A. Girard

Inelastic neutron scattering study of structural phase transitions in polyphenyls

Abstract Inelastic neutron scattering was used to study structural phase transitions in polyphenyls. These transitions result from the stabilization of the non-planar molecular configuration with respect to a torsional angle between the planes of phenyl rings. For biphenyl the zone boundary mode shows a pronounced softening in the temperature range 60–200 K and becomes overdamped close to the transition temperature. For p-terphenyl the spectrum contains a quasi-elastic anisotropic central component diverging on lowering the temperature.

Raman Scattering Study of the order—disorder phase transition in para-terphenyl

Abstract We have investigated the effect of the order—disorder phase transition on the Raman spectrum of para-terphenyl in the spectral range 3–170 cm−1. The Raman spectrum does not present a sudden change at the transition temperature but a progressive evolution in the low temperature phase. The low temperature spectra (15 K) exhibit external modes and torsional internal modes.

Influence of pressure on structural phase transitions in P-polyphenyls

Abstract The influence of pressure on structural phase transitions in p-polyphenyls is discussed and the results of neutron diffraction and Raman scattering experiments on biphenyl and p-terphenyl are presented.

Low-frequency Raman study of para-terphenyl influence of the isotopic substitution and interpretation of the spectra in both phases☆

Abstract We report a low-frequency Raman study of deuterated para-terphenyl versus temperature in both phases. From observed and calculated deuteration effects and potential calculations, we have interpreted the spectra of the low-temperature phase. The features of the high-temperature spectra are analysed taking into account orientational disorder. The particular behaviour of the lowest-frequency line is carefully analysed in both phases.

Optical and calorimetric studies on the role of lattice mode softening in assisting a thermally enhanced solid state reaction

Abstract The concept of phonon-assisted solid-state chemical reactions is analyzed in the case of the thermal rearrangement of methyl p -dimethylaminobenzenesulfonate (MSE) to a zwitterionic product p -trimethylammoniumbenzenesulfonate. The experimental Raman scattering reinvestigation of MSE as a function of temperature shows that mode softening does not play an important role in the thermal reaction of MSE. A series of structural phase transitions in the [90–300 K] temperature range is shown in the MSE crystal by calorimetry, infrared absorption and Raman scattering.

Pressure dependence of conformational instabilities in crystalline p-terphenyl

Abstract Recent inelastic neutron scattering measurements in pterphenyl under pressure are presented. They reveal a continuous changeover of the high temperature structural instability from an order-disorder regime to a displacive one. Static and dynamic aspect of the phase 111 of p-terphenyl is discused.

The neutral-to-ionic phase transition of TTF-CA : a Raman study of the influence of temperature and hydrostatic pressure

A low- and high-frequency Raman scattering study of the neutral-to-ionic phase transition of TTF-CA as it relates to the temperature and hydrostatic pressure is reported. Around the transition, the variations of the intensities of several antisymmetric modes permit the determination of the phase diagram and therefore the confirmation of previous results. Under pressure, the more continuous character of the transition is confirmed unambiguously. On the other hand, an evolution under pressure of the mechanism of the transition is clearly shown. The soft mode is still observed at 4 kbar but with a softening which is less critical. Moreover, in the high-temperature phase, the broadening of several totally symmetric modes favours an order - disorder mechanism. So, our study performed under pressure clearly shows that the mechanism of the transition moves from a displacive mechanism to an order - disorder one.

Elastic and anharmonic properties of pyridinium tetrafluoroborate ([C5NH6]+[BF4]- ≡ PyBF4): light scattering study of the phase transitions at various pressures

The vibrational properties of pyridinium tetrafluoroborate crystals were studied by means of Brillouin scattering at atmospheric pressure, Raman scattering and neutron diffraction under high pressure. It is shown that the elastic properties strongly reflect the molecular anisotropy of the pyridinium ions. The compressibility obtained from the Brillouin scattering study is in reasonable agreement with the results from neutron crystallography under pressure. Brillouin spectra reveal a small quasielastic component at room temperature, which is described by a coupled-susceptibilities formalism applied to the longitudinal mode and a relaxation mode. The mode Gruneisen parameters for all three phases that are found are determined from the pressure and temperature dependence of the Raman spectra and from neutron experiments.

Low Temperature Raman Spectra of Dibenzyl; Comparison with Biphenyl and Paraterphenyl

Abstract The Raman spectra of crystal dibenzyl have been recorded from 293 down to 10 K. No typical temperature dependence which usually accompanies phase transitions has been observed in frequencies, intensities or background. A comparison is made with biphenyl and paraterphenyl whose evolution of Raman spectra with the temperature looks similar. The very broad bands observed at room temperature result from the superposition of the different modes which appear well separated at 10 K. This behavior is consistent with the large atomic thermal motion observed by X-ray analysis. Vibrational frequencies have been calculated using the intermolecular potential of the atom-atom type in the rigid phenyl approximation and compared with experimental data.

A RAMAN STUDY OF THE DISORDER INDUCED BY POLYMER CHAINS IN MIXED MONOMER-POLYMER CRYSTALS OF THE DIACETYLENE PTS-D

Abstract The evolution of the high frequency modes of polymer chains in mixed monomer-polymer crystals of fully deuterated diacetylene 2,4-hexadiynylene bis( p -toluenesulfonate) has been studied as a function of the temperature by Raman scattering using IR sources. This compound undergoes topochemical complete polymerization in the crystalline state: that gives the possibility to control the polymer content x from 0 to 100%. In both pure monomer and polymer crystals, an antiferroelectric phase transition occurs at T c = 182 K in the polymer and, through an intermediary incommensurate phase, from 155 to 195 K in the pure monomer. The evolution of an elongation mode of the conjugated diacetylene backbone is reported in mixed monomer-polymer crystals as a function of the temperature and polymer content from x =20% to x =90%. This mode is coupled to the order parameter. Our results are compared with the previous data obtained for the pure polymer. The influence of disorder induced by polymer chains in mixed monomer-polymer crystals is discussed in a qualitative way.