[en] We study both theoretically and experimentally the structure of the doped silicon clathrate II Na_xSi₃₄. We find that contrary to published works, the sodium atoms do not retain the T_d symmetry inside the Si₂₈ cages and move about 1 A away from the center of the cage. This displacement, in conjunction with that of a sodium atom in an adjacent Si₂₈ cage, leads to a 'dimerization' of sodium atoms. As a consequence, Rietveld refinements of x-ray diffraction spectra and transport, vibrational, and electronic properties must be revisited

[en] Ti-doped borosilicate and disilicate glasses were irradiated with high-energetic electrons in order to study Ti(IV) reduction processes. Reduction efficiency was actually confirmed by an important Ti(III) EPR signal around g=1.93 in irradiated samples. The Ti(III) amount increases with the irradiation dose. Moreover, the Ti(III) environment depends on the glass composition. Hence, the reduction process efficiency is strongly correlated to the Ti(IV) environment in the pristine glass. The Ti environment was analyzed by Ti K-edge XANES in order to understand the reduction mechanisms. Pre-edge analysis of non-irradiated samples indicates the presence of four, five and six coordinated tetravalent Ti sites in borosilicate glasses, whereas in disilicate the Ti(IV) coordination is a mixture between five-fold and six-fold. In irradiated samples, the Ti pre-edge shows a slight low-energy shift associated to an intensity decrease in disilicate glasses. Correlation between EPR and XANES experiments allows for interpretation of the pre-edge variation in terms of Ti(III) formation. Profiles (μm resolution) were also performed to measure the proportion of Ti(III) on different points across the irradiated DS. No significant difference in XANES spectra was observed suggesting that the production of Ti(III) is homogeneous in the volume

[en] XANES spectra were collected at the Si-, Al-, and Na K-edge in hydrous silicate glasses to understand the effect of water on the local structure around these cations. Around network forming Si and Al, no drastic changes are observed. Around Na, the dissolution of water creates more ordered environments in Al-bearing glasses and less ordered environment in Al-free glasses. Ab-initio XANES calculations were undertaken to understand the structural origins for these features. Based on these results, a bond valence model was refined that considers not only the present XANES experiments and models but also NMR information. The double percolation model refined explains, among others, the explosive properties of water-bearing hydrous melts, at the origin of a number of cataclysmic eruptions in subduction zones

[en] The structure of crystals and melts were obtained at high temperature using X-ray absorption at the Ca K-edge on CaMgSi2O6 (diopside), CaAl2Si2O8 (anorthite), Ca3Al2O6 (C3A) and CaAl2O4 (CA) compositions. Important changes are observed above the liquidus temperature particularly for the C3A composition where all oscillations in the XANES spectra disappear. Important changes in the Ca K-edge XANES are also visible in the pre-edge region, with increasing temperature, for crystalline CaMgSi2O6

[en] Using X-ray absorption at the Al K-edge at high temperature, structural information was determined on Al2O3, CaAl2O4 (CA), Ca3Al2O6 (C3A) and CaAl2Si2O8 (anorthite) in the crystalline and liquid states (2380 K). Important changes are observed for Al2O3 where all oscillation in the XANES spectra disappear above the liquidus temperature. For the three other compositions some modifications of the XANES spectra can be attributed to changes in the Al coordination

[en] Experimental Mg K-edge XANES spectra were obtained for crystals and synthetic glasses. To interpret the experiments, two different ab initio XANES calculation methods were employed. The first one is based on multiple-scattering calculations and muffin-tin potentials (FEFF package). The second uses a plane-wave basis set, norm-conserving pseudo-potentials, periodic boundary conditions. The resulting calculations for reference models with three different Mg coordinations are used to derive the best way to analyze the experimental XANES spectra for the glasses

[en] This collective publication proposes a comprehensive synthesis on instruments, methodology, applications and works in which the interaction between X rays and condensed matter is implemented. This fourth release mainly focuses on the use the synchrotron radiation which is largely used by scientists to explore matter in new directions. After a presentation of X ray sources and of the issue of coherence, the chapters address some specific issues: small angle scattering of X rays, the EXAFS (Extended X-ray Absorption Fine Structure) as a local synchrotron technique, the state of art and evolution of X ray tomography in 2012, structural studies of complex ordered states in material science, studies of relationships between structures and properties in oxide ion based conducting materials, the possible explanation and prediction of recrystallisation from the characterization of the deformed state, and the applications of X rays to the study of metallic contaminants in the environment