C. Ecolivet

Brillouin scattering in polyphenyls. I. Room temperature study of p‐terphenyl (H14, D14) and biphenyl (D10)

We report the results of Brillouin scattering measurements of the elastic constants of two elements from the polyphenyl series : biphenyl (D10) and p‐terphenyl (D14, H14). These values are found to differ mainly for C33 : 18.06×109 N/m2 in biphenyl and 26.42×109 N/m2 in p‐terphenyl. A simple chain model is given which accounts for this difference whereas elastic constants computed from atom–atom potential parameters are found in moderate agreement with our measured data.

Brillouin scattering in polyphenyls. II. Through the incommensurate phases of biphenyl

Brillouin scattering experiments from room temperature down to liquid helium temperature are reported. Two phase transitions occur in this temperature range at TI=40 K and TII=17 K, but elastic anomalies only happen at TI. Several attempts for the direct phason detection have been found unsuccessful. The elastic anomalies are qualitatively interpreted inside the frame of the Landau theory with simple group theory arguments.

Temperature-pressure phase diagram of an aperiodic host guest compound

This letter reports on the structural instabilities of an aperiodic composite crystal under pressure. The (P, T) phase diagram up to 0.55 GPa of nonadecane-urea is reported showing various symmetry breakings in crystallographic superspaces, towards three different orthorhombic phases. These structural phase transitions are characterized by a change in the intermodulation and are described by increasing the rank of the crystallographic superspaces.

Studies of phase transitions in new ferroelectric crystal [nh(ch3)3]3sb2cl9

Abstract A new phase transition with second order character has been found in [NH(CH3)3]3Sb2Cl9 crystals at 364.0 K close to the ferro-paraelectric first order transition at 363 K. The transitions were studied by DSC, dielectric and pyroelectric measurement techniques. The phase situation and possible nature of transitions observed in this compound are discussed.

Study of elastic properties of sodium, potassium and rubidium acid phthalates by brillouin scattering

Abstract Elastic properties of three, isomorphous, orthorhombic (mm2) molecular-ionic crystals of sodium, potassium and rubidium acid phthalates have been studied employing the Brillouin scattering technique. The values of all elastic constants: stiffnesses cαβ and compliances sαβ were determined at 294 K using the measured longitudinal and transverse sound velocities for different phonon directions.

Brillouin scattering in p‐polyphenyls. III. The improper ferroelastic phase transition of p‐terphenyl

The elastic properties of p‐terphenyl were studied in its high and low temperature phases. The 13 elastic constants were determined in the monoclinic phase, whereas 16 elastic constants were found sufficient to describe the elastic properties of the triclinic phase. In this phase the main change in the Brillouin spectra is a splitting of a quasilongitudinal line. This splitting varies like the square of the order parameter. A quasielastic component is observed in the vicinity of the transition temperature. Deuterated and hydrogenated samples revealed the same behavior. Elastic anomalies are tentatively interpreted inside the frame of the Landau theory.

Confined linear molecules inside an aperiodic supramolecular crystal: The sequence of superspace phases in n-hexadecane/urea

High-resolution studies of the host-guest inclusion compound n-hexadecane/urea are reported at atmospheric pressure, using both cold neutrons and x-ray diffraction. This intergrowth crystal presents a misfit parameter, defined by the ratio c(h)/c(g) (c(host)/c(guest)), which is temperature independent and irrational (γ = 0.486 ± 0.002) from 300 to 30 K. Three different structural phases are reported for this aperiodic crystal over this temperature range. The crystallographic superspaces are of rank 4 in phases I and II, whereas phase III is associated with an increase in rank to 5, with a supplementary misfit parameter (δ = 0.058 ± 0.002) that is constant throughout this phase. The superspace group of phase I is hexagonal P6(1)22(00γ) down to T(c1) = 149.5 ± 0.5 K; phase II, which persists down to T(c2) = 127.8 ± 0.5 K is orthorhombic P2(1)2(1)2(1)(00γ), and phase III is orthorhombic P2(1)2(1)2(1)(00γ)(00δ).

Pressure, temperature and light influence on spin transition solids

Abstract Present paper is an overview of our efforts during the past few years to understand complicated corelations of physical phenomena related to pressure in Fe(I1) solid state spin transition systems. Some principal results concerning p, T, λ-experiments are extracted. In the context of correlation of the crystallographic phase transition with simultaneous HS → LS relaxation and LS → HS photopopulation, we show the latest results: Brillouin and magnetic measurements on the crystal [Fe(pt6](BF6)2.

Structure and relaxation processes of an anisotropic molecular fluid confined into 1D nanochannels

Structural order parameters of a smectic liquid crystal confined into the columnar form of porous silicon are studied using neutron scattering and optical spectroscopic techniques. It is shown that both the translational and orientational anisotropic properties of the confined phase strongly couple to the one-dimensional character of the porous silicon matrix. The influence of this confinement induced anisotropic local structure on the molecular reorientations occurring in the picosecond timescale is discussed.

Sliding and translational diffusion of molecular phases confined into nanotubes

The remaining dynamical degrees of freedom of molecular fluids confined into capillaries of nano to sub-nanometre diameter are of fundamental relevance for future developments in the field of nanofluidics. These properties cannot be simply deduced from the bulk one since the derivation of macroscopic hydrodynamics most usually breaks down in nanoporous channels and additional effects have to be considered. In the present contribution, we review some general phenomena, which are expected to occur when manipulating fluids under confinement and ultraconfinement conditions.