Actinide speciation in glass leach-layers: An EXAFS study

Uranium L{sub 3} X-ray absorption data were obtained from two borosilicate glasses, which are considered as models for radioactive wasteforms, both before and after leaching. Surface sensitivity to uranium speciation was attained by a novel application of simultaneous fluorescence and electron-yield detection. Changes in speciation are clearly discernible, from U(VI) in the bulk to (UO{sub 2}){sup 2+}-uranyl in the leach layer. The leach-layer uranium concentration variations with leaching times are also determined from the data

Past Achievements and Future Challenges in 3D Photonic Metamaterials

Photonic metamaterials are man-made structures composed of tailored micro- or nanostructured metallo-dielectric sub-wavelength building blocks that are densely packed into an effective material. This deceptively simple, yet powerful, truly revolutionary concept allows for achieving novel, unusual, and sometimes even unheard-of optical properties, such as magnetism at optical frequencies, negative refractive indices, large positive refractive indices, zero reflection via impedance matching, perfect absorption, giant circular dichroism, or enhanced nonlinear optical properties. Possible applications of metamaterials comprise ultrahigh-resolution imaging systems, compact polarization optics, and cloaking devices. This review describes the experimental progress recently made fabricating three-dimensional metamaterial structures and discusses some remaining future challenges

DETERMINATION OF STRUCTURE AND VALENCE IN VANADIUM BRONZE BY X-RAY ABSORPTION SPECTROSCOPY

We used XANES to determine coordination geometry and charge state in a series of mixed valence vanadium bronzes, NaXV2O5 where x = 0.25, 0.40, and 0.75. Simple chemical arguments suggest that each additional positive charge added to the V205 lattice will switch one V5+ to V4+. Our results sugaest that this is generally true. The energy position and splitting of the V K-edge prepeak, 1s-to-3d, was a sensitive indicator of V valence(s). The intensity of the brepeak decreased as the first neighbor bonds increased in length

Investigation of annealed and metamict pyrochlore minerals by x-ray absorption spectroscopy. [Synthetic pyrochlores with general formula (RE)/sub 2/Ti/sub 2/O/sub 7/, RE = Er, Y/sub 2/, Gd/sub 2/, Dy, La]

Materials of the pyrochlore structure type exhibit a variety of interesting properties including phases capable of acting as hosts for actinides in radioactive wastes. Studies of curium doped gadolinium titanate phases (Gd/sub 2/Ti/sub 2/O/sub 7/) have been made which showed that the radiation damage ingrowth followed an exponential relationship. For the study reported here a series of synthetic pyrochlores were produced having the titanate phase with the general formula (RE)/sub 2/Ti/sub 2/O/sub 7/, RE = Er, Y/sub 2/, Gd/sub 2/, Dy, La. Additionally a set of metamict (radiation damaged) pyrochlores was examined in both a natural and post temperature annealed state. Experiments were conducted on these samples using the Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) techniques. In summary, these studies show that in pyrochlore structure types the Ti-O cage undergoes changes due to radiation damage. The individual Ti-O bonds become more disordered which leads to a loss of short and long range order and, ultimately, to expansion of the bulk material. 2 refs., 2 figs