No abstract available

Structural and physical consequences of downsizing to form nanopowders or nanoceramics are very important both from applicative and fundamental point of view. We will discuss, using ab‐initio‐based calculation and Landau theory the origins and the consequences on structural properties when synthesizing nano‐objects with BaTiO3 and SrTiO3 , and show how to use nanostructuration to optimize properties such as electrocaloric effects or energy storage. For instance calculations in BaTiO3 show the possibility to form new phases (fig.1) in which vortices of polarization compete with homogeneous polarization or in which (in nanotorus) homogeneous hypertoroidization of nanotori coexists with axialhomogeneous toroidization. In SrTiO3 a new low temperature ground state, evidenced by synchrotron and neutron diffraction is discussed within the Landau theory and compared with the situation in thin films and during pressure experiments. The crucial competition between particles shape anisotropy, surface tension and shear strain is analyzed. Inducing a specific core‐shell structure is therefore a new and easy way to tailor structural properties and to stabilize new phases that cannot exist in bulk material, just like film deposition on substrate.

We present accurate measurement of the VV- and VH-polarized Raman spectra of vitreous SiO2 pressurized in the range 0 – 8 GPa in argon or helium atmosphere by means of diamond anvil cell. A fine quantitative analysis of the behavior of the Raman modes in function of pressure is achieved with the help of an old central-force model by Sen and Thorpe describing the dynamics of covalently bonded networks. The model predicts the effect of changes in force constants and bond angles on the vibrational frequencies of the coupled SiO4 tetrahedral network. We show that the pressure shift of the Raman bands cannot be understood without encompassing the variations of both parameters. Investigating two different pressurizing fluids including He which is adsorbed into the silica network, allows revealing the role of each parameter in the compression mechanism of the glass.

La diffusion hyper-Raman (DHR) est une spectroscopie optique des vibrations qui présente des règles de sélection différentes du celles du Raman et de l'infrarouge. Après avoir décrit les principes de la DHR, nous ferons un état de l'art de la dynamique vibrationnelle (IR, Raman, neutrons) du système relaxeur modèle Pb1/3Mg2/3NbO3 (PMN). Nous insisterons sur le comportement des modes mous polaires dont l'observation est uniquement possible par diffusion hyper-Raman. Les résultats ont été capturés dans un modèle qui offre une vision cohérente des propriétés dynamiques de toute une famille de matériaux. Ils relient par exemples les vibrations à la structure polaire locale, et donnent une origine microscopique aux propriétés diélectriques géantes de ces systèmes.

Une introduction à la théorie de la réponse linéaire nous permettra dans un premier temps de définir les notions de susceptibilité, de spectre des fluctuations, de fonction d’auto-corrélation,… et de relier ces grandeurs à travers le théorème de fluctuation-dissipation. Nous déterminerons ensuite l’origine de la diffusion de la lumière et calculerons l’intensité Raman et Brillouin diffusée dans le cadre d’une théorie macroscopique (classique). Ces calculs feront apparaitre des règles de sélection qui seront mises en pratique à travers quelques exemples simples pris parmi les solides cristallins et les liquides.

The fluctuations of electric polarization in a disordered ferroelectric substance, relaxor crystal PbMg_{1/3}Nb_{2/3}O_{3} (PMN), were studied using a nonlinear inelastic light-scattering technique, hyper-Raman scattering, within a 5-100  cm^{-1} spectral interval and in a broad temperature range from 20 to 900 K. The split ferroelectric mode reveals a local anisotropy of up to about 400 K. Spectral anomalies observed at higher temperatures are explained as due to avoided crossing of the single primary polar soft mode with a temperature-independent, nonpolar spectral feature near 45  cm^{-1}, known from Raman scattering. The temperature changes of the vibrational modes involved in the measured fluctuation spectra of PMN were captured in a simple model that accounts for the temperature dependence of the dielectric permittivity as well. The observed slowing down of the relaxational dynamics directly correlates with the huge increase of the dielectric permittivity.