Hyper-Raman scattering is a non-linear inelastic spectroscopy sensitive to polar excitations of solids whatever the crystalline symmetry. This selection rule is very attractive in particular for centrosymmetric structures for which polar modes are inactive in Raman. HRS has been performed in the cubic relaxors PbMg1/3Nb2/3O3 (PMN) and PbMg1/3Ta2/3O3 (PMT), with particular attention to the low frequency region, down to about 2 cm-1. Spectra have been recorded over a wide temperature range (900K-30K in PMN) and the results compared to existing infrared and neutron scattering data. This complete set of vibrational information provides the framework for a detailed analysis of the polarization dynamics in these systems. In particular, in both materials, the soft mode response exhibits a doublet structure up to the highest temperature investigated [1,2] emphasizing therefore a common property of cubic relaxors. The lowest frequency component is assigned to the primary soft mode of symmetry F1u, while the second likely originates from a local disorder persisting until very high temperatures, and which lifts the cubic selection rules. On cooling, the soft F1u-mode of PMN becomes overdamped at the onset of the Burns temperature but very interestingly, it splits between 600K and 400K into two components which harden on decreasing further the temperature. This behavior likely highlights the onset of a local anisotropy of the polarization and provides therefore new insight about the relaxor nature.

Ferroelectric nano-objects display a rich variety of shapes, among them nanocubes (bulk or hollow),nanospheres, nanotori etc with fascinating morphogenesis mechanisms1. Structural and physicalconsequences of ferroelectric downsizing to form nanopowders or nanoceramics are very important bothfrom applicative and fundamental point of view (see ref. 2 for a good review). Indeed many interestingeffects have already been reported such as diminishing of permittivity and increase of diffusivity2,3,cancelation of dielectric relaxation in relaxors3,4, lowering of ground states, increase and rotation ofpolarization in relaxors and morphotropic compounds3,5 etc. In order to explain these experimental results,one must take into account not only size and microstrains effects but also existence of core-shellstructure2,5-8. Modeling also shows that there is optimal nanostructurations to maximize energy storage6 andelectrocaloric effect8. Calculations also show the possibility to form new phases in which vortices ofpolarization compete with homogeneous polarization8 or in which homogeneous hypertoroidization ofnanotori coexists with axial homogeneous toroidization10. After a survey of these effects, we will give newexperimental results observed in SrTiO3 showing at small size decoupling of oxygen octahedra tiltings anddistortion, strong shift of ferroelastic critical temperature11,12 and lowering of the tetragonal structuretowards orthorhombic ground state. Possible connection with a core-shell structure will also be discussed.

Hyper-Raman spectroscopy has been performed in PbMg1/3Nb2/3O3 (PMN) from 800K to 50K, with particular attention to the low frequency region, down to about 5 cm-1 [1]. The high temperature spectra exhibit two resolved peaks at low frequency confirming the doublet structure of the soft mode of PMN, as previously anticipated [2,3]. An even more pronounced doublet structure has also been observed in PbMg1/3Ta2/3O3 (PMT) emphasizing therefore a common property of cubic relaxors. In PMN the lowest frequency component becomes overdamped at the onset of the Burns temperature Td but very interestingly, it hardens again on cooling down to 50 K. The relaxational frequency 02/ mimics the behavior of the inverse dielectric constant over the full temperature range explored: it extrapolates to the Curie temperature T0  390 K in the high temperature region, and close to the freezing temperature Tf  220K in the low temperature region. In between, the dynamics is controlled by a strongly overdamped soft excitation defining thereby a relaxational regime of the polarization fluctuations, characteristic of the relaxor’s state.

Raman scattering is performed in three sets of aluminosilicate glasses with light cations and concentrations varying from peralcaline to peraluminate domain. The depolarized spectra highlight two cation modes below ∼400 cm(-1). Comparison with infrared data reveals very stringent selection rules providing as much additional information for a vibrational analysis. The latter suggests in-phase (network-coupled) and out-of-phase (network-decoupled) displacements of the cations relative to their adjacent negatively charged structures. The low frequency vibration involves all cations whatever their role in the glass, network modifiers or charge compensators. Very interestingly, the second mode originates mostly from cations at modifier's places, providing thereby a new support for structural and chemical analysis of silicate glasses using Raman scattering.

Gallium arsenate (GaAsO4) has the highest piezoelectric properties among α-quartz-type materials. We describe first-principles calculations of the linear and second-order nonlinear optical properties of GaAsO4 combined with measurements of its second harmonic generation (SHG). Our calculations show that GaAsO4 has a SHG efficiency between 7.1 (LDA) and 12.3 pm/V (GGA), which is in agreement with our experiments (7.5 ± 1.5 pm/V) using the technique of Maker fringes. This SHG efficiency, attributed to an electronic transition from O 2p to As 4 s states, is higher than that observed in GaPO4 due to a better polarizability of As atoms. Thus, the thermal stability of GaAsO4 (no phase transition up to its thermal decomposition at 1303 K) associated with its high laser damage threshold (0.93 GW/cm2) make this compound a promising bifunctional material for piezoelectric and nonlinear optical applications at high temperatures.

The vibrational properties of silica glass have been intensively studied experimentally andtheoretically during the last four decades. However there are few theoretical studies of theevolution of the vibrational properties under pressure.We have calculated the parallel and perpendicular Raman spectra of the vitreous silica. The studyhas been carried out using first principles calculations within the density functional theoryframework. The calculated spectra at ambient pressure are in good agreement with theexperimental ones as well as with previous calculations reported in the literature. Themodifications of the Raman spectra under pressure also reproduce the experimental data. Apreliminary analysis of the resulting structural modifications and their correlations with the changesseen in the spectra will be reported.