ETUDE SOUS PRESSION ET TEMPERATURE DES TRANSITIONS DE PHASE DE KNBO3 PAR SPECTROSCOPIE RAMAN, DIFFRACTIONS X ET NEUTRONS

PARIS-BIUSJ-Physique recherche (751052113) / SudocCentre Technique Livre Ens. Sup. (774682301) / SudocSudocFranceF

Réactivité chimique de composés moléculaires sous l'effet de très haute pression et de la radiation laser

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Détermination calorimétrique de l’entropie dans une onde de choc

On propose une nouvelle méthode calorimétrique de mesure au tube à choc. Elle est fondée sur le fait que l’entropie acquise par un fluide soumis à l’action d’une onde de choc, est conservée après retour à l’équilibre mécanique de ce dernier. L’entropie dans l’onde est déterminée d’après la quantité de chaleur alors échangée par le fluide pour revenir dans son état initial. La méthode est éprouvée (aux surpressions modérées) dans le cas d’un gaz parfait et l’on montre son intérêt dans le cas général

Crystal Structure of Nitromethane up to the Reaction Threshold Pressure

International audienceAngle dispersion X-ray diffraction (AXDX) experiments on nitromethane single crystals and powder were performed at room temperature as a function of pressure up to 19.0 and 27.3 GPa, respectively, in a membrane diamond anvil cell (MDAC). The atomic positions were refined at 1.1, 3.2, 7.6, 11.0, and 15.0 GPa using the single-crystal data, while the equation of state (EOS) was extended up to 27.3 GPa, which is close to the nitromethane decomposition threshold pressure at room temperature in static conditions. The crystal structure was found to be orthorhombic, space group P212121, with four molecules per unit cell, up to the highest pressure. In contrast, the molecular geometry undergoes an important change consisting of a gradual blocking of the methyl group libration about the C-N bond axis, starting just above the melting pressure and completed only between 7.6 and 11.0 GPa. Above this pressure, the orientation of the methyl group is quasi-eclipsed with respect to the NO bonds. This conformation allows the buildup of networks of strong intermolecular O‚‚‚H-C interactions mainly in the bc and ac planes, stabilizing the crystal structure. This structural evolution determines important modifications in the IR and Raman spectra, occurring around 10 GPa. Present measurements of the Raman and IR vibrational spectra as a function of pressure at different temperatures evidence the existence of a kinetic barrier for this internal rearrangement

A combined X-ray diffraction and Raman scattering study of the phase transitions in Sr<SUB>1-x</SUB>Ca<SUB>x</SUB>TiO<SUB>3</SUB> (x=0.04, 0.06, and 0.12)

Results of powder X-ray diffraction and Raman scattering studies on the phase transitions in Sr1-xCaxTiO3 (SCT) are presented for x=0.04, 0.06 and 0.12 in the temperature range 8-473 K. It is proposed that the space group of SCT in the composition range 0.06&#8804;x&#8804;0.35 is Imma with a0b-b- tilt system and not I4/mcm with a0a0c- tilt system, as assumed by earlier workers. The lowering of the crystal symmetry from I4/mcm to Imma is supported by the observation of additional Raman lines, in agreement with the factor group analysis for the Imma space group. The structural Eg mode, characteristics of the non-cubic phase, is shown to be present even in the cubic phases of x=0.06 and x=0.12 but not of x=0.04 indicating the change in the local structure of the cubic phase of SCT for x&#8805; 0.06. The presence of symmetry forbidden TO2 mode in the Raman spectra of the cubic phase of SCT for x&lt;0.06 and its absence for x&#8805;0.06 provides yet another characteristic feature distinguishing the I4/mcm and Imma space groups. The implications of the change in the tilt system from a0a0c- to a0b-b- on the development of the polar order is also discussed

Resolving the controversies about the 'nearly cubic' and other phases of Sr<SUB>1-x</SUB>Ca<SUB>x</SUB>TiO<SUB>3</SUB> (0≤x≤1): II. Comparison of phase transition behaviours for x = 0.40 and 0.43

The phase transition behaviour of two 'nearly cubic' compositions, x = 0.40 and 0.43, of Sr<SUB>1-x</SUB>Ca<SUB>x</SUB>TiO<SUB>3</SUB>, representative of Pbnm and Pbcm space group structures, has been investigated using temperature dependent dielectric, x-ray diffraction and Raman scattering techniques. For x = 0.40, a first order antiferroelectric phase transition occurs around 364 K followed by an antiferrodistortive phase transition involving the R point (q=1/2 1/2 1/2) instability at ~800 K. For x = 0.43, on the other hand, there is no antiferroelectric phase transition. Instead, two antiferrodistortive transitions around 463 and 850 K linked with the M (q=1/2 1/2 0 ) and R (q=1/2 1/2 1/2) point instabilities, respectively, are observed. Both the compositions also exhibit qualitatively different types of phase transitions below room temperature. Evidence for a new phase transition at low temperatures is presented for x = 0.43