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.

X-ray-absorption studies of zirconia polymorphs. I. Characteristic local structures.

Abstract: Characteristic extended x-ray-absorption fine structure and x-ray-absorption near-edge structure spectra of zirconia polymorphs (monoclinic, orthorhombic, tetragonal, and cubic) have been identified at 10 K. For the host cations, these spectra are phase dependent but dopant independent because the dopant perturbation is short ranged. In the tetragonal structure, the first Zr-O shell is bifurcated into two tetrahedra, with outer O ions loosely bound and severely distorted. Although the tetragonal Zr cation network is nearly fcc and higher-order Zr-Zr coordinated shells are resolvable at distances up to 9 \AA{}, other evidence points to incoherent-distortion reflecting phase instability. In monoclinic, orthorhombic, and cubic structures, a similar Zr-O polyhedron with sevenfold coordination and a comparable bond length is observed, along with a large distortion or splitting of the next-nearest-neighbor Zr-Zr shell. Progressive departure from the ideal fluorite structure by reducing the Zr-O coordination and dilating or distorting the local Zr-Zr network is seen to be correlated to increased phase stability but not to long-range crystal symmetry.

Investigation of the Structure of Microporous Ti-Si Mixed Oxides by X-ray, UV Reflectance, FT-Raman, and FT-IR Spectroscopies

Abstract: A series of high surface area, microporous, Ti-Si mixed oxides having a wide range of elemental compositions was prepared by a sol-gel method. Since the materials were noncrystalline, X-ray absorption spectroscopy at the Ti K edge was used to probe the first-shell coordination environment around Ti. Incorporation of Si into the mixed oxide shortened the average Ti-O bond distance derived from EXAFS and disrupted the normal octahedral coordination of pure titania. The shortest Ti-O bond distance was 1.82 [angstrom] and was attributed to Ti primarily residing in tetrahedral sites, which was also indicated by a significant pre-edge peak present in Ti K edge XANES. Shifts in the UV absorption edges for the mixed oxides were attributed to titania domain sizes decreasing with increasing Si content in accord with the well-known quantum size effect. The smallest domain size was estimated as less than 1 nm for a sample with a Ti:Si atomic ratio of 1:8. Lattice vibrations in the materials were probed with FT-Raman and FT-IR absorption spectroscopies, and the results were consistent with a shortening of the Ti-O bond and the formation of Ti-O-Si linkages in the mixed oxides. In situ experiments showed that adsorbed water profoundly affects the localmore » environment around surface Ti atoms. 44 refs., 11 figs., 3 tabs.« less

Determination of visible near-IR absorption coefficients of mammalian fat using time- and spatially resolved diffuse reflectance and transmission spectroscopy.

Abstract: In-vivo optical spectroscopy and the determination of tissue absorption and scattering properties have a central role in the development of novel optical diagnostic and therapeutic modalities in medicine. A number of techniques are available for the optical characterization of tissue in the visible near-IR region of the spectrum. An important consideration for many of these techniques is the reliability of the absorption spectrum of the various constituents of tissue. The availability of accurate absorption spectra in the range 600 to 1100 nm may allow for the determination of the concentration of key tissue constituents such as oxy- and deoxy-hemoglobin, water, and lipids. The objective of the current study is the determination of a reliable absorption spectrum of lipid(s) that can be used for component analysis of in-vivo spectra. We report the absorption spectrum of a clear purified oil obtained from pig lard. In the liquid phase above 36°C, the oil is transparent and thus suitable for collimated transmission measurements. At room temperature, the oil is a solid grease that is highly scattering. The absorption and scattering properties in this solid phase are measured using time- and spatially resolved diffuse reflectance spectroscopy. Using these three independent measurement techniques, we have determined an accurate estimate for the absorption spectrum of mammalian fat.