Cerium LIII-Edge X-ray Absorption Study of the CeₓFe\u3csub\u3e4-y\u3c/sub\u3eCo\u3csub\u3ey\u3c/sub\u3eSb₁₂ Skutterudites

The cerium LIII-edge x-ray absorption near-edge spectra of the CexFe4-yCoySb12 compounds have been obtained at 295 K and unambiguously indicate that cerium is in the 4f1 electronic ground state for all values of 0.22≤x≤0.98 and 0.0≤y≤3.5. This stable trivalent state of cerium is in agreement with the proposed (CeFe4Sb12) 1-α(Co4Sb12)α, solid solution structure, in which the cerium atoms are always surrounded by twelve antimony first neighbors and six iron second neighbors, the observed magnetic properties of CeFe4Sb12 and Ce0.9Fe3CoSb12, and the electronic structure of CeFe4Sb12 obtained from band-structure calculations

Dispositif d'étude des surfaces solides ou liquides par réflexion spéculaire des rayons X

On décrit un dispositif permettant de tester des modèles de profils de densité au voisinage d'une interface solide (ou liquide) -vapeur. Le principe de la méthode consiste à mesurer, en fonction de l'angle d'attaque (de l'ordre de quelques mrad), l'intensité réfléchie spéculairement par une surface plane lorsque celle-ci reçoit un faisceau de rayons X. On donne quelques exemples d'application

Heteroepitaxy of Chemically Deposited CdS on Large Lattice Mismatched (111) GaP

Heteroepitaxial CdS films (60 nm) have been deposited at $\rm 85\ ^\circ C$ on GaP from aqueous solutions of ammonia containing cadmium ions and thiourea precursors. Structural characterizations have been performed with RHEED, five circle XRD and TEM observations of cross sections. The large lattice mismatch results in the formation of a large density of stacking faults in the CdS leading to a polytype structure with an equal proportion of cubic and hexagonal modifications. Changes in the lattice parameter are observed in the CdS near the interface.

Diffractomètre θ-θ à axe horizontal pour étude aux rayons X jusqu'à très basse température

An automatic θ-θ diffractometer has been built in which X-ray source and detector rotate about a fixed horizontal specimen, in such way that measurements can be carried at equally spaced values of the scattering vector modulus. This especially designed diffractometer for the study of liquids and glassy materials, is provided with a cryostat allowing diffraction measurements down to very low temperatures. Some results are reported concerning liquid gallium at 323 and 203 K.On a réalisé un diffractomètre automatique θ-θ où tube à rayons X et compteur se déplacent autour d'un échantillon fixe, horizontal, de façon à faire des mesures sensiblement équiréparties dans l'espace réciproque. Ce diffractomètre, particulièrement adapté à l'étude des liquides et des substances vitrifiables, a été équipé d'un cryostat qui permet d'effectuer des mesures de diffraction jusqu'à très basse température. Quelques résultats concernant le gallium liquide à 323 et 203 K sont donnés.Bizid Abdelmalek, Bosio Louis, Cortès Robert, Defrain André. Diffractomètre θ-θ à axe horizontal pour étude aux rayons X jusqu'à très basse température. In: Bulletin de la Société française de Minéralogie et de Cristallographie, volume 100, 1, 1977. pp. 9-14

X-ray and neutron scattering studies of liquid formic acid DCOOD at various temperatures and under pressure

A structural investigation of fully deuterated liquid formic acid was performed by neutron scattering at pressure up to 3 kbar. The molecular pair correlation function was also deduced from x-ray study of DCOOD at ambient pressure and at 294 K. The results could be explained in terms of an open-chain structure with only two H bonds per molecule. The mean O⋯O distance is about 2.72 Å. The effect of both temperature and pressure on the hydrogen bond network is examined

Electrocrystallization of Epitaxial Zinc Oxide onto Gallium Nitride

International audienceWe have studied in detail the structure and the nucleation and growth process of electrodeposited epitaxial hexagonal ZnO on GaN(0001). The crystallites grow with the c-axis perpendicular to substrate and the in-plane relationship is ZnO[100] || GaN[100]. We show that the deposit aspect progressively changes with the deposition time from isolated dispersed dots toward fully covering flat zinc oxide single crystals. A simple model is presented which explains these morphological changes. The nucleation is instantaneous and the growth rate is controlled by the crystal surface reactions. The velocity of crystal growth is found dependent on the crystallographic face of the grain: it is approximately five times faster on the {0001} planes than on the {101̄0} and {011̄0} families of planes. We show that nucleation problems are encountered with high-quality GaN presenting a low surface defect density. The difficulty has been overcome by chemically treating the substrate with an aqueous ammonium hydroxide solution prior to the deposition. This treatment allows the nucleation of zinc oxide but has a detrimental effect on the deposit “mosaicity” which is better in the case of GaN with high defect density

Introduction of Cobalt Ions in γ-Fe 2 O 3 Nanoparticles by Direct Coprecipitation or Postsynthesis Adsorption: Dopant Localization and Magnetic Anisotropy

International audienceThe influence of cobalt doping on the magnetic anisotropy of γ-Fe2O3 nanoparticles has been investigated using two different approaches: (i) simultaneous precipitation of Fe2+, Fe3+, and Co2+ precursors in water and (ii) adsorption of Co2+ ions onto the surface of preformed iron oxide particles followed by diffusion in the solid phase upon heat treatment. The incorporation of small amounts of Co dopants, less than 1 at %, was monitored by magnetization measurements combined with X-ray absorption spectroscopy experiments at the Co K-edge. These latter measurements were carried out in fluorescence mode using a crystal analyzer spectrometer for enhanced sensitivity. Analyses of the X-ray absorption fine structures allowed for unraveling the differences in local atomic structure and valence state of Co in the two series of samples. A thermally activated diffusion in the spinel lattice was observed in the 250−300 °C range, leading to a substantial increase in magnetocrystalline anisotropy. At higher annealing temperature, magnetic anisotropy was still found to increase due to an enhanced surface contribution associated with the dehydroxylation of terminal Fe atoms. This study not only provides direct correlations between magnetic anisotropy and dopant localization in Co-doped γ-Fe2O3 but also demonstrates for the first time that simultaneous coprecipitation of Fe2+, Fe3+, and Co2+ may actually lead to heterogeneous doping, with a significant part of the Co dopants adsorbed at the particle surface