Crystal structure of synthetic Mg3Cr2Si3O12, the high-pressure Cr end-member of the knorringite-pyrope garnet series

Knorringite, the Cr-end-member of the pyrope garnet series (Nixon et al. 1968), often occur in high proportions in kimberlite garnets and is thus used for tracing high-pressure deep-earth conditions favorable to the formation of diamonds, in which knorringite-rich garnet can occur as inclusions. However, although the synthesis of knorringite is reported in the literature (Ringwood 1977; Irifune et al. 1982; Taran et al. 2004), the structure of the pure end-member has not been yet determined from experimental data. In this study, the crystal structure of knorringite, Mg3Cr2(SiO4)3, has been refined from high resolution synchrotron X-ray powder diffraction data recorded under ambient conditions on a polycrystalline sample synthesized at 12 GPa in a multi-anvil apparatus. The structure is cubic, space group Ia-3d, a = 11.5935(1), V = 1558.27(4) {\AA}3, dcalc = 3.97 g.cm-3. The Cr-O distance of 1.957(2) {\AA} is consistent with EXAFS results on the same sample. This short distance indicates a substantial compression of the CrO6 octahedron, compared to ambient pressure Cr3+-minerals such as uvarovite ( = 1.99 {\AA}, Andrut and Wildner 2002). Our experimental results thus confirm early empirical predictions based on series of high-pressure Cr-garnet end-members (Fursenko 1981), showing that the values of the Cr-O distance and the Cr-O-Si angle decrease with the augmentation of pressure and with the diminution of the size of the divalent cation

Etude de la conformation du nonamethylimidodiphosphoramide par diffraction des rayons X a l'etat libre et complexe

SIGLET 55377 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Increase of CYP1B1 Transcription in Human Keratinocytes and HaCaT Cells after UV-B Exposure

International audienceNonmelanoma skin cancers represent the most common malignant neoplasms in humans. UV-B play a major role in the etiology of these tumors, but exposure to environmental procarcinogens is also involved. CYP catalyzes numerous chemical carcinogen bio-activations and effects of UV-B on their expression are poorly understood. The aim of this study was to explore the molecular events involved in the induction of CYP1B1, a major cutaneous CYP, by UV-B. Our results demonstrated that unique UV-B exposure (20 mJ/cm 2) increases human CYP1B1 transcript in primary keratinocytes and HaCaT cell cultures. Among 20 human samples studied, we observed a large interindividual variability of CYP1B1 mRNA induction (1.1-to 4.5-fold). Pretreatment with an antioxidant, N-acetylcysteine, repressed CYP1B1 increase, suggesting the involvement of UV-B photoproducts.-Amanitin inhibition studies and CAT assays demonstrated that CYP1B1 mRNA induction is associated with a transcriptional activation of its expression.-Naphthoflavone inhibition studies and CAT assays performed after directed mutagenesis of xenobiotic responsive element sites showed the involvement of Ah receptor. Taken together, these data demonstrated that UV-B induces CYP1B1 gene expression after an activation of its transcription, which involves Ah receptor

Phase diagram and order-disorder transitions in Y0.9Gd0.1Fe2Hx hydrides (x ≥ 2.9)

International audienceY0.9Gd0.1Fe2, which crystallize in a C15 cubic structure, can absorb up to 5 H/f.u. and its pressure-composition isotherm displays a multiplateau behavior related to the existence of several hydrides with different crystal structures. At room temperature Y0.9Gd0.1Fe2Hx hydrides (2.9 ≤ x ≤ 5) crystallize in three phases with cubic structure (C1, C2 and C3), two phases with monoclinic structures (M1 and M2), and one phase with orthorhombic structure (O), with the following sequence for increasing H concentration: C1, M1, C2, M2, C3, O. Each phase exists as single phase within a H homogeneity range, and they are separated from each other by two-phase domains. The reductions of crystal symmetry are related to various hydrogen orders into interstitial sites. Weak superstructure peaks were indexed by doubling the cubic cell parameter of the cubic C2 phase. Upon heating, the monoclinic M1 and M2 and the cubic C2 phases undergo order-disorder (O-D) transitions toward a disordered cubic structure CDis. These O-D transitions are reversible with thermal hysteresis effects. The cubic C3 and orthorhombic O phases transform into a disordered cubic phasecompanied by H desorption

Correlation among Structure, Microstructure, and Electrochemical Properties of NiAl-CO3 Layered Double Hydroxide Thin Films

International audienceWe studied a series of NiAl-CO3 layered double hydroxides (LDHs) of various degrees of crystallinity prepared by a glycine-assisted hydrothermal method. The structures and the microstructures were determined by Rietveld refinement with a spherical harmonic-implemented algorithm using high-resolution synchrotron powder X-ray diffraction (XRD) data. These XRD results combined with transmission electron microscopy (TEM) observations indicate that the broadening of 00l diffraction lines is mainly due to size effects, while both size and strain effects contribute to the anisotropic broadening of the other hkl reflections. The in-plane TEM dimensions of LDH platelets are found larger than the coherent lengths in the 110 direction, indicating a noncoherent coalescence of domains during crystal growth. The DIFFaX program was also used to model structural defects, i.e., CO32-/SO42- interstratification and intergrowth between rhombohedral 3R(1) and hexagonal 2H(1) polytypes. Furthermore, the distribution of the cations within the hydroxide layers has been determined by the atomic pair distribution function technique, indicating a disordered distribution of the cation in all samples with absolutely identical cationic coordination spheres. The electrochemical characterization of these samples as thin-film modified electrodes, by means of cyclic voltammetry and electrochemical impedance spectroscopy measurements, reveals complex behaviors which are the result of competing effects between the coherent domain size and the particle size, the aggregation state of LDH particles, the Ni bulk concentration, and the presence of structural defects. Remarkably, the presence of the 2H(1) stacking motifs in the 3R(1) LDH matrix results in an increased electrochemical signal. Either the location of metal cations exactly on top of the others in the 2H(1) polytype or the defects associated with the intergrowth of 2H(1)-3R(1) polytypes may be responsible for the enhanced electroactivity of Ni centers

Etude in situ par DRX sur poudre et RMN du solide de catalyseurs industriels vanadophosphates : focus sur la phase ω-VOPO4

International audienc

High-pressure polymorph of LuFe2O4 with room-temperature antiferromagnetic order

International audienceBranded for its potential electronic ferroelectricity, charge-ordered LuFe2O4 has a layered Fe triangular framework, whose topology is a source of degeneracy, both at the charge and spin levels. Here we present an in-depth characterization of LuFe2O4-hp, the high-pressure (hp) polymorph of LuFe2O4, using electron, x-ray, and neutron diffraction, combined with transport and magnetization measurements. We show that LuFe2O4-hp is characterized by a misfit-related monoclinic structure, accommodating a buckled triangular [Lu](infinity) layer and two shifted adjacent rectangular [Fe](infinity) planes belonging to a distorted rock salt-type layer. The release of the geometric frustration of the Fe magnetic lattice in the hp form leads to collinear antiferromagnetic ordering at T-N = 380 K. Possible coexistence of charge and magnetic orders in this material opens research pathways for the design of tunable multifunctional devices using high-pressure techniques