Y. Delugeard

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.

Heat capacity measurements of Mg–Y–Si–Al–O–N glasses

Abstract Heat capacity measurements on two glasses in the Mg–Y–Si–Al–O–N system have been carried out by differential thermal analysis in the range from room to the glass transition temperatures. The substitution of oxygen by nitrogen in the anionic network, the cationic one remaining the same, leads to an increase in the solid heat capacity measured at a constant pressure, over the whole range of temperature. Furthermore, the difference between the Cp of the solid state and the liquid state is close to 10 kJ per mole in the two glasses. The determination of the Debye temperature gives 930 K for the oxide glass and 952 K for the oxynitride one, the corresponding Debye frequencies being 647 and 663 cm−1, respectively. Moreover, we determined the activation energies for relaxation of these glasses: 836 and 962 kJ/mol for the oxide glass and the oxynitride glass, respectively.

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.

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.

Proton magnetic resonance study of some crystalline p-polyphenyls: Evidence of relaxation by slow molecular motions above room temperature

Abstract A NMR study of three p -Polyphenyls ( p terphenyl C 18 DH 14 , Selectively deuterated p -terphenyl C 18 D 10 H 4 and p quaterphenyl C 24 H 18 ) has been performed from about room temperature to their melting temperatures. The second moments and the spin-lattice relaxation times indicated similar dynamical processes for the three compounds: evidence of two slow molecular jump processes governed by high energy activations (about or larger than 20 kcal/mole). The geometry of these motions is described by large amplitude rotational jumps (≈ 180°) of respectively the external and the central phenyl rings of the molecules, along their long molecular axis.

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.

Dynamical study by Raman scattering of the ferroelectric phase transition of the disubstituted diacetylene 1,6-bis(2,4-dinitrophenoxy)-2,4-hexadiyne (DNP)

Abstract Dynamical study by Raman scattering of single monomer crystals of 1,6-bis-(2,4-dinitrophenoxy)-2,4-hexadiyne (DNP) has been performed in the low- and high-temperature phase ( T c ≈ 46 K). A soft mode characterized by a weak and constant damping is clearly observed in the low-temperature phase; this structural instability presents a displacive character. The critical exponent has been determined from the variation of other low-frequency modes which are coupled with the order parameter. Interesting additional information on the molecular conformation is given by the study of high-frequency modes. These results are in good agreement with dielectric and calorimetric measurements and they indicate either a tricritical or a weakly first order transition.

Experimental Study of the Liquid-Glass Transition in an Inorganic Polymer Li 0.5 Na 0.5 PO 3

New experimental results obtained with various techniques on a less-studied glass-forming system are presented. At low frequency, a secondary β slow -process, decoupled from the viscous flow, is observed by 3 1 P NMR. Raman scattering spectra and coherent neutron scattering spectra has been obtained in wide frequency and temperature ranges showing the same qualitative features for the Boson peak while the quasielastic contribution seems to differ markedly.

Raman scattering study under pressure at 6 K of the N = 4 incommensurate phase II of biphenyl

Abstract Pressure lowers the domain of existence of the incommensurate phase II of biphenyl. This allows a study of the characteristic dynamics of an incommensurate phase with n = 4 in optimal conditions i.e. at very low temperature. Raman scattering in such extremal conditions have allowed to give complementary results with respect to previous neutron scattering experiments and so to extend the knowledge of the characteristic phason and amplitudon branches by investigating a larger range of frequency with a good resolution.

Neutron and Raman studies of the dynamics of the n = 4 incommensurate biphenyl

Abstract Dynamics of an n = 4 incommensurate system, the phase II of biphenyl, is investigated by two complementary techniques, neutron and Raman scattering. The experiments are performed under pressure at very low temperature when the damping is considerably reduced.