Differences between bond lengths in biogenic and geological calcite.

8 pagesInternational audienceWe used high-resolution neutron powder diffraction to accurately measure the atomic positions and bond lengths in biogenic and geological calcite. A special procedure for data analysis was developed in order to take into account the considerable amounts of magnesium present in all the investigated samples. As a result, in biogenic calcite we found some atomic bonds to have significantly different lengths as compared to those in geological calcite, after the contribution of magnesium is accounted for. The maximum effect (elongation up to 0.7%) was found for the C−O bonds. We also analyzed changes in frequencies and spectral widths of normal vibrations of carbonate groups in biogenic calcite (as compared to geological calcite) measured by Raman and Fourier transform IR techniques. Surprisingly, the frequency shifts after subtracting the magnesium contribution are close to zero. At the same time, substantial spectral broadening (up to 1.2%) in biogenic calcite as compared to geological samples was detected. Possible explanations for the experimental findings are discussed

Effect of Temperature on Hybridization and Magnetism in U2Pd2Sn and U2Ni2In.

Abstract not availableJRC.E-Institute for Transuranium Elements (Karlsruhe

Applications of pulsed neutron powder diffraction to actinide elements. [Pu-Al]

We have been using the technique of pulsed neutron powder diffraction to study several problems in the physics and chemistry of the actinide elements. In these elements one often encounters very complex structures resulting from polymorphic transformations presumably induced by the presence of 5f-electrons. For example, at least five distinct structures of plutonium metal are found between room temperature and its melting point of 640/sup 0/C, and two of the structures are monoclinc. The determination of the crystal structure of beta-uranium (tetragonal, 30 atoms per unit cell) which has finnaly been shown to be centrosymmetric, after decades of uncertainty is discussed. Some preliminary results on the structure of alpha-plutonium (which confirm Zachariasen's original determination of the monoclinic structure) are presented. Pu-Al alloys were also studied. 12 refs., 18 figs

Elastoplastic deformation of 316 stainless steel under tensile loading at elevated temperatures

The response of 316 stainless steel has been examined under uniaxial tensile loading during a range of tests carried out between 20 °C and 650 °C. In-situ neutron diffraction was used to measure internal elastic strain in subsets of differently oriented crystallites within the polycrystal aggregate. This allowed the determination of diffraction elastic constants. Further, results have been compared with predictions from a slip-based elasto-plastic self-consistent model. Good agreement is obtained during both conventional slip and when dynamic strain aging (DSA) is evidenced. The quality of agreement was reduced in the higher temperature regime, where it is expected that other mechanisms become active