Study of the influence of pressure on the p-terphenyl phase transition by Raman scattering

The influence of pressure on the p-terphenyl phase transition is analysed by Raman Scattering. This study improves the understanding of the mechanism of this structural instability which is related to competing intramolecular and intermolecular forces. So, as expected, the phase diagram shows that the transition temperature decreases with increasing pressure. The observation of a soft mode, at high pressure in the low temperature phase, showing a change-over from an order-disorder regime to a displacive one, also agrees with the transition mechanism. Furthermore, our study indicates that the transition becomes clearly discontinuous at high pressure.L'influence de la pression sur la transition de phase du p-terphényle est analysée par diffusion Raman. Cette étude améliore la compréhension du mécanisme de cette instabilité structurale reliée aux forces antagonistes inter et intramoléculaires. Aussi, comme prévu, le diagramme de phase montre que la température de transition décroît quand la pression augmente. L'observation d'un mode mou, à haute pression dans la phase basse température, indiquant que la transition passe d'un régime ordre-désordre à un régime displacif, est aussi en accord avec le mécanisme de transition proposé. Par ailleurs, notre étude montre que la transition devient clairement discontinue à haute pression

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

TRANSITION STRUCTURALE DANS LE PARATERPHÉNYLE

Une transition antiferroréorientationnelle a été mise en évidence pour le paraterphényle. L'absence de discontinuité du paramètre d'ordre et l'existence d'une forte diffusion critique anisotrope soulignent la nature critique de cette transition.An antiferro-reorientational phase transition has been seen in para-terphenyl. The absence of any discontinuity in the order-parameter and the presence of strong critical scattering emphasizes the critical nature of the transition

Vibrational properties of polymer and quenched CsC

We report new results on the vibrational dynamics of polymer and quenched CSC60 phases. Both the splitting and the structure of new activated lines of polymer CsC60 are in agreement with the lowering of C60 molecular symmetry from Ih to D2h. An inelastic neutron investigation on polymer and quenched CsC60 is reported. It allows one to identify the vibrational signature of the low-temperature (T < 150 K) ordered monomer phase on quenched CsC60