease from magnetite (4.06 {mu}{sub B}) to USP45 (1.5 {mu}{sub B}) and then a slower decrease towards the value for ulvoespinel (0 {mu}{sub B}). Two of the synthesized samples were also partially maghemitized by re-equilibrating with an oxidizing Ni-NiO buffer and XMCD showed that Fe{sup 2+} oxidation only occurred at Oh sites, with concomitant vacancy formation restricted to this site. This study shows the advantage of using XMCD as a direct measurement of Fe oxidation state in these complex magnetic spinels. These results can be used to rationalize the magnetic properties of titanomagnetites, and their oxidized titanomaghemitized analogues, in Earth's crustal rocks.« less

Structural Characterization of Solar Cell Prototypes Based on Nanocrystalline TiO2 Anatase Sensitized with Ru Complexes. X-ray Diffraction, XPS, and XAFS Spectroscopy Study

Abstract: Prototype solar cells made up of nanocrystalline anatase supported on a conducting glass and sensitized with two Ru2+ complexes (cis-dithiocyanatobis(2,2‘-bipyridine-4,4‘-dicarboxylate)ruthenium(II), or red dye, and (4,4‘,4‘ ‘-tricarboxy-2,2‘:6‘,2‘ ‘-terpyridine)ruthenium(II), or black dye) are characterized with out-of-plane X-ray diffraction, X-ray photoelectron spectroscopy, and Ti K-edge XAFS (XANES and EXAFS), as well as by powder X-ray diffraction of the pure dyes used for a coating of a TiO2 layer. The data indicate, within the sensitivity limits of the methods applied, that the dyes most probably form (partial coverage) a monolayer at the surface of anatase nanoparticles. Coating with the dyes leads to a stronger distortion of the local environment of Ti atoms in anatase nanoparticles.

Local structural order in the disordered vanadium tetracyanoethylene room-temperature molecule-based magnet

Abstract: We determined the vanadium oxidation state and local coordination environment in disordered samples of magnetic $\mathrm{V}{[\mathrm{TCNE}]}_{x}(x\ensuremath{\approx}2)$ prepared by chemical vapor deposition (CVD). Systematic studies of the x-ray absorption near-edge structure (XANES) in this material and reference compounds show that V ions have a valence state near $2+$. Extended x-ray absorption fine structure (EXAFS) analysis shows that vanadium ions are coordinated by $6.04\ifmmode\pm\else\textpm\fi{}0.25$ nitrogen atoms at a room-temperature average distance of $2.084(5)\mathrm{\AA{}}$. The local environment is well defined with a distribution of V-N bond lengths comparable to that commonly found in ordered compounds. This distribution is mostly vibrational in origin, with static contributions being at least four fold smaller. The small disorder in V-N distances is a consequence of strong binding between $\mathrm{V}$ and TCNE, with an effective local force constant of $k=87\phantom{\rule{0.3em}{0ex}}\mathrm{N}∕\mathrm{m}$. This strong bonding leads to strong nearest neighbor coupling, which for the extended structure of $\mathrm{V}{[\mathrm{TCNE}]}_{x}$ with six $\mathrm{N}$ nearest neighbors results in magnetic ordering above room temperature. The strong V-N bonding explains in part the insoluble nature of this compared to other molecule-based magnets. The room-temperature XANES and EXAFS results for the CVD-prepared samples are compared to those for $\mathrm{V}{[\mathrm{TCNE}]}_{x}$ prepared as a powder from ${\mathrm{CH}}_{2}{\mathrm{Cl}}_{2}$ solvent, which has a similar magnetic ordering temperature but a magnetization that is more strongly temperature dependent. This comparison suggests that coordination of 6 nitrogens around each V(II) with little variation in the V-N distances is important for achieving the high magnetic ordering temperature of $400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ associated with samples made by both the CVD and ${\mathrm{CH}}_{2}{\mathrm{Cl}}_{2}$ solution methods.