D. Raoux

X-ray absorption spectroscopy studies of ionic association in aqueous solutions of zinc bromide from normal to critical conditions

Ion-pairing and dehydration phenomena occurring in ZnBr2 aqueous solutions from normal to critical T, P conditions were investigated by x-ray absorption spectroscopy. The respective influences of temperature, pressure, and concentration were studied. The evolution of the density of solute ions, probed by the height of the absorption edge, allowed us to get information on phase diagrams and salt precipitation. The average structural evolution deduced from extended x-ray absorption fine structure was related to the formation of complexes identified from x-ray absorption near edge structure analysis. Consequently, in noncritical conditions, an increase of temperature or concentration produces dehydration and ion-pairing, while a rise of pressure destroys the ion-pairs. In contrast, concentration and pressure have weaker effects on the local order in high P, T conditions. Moreover, ion pairing formation is found not to be specifically enhanced when the fluid is close to supercritical conditions as it also occ...

Structure of aqueous ZnBr2 solution probed by x-ray absorption spectroscopy in normal and hydrothermal conditions

Local-order evolution around ions in aqueous solutions has been investigated between normal and hydrothermal conditions. The behavior of cations and anions in aqueous ZnBr2 solution were studied by performing x-ray absorption spectroscopy experiments at both Br and Zn edges. Extended x-ray absorption fine structure analyses are made on account of anharmonic treatment and multiple scattering contributions involving H atoms at the Br edge. The extended x-ray absorption fine structure results are coupled to x-ray absorption near-edge structure simulations in order to identify the complexes formed under normal and hydrothermal conditions. It appears that both Zn and Br ions are largely hydrated under normal conditions and that Zn–Br pairs are formed in hydrothermal conditions. This is related to an octahedral-to-tetrahedral evolution of the Zn local environment, the majority of Zn atoms being surrounded by water octahedra in normal conditions and by distorted tetrahedra involving Br and O atoms in hydrotherma...

Local and medium range order in germanium chalcogenide glasses

The results of EXAFS measurements on GeXx X = S, Se and 1.5 ⩽ x ⩽15) binary are reviewed. This structural study is completed by anomalous wide angle X-ray scattering experiments for vitreous GeSe3 and GeSe5. Thus, the local and the medium range order of these glasses have been charaterized.

An apparatus for temperature-dependent soft X-ray resonant magnetic scattering.

Interest in the use of soft X-ray resonant magnetic scattering techniques to probe the distribution of magnetic moments in thin films has exploded during the last few years. In this paper a novel diffractometer devoted to temperature-dependent soft X-ray resonant scattering is described. The principal features of the diffractometer are presented and illustrated through experiments performed at LURE during the commissioning phase.

Local atomic order in amorphous III-V semiconductors by EXAFS and X-ray anomalous scattering

Abstract Short range ordering is compared in three amorphous III-V compounds: InP, GaP and GaAs. For a-InP and a-GaP, a high degree of chemical disorder is evident from an extended X-ray absorption fine structure (EXAFS) study. In the case of a-GaAs, the three partial pair distribution functions deduced from anomalous X-ray diffraction experiments indicate strong chemical ordering. A joint analysis of EXAFS and anomalous X-ray diffraction data reveals the presence of threefold coordinated As which is shown to play an important role in the relaxation of the network as demonstrated by the anomalous X-ray diffraction measurements.

Correlation between density variation and electrical conductivity in supercritical selenium probed by small angle x-ray scattering

Small angle x-ray scattering experiments were performed in order to investigate the semiconductor–metal and metal–nonmetal transitions that take place in supercritical fluid selenium. At the semiconductor to metal transition, we do not observe any density fluctuations indicating the homogeneity of the fluid at this scale. On the other hand, we observe the onset of density fluctuations and a sharp density variation in the pressure-temperature range of the metal–nonmetal transition. We show that the supercritical domain can be separated into two regions: a crossover takes place between a liquidlike density region and a gaslike one. The onset of density fluctuations, i.e., the development of rare and dense domains, forbids any percolation and induces the loss of the metallic character.

X-ray absorption fine structure studies on expanded fluid Se: from liquid to dense vapor

X-ray absorption fine structure (XAFS) measurements for expanded fluid Se at high temperatures and pressures up to 1650°C and 600 bar have been carried out to investigate the change of local structure with volume expansion. The electronic property of fluid Se changes from a semiconducting one to metallic with increasing temperature and pressure. When the volume is further expanded, the fluid becomes non-metallic vapor. XAFS spectra of the non-metallic vapor shows that the vapor rarer than the critical density consists mainly of Se dimers. When the volume contracts exceeding the critical density, the first peak in the radial distribution function is shifted towards larger distance and is increasingly asymmetrical.

Structural characterization of germanium selenide glasses by differential anomalous X-ray scattering

Abstract The GeSe 3 and GeSe 5 binary glasses have been measured by anomalous wide angle X-ray scattering. The structure of g-GeSe 3 is found to be mainly based on that of the crystalline GeSe 2 compound with the presence of edge- and corner-sharing GeSe 4 tetrahedra. For g-GeSe 5 , the glassy structure is built up from isolated GeSe 4 tetrahedra interconnected by short chains of chalcogen atoms in agreement with the ‘crossing chain model’. Finally, the ‘first sharp diffraction peak’, encountered on the total structure factors around 1 A −1 , seems to be due to Ge … Ge correlations in the intermediate range order (6–7 A).

Structural study of coordination defects in amorphous GaAs by a combination of EXAFS and X-ray anomalous scattering experiments

The short range order around Ga and As sites in a-GaAs films has been investigated by EXAFS and anomalous X-ray scattering. The three partial pair distribution functions indicate a strong chemical ordering. The presence of three-fold coordinated As is shown to play an important role in the relaxation of the network.

Differential anomalous X-ray scattering studies of amorphous Cd59As41 and Cd26As74

Abstract The local structure of amorphous cadmium arsenide semiconducting films has been studied by differential anomalous X-ray scattering. Intensity measurements were carried out on two samples, containing 41 and 74 at.% As, in the vicinity of the absorption K edges of both constituents using synchrotron radiation. The computational procedure, similar to that proposed by Warren for an amorphous sample with more than one kind of atom, was applied to obtain the structural parameters from the experimental data. It has been found that atoms in the amorphous CdAs films remain almost tetrahedrally coordinated and that the investigated films are chemically ordered. The structural changes going from cadmium- to arsenic-rich composition have been revealed. The differential anomalous X-ray scattering technique proved to be effective, providing the evidence for the CdCd and AsAs near neighbour correlations in Cd 59 As 41 and Cd 26 As 74 , respectively. The simulations of the differential radial distribution functions have shown that for the amorphous film containing 41 at.% As the distorted tetrahedral structure, intermediate between the CdAs and Si III type structures, is adequate to account for the experimental data. At 74 at.% As, the atomic arrangement can be described satisfactorily by the structural model based on the tetragonal CdAs 2 structure. The structural parameters obtained from the present study and those previously derived using the extended X-ray absorption fine structure and conventional large angle X-ray scattering techniques are compared.