HIGH DENSITY REFRACTIVE INDEX OF XENON

Nous avons mesuré l'indice de réfraction du Xénon sur un grand domaine de densité (jusqu'à 0,06 mole cm-3. Le facteur de Lorentz-Lorentz décroit fortement quand la densité augmente. Une discussion critique du critère de métallisation d'Herzfeld est faite.The refractive index of Xenon is determined over a large range of density (up to 0,06 mole cm-3). The Lorentz factor decreases strongly with increasing density. A critical discussion on Herzfeld criterion of metallization is made

Application of synchrotron radiation to the study of high-pressure zeolite phases. Case of scolecite

Pressure-induced amorphization has already been observed in some framework silicates like quartz and feldspars. A similar crystal to highly disordered solid transition is reported in this study fora zeolite named scolecite, which is the calcium end-member of the natrolite group. Pressure-induced amorphization is demonstrated using in-situ X-ray diffraction and Raman spectroscopy. This transformation is preceded by polymorphic transitions and occurs below 10 GPa. The crystal-amorphous transition is irreversible and the pressure quenched samples exhibit Raman spectra which bear strong resemblance with alumino-silicate glasses obtained by quenching a liquid. In some cases however, decompression is accompanied by a rim recrystallisation of the sample. The role of shear stresses seems thus to be crucial during the decompression of the samples

VARIATION OF THE OPTICAL ABSORPTION EDGE OF CsI AT HIGH PRESSURE

L'absorption optique d'un monocristal de CsI a été mesurée à température ambiante jusqu'à des pressions de 58 GPa, en utilisant le xénon comme milieux transmetteur de pression. La forme et la position du front d'absorption mesuré diffèrent de façon significative des résultats obtenus sur des poudres, alors que seule la forme diffère des résultats précédents obtenus sur des monocristaux.Optical absorption was measured in single crystals of CsI at room temperature at pressures up to 58 GPa, using xenon as a pressure transmitting medium. The shape and position of the absorption edge are shown to differ significantly from results obtained on powders, whereas only the shape differs from results obtained on single crystals

Phase transitions in yttrium oxide at high pressure studied by Raman spectroscopy

Raman spectra of Y2O3 single crystal were recorded at room temperature in a diamond anvil cell up to a pressure of 22 GPa. Two phase transitions were detected at pressures of 12 and 19 GPa. The different phases were identified with the three structures exhibited by rare-earth (RE) sesquioxides: cubic (C-type), monoclinic (B-type), and rhombohedral (A-type) phases. The first transition corresponds to the C-->B transformation and the second transition to the B-->A transformation. The reversibility of the two phase transitions as well as the evolution of the Y-O bonds under high pressure is discussed. (C) 2000 Elsevier Science Ltd

HIGH PRESSURE XAS ON BROMINE IN THE DISPERSIVE MODE

Br2 a été étudié sous haute pression par spectroscopie d'absorption X en dispersion d'énergie jusqu'à 57,5 GPa. Les avantages du montage en dispersion d'énergie sont présentés. Les résultats de XANES et d'EXAFS sur le brome sont discutés en termes de dépendance avec la pression de la longueur de la liaison moléculaire et du potentiel interstitiel moyen.High pressure energy dispersive x ray absorption spectroscopy has breen performed on Br2 up to 57.5 GPa. The advantages of the energy disprsive set up are presented. The XANES and EXAFS results on bromine are discussed in terme of bond length and average interstitial potential pressure dependence

High pressure structural and magnetic studies of LaFe 12 B 6

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Thermal pressure in the laser-heated diamond-anvil cell: An X-ray diffraction study

Using X-ray diffraction with synchrotron radiation, we have studied the pressure changes induced by laser heating on samples compressed in a diamond-anvil cell. The method has been to compare experimentally observed phase transitions of Mg2SiO4 and SiO2 polymorphs with well-constrained phase diagrams and equations of state reported in the literature. Our results clearly demonstrate an increase of pressure in the laser hot spot with respect to the nominal pressure measured from the ruby fluorescence at room temperature. At 2200 +/- 100 K, for instance, wadsleyite has been synthesized from forsterite at a nominal pressure of 11 Gpa, which is 4 GPa lower than the reported transition pressure. In addition, the measured high-pressure, high-temperature molar volumes of forsterite and wadsleyite appear much smaller than those calculated from available thermoelastic data. Taking into account this pressure increase, we reconcile conflicting experimental determinations of the coesite-stishovite transition made with multi-anvil press and diamond-anvil cell. The pressure change induced by laser heating is a function of the product of the thermoelastic coefficients alpha (thermal expansion) and K-T (bulk modulus) of the sample. We thus stress the need for an internal pressure standard, such as Pt, Au or MgO for determining equations of state and phase equilibria under the P-T conditions prevailing in the Earth's mantle and core

Psiché : A synchrotron tomography beamline at Soleil designed for Material Science

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