LINEAR CHAIN MODELS AND SOUND VELOCITIES IN MOLECULAR ORGANIC CRYSTALS

Longitudinal sound velocities in crystals of elongated aromatic molecules have been measured by Brillouin scattering and are found to be faithfully described by a simple linear chain model, whereas a similar behaviour is observed for compressibility and thermal expansion. A particular attention has been paid to the p-polyphenyls and polyacenes series

Dynamics in the supercooled liquid

The temperature and wave number dependences of the dynamical structure factor obtained in a nonfragile glass-forming liquid by Brillouin scattering and coherent neutron scattering experiments are for the first time described by the solutions of a schematic mode coupling theory model over a wide frequency range. In intermediate regime, the crossover between vibrational excitations and fast relaxational processes observed above the susceptibility minimum in neutron spectra is well reproduced by the theoretical model. At lower Q-values, the temperature dependence of the Brillouin lineshape is also accurately described. Besides, the dynamical transition is shown to occur between 601 K and 637 K ($T_{\rm g} = 515\;{\rm K}$, $T_{\rm m} = 749\;{\rm K}$) in this supercooled liquid

External pressure and light influence on internal pressure in a spin-crossover solid [Zn : Fe(ptz)6](BF4)2

The relation between the internal pressure during spin-crossover is compared to the chemical pressure induced by dilution with zinc. Further, the light of a specific LIESST (Light Induced Excited Spin State Trapping) wavelength is used to induce partial stabilisation of high-spin state and thus shift temperature of the spin-crossover towards lower values. The de-coupling of the spin-crossover and structural phase transition is discussed

BRILLOUIN SCATTERING STUDY OF PHASE TRANSITIONS IN p-POLYPHENYLS

A Brillouin scattering study of the various phase transitions in p-polyphenyls reveals different kinds of elastic anomalies depending on the nature of these transitions

ORDERING PROCESS OF SPUTTERED AMORPHOUS THIN FILMS OF Al Mn UNDER THERMAL TREATMENTS

We report here preliminary results on an attempt to prepare Al-Mn quasi crystalline thin films by cathodic sputtering. Starting from as deposited amorphous materials we have carried out successive in situ annealings of the sample in the electron microscope. The result is that, if moderate heating leads to crystalline phases, an a posteriori strong local irradiation by the electron beam can transform these crystals into a T phase with numerous defects.Nous reportons ici des résultats préliminaires concernant la préparation de Al Mn quasicristallin à partir de couches minces fabriquées par pulvérisation cathodique. L'échantillon déposé amorphe est soumis à une succession de recuits dans le microscope électronique ; cela induit, pour un chauffage modéré l'apparition de phases cristallines, qui lors de fortes irradiations localisées, par le faisceau d'électrons, se transforment en phase T avec de nombreux défauts

High resolution Raman study of the soft mode in the incommensurate bis(4-chlorophenyl)sulfone

The soft optical mode of bis(4-chlorophenyl)sulfone is analysed up to the incommensurate transition temperature $T_{\rm i}$ with both a tandem of Fabry-Perot interferometers and a triple monochromator. High resolution spectra ($\sim 4$ GHz ) of the soft mode reveal a mean field behaviour up to $T_{\rm i} - 4$ K, but between $T_{\rm i}$ and this temperature the frequency of the soft mode saturates whereas its linewidth increases and its intensity decreases. The occurrence of second order scattering with a very slow decrease of its intensity above $T_{\rm i}$ precludes the explanation of the observed soft mode blocking by a divergence of the second order scattering. Different spectral profiles of the soft mode have been recorded in the (bb) and ($\gamma \gamma$) polarizations, $\gamma$ being the principal dielectric axis parallel to the molecular long axis. Within single mode analysis, excess intensity is found between the soft mode and the longitudinal acoustic mode in the ($\gamma \gamma$) polarization. This intensity is assigned to a coupling effect between these modes whereas a phason contribution is ruled out for reasons of symmetry and temperature behaviour