Due to their exceptional ferroelectric properties, lead-based perovskite materials (PbB'B''O3) have been the subject of numerous studies. The different solid solutions exhibit their highest electromechanical responses for compositions lying at the so-called morphotropic phase boundary (MPB) between the rhombohedral (R3m) and tetragonal (P4mm) phases. Ferroelectric properties are dependent on 3 variables: applied electric field, temperature and mechanical stress σij. The effect of hydrostatic pressure on the physical properties of ferroelectrics is actually under intense investigations. In technological applications, large stress fields can be observed in miniaturized components or in ferroelectric dots. Additionally in the literature, experiments and theoretical calculations have indicated the existence of an intrinsic short range dynamical disorder over nearly the entire lead-based perovskite materials systems. Therefore the structural and dynamical behavior of lead-based perovskite materials as a function of P,T,E has to be entirely considered in the fundamental understanding of the ferroelectric-to-ferroelectric and ferroelectric-to-paraelectric phase transitions of such important technological materials. Based on neutron diffraction, X-ray diffraction, dielectric measurements, resonance Raman spectroscopy and X-Ray absorption Spectroscopy, we will present an overview of our new results.

In the past few years, we have tried to employ the Hyper'Raman Scattering technique in the investigations of the prototype relaxor crystal ' lead magnoniobate, PMN. This nonlinear technique is usually applicable only in the paralecric phase of ferroelectric substances, but is is often quite conveninent as the Hyper'Raman selection rules allways allow scattering by polar modes. In case of realoxors, investigations can be pursued well below the Burns temperature. Recently, the technique has been sucessfully applied to the investigation of the soft polar mode. We shall demonstrate that the lowest'frequency polar mode observed is the \"primary\" polar soft mode of PMN, responsible for the Curie'Weiss behavior of its dielectric permittivity above the Burns temperature.

Hyper-Raman scattering is a non-linear-optic spectroscopy where two incident photons scatter one photon after interaction with an excitation in the media [1]. One major interest of this technique is its high sensitivity to all polar vibrations, including soft polar modes [2], and to excitations that are inactive in both infrared absorption and Raman scattering. These specificities provide hyper-Raman with a very powerful tool for the investigation of local and average structure of ferroelectric-type materials such as relaxors or multiferroic systems. We will focus on hyper-Raman results obtained on three single crystals of the relaxor PbMg1/3Nb2/3O3 (PMN)[3,4]. The relative scattering intensities of the band near 250 cm−1 in various polarization geometries are fully compatible with the hyper-Raman tensor of the F2u ‘silent' mode of the parent Oh cubic structure. The temperature dependence of the three F1usymmetry polar modes was investigated between 20 K and 800 K. Some of the transverse (TO) and longitudinal (LO) components are splitted up to the highest temperatures, confirming the existence of local lattice distortions from the cubic symmetry well above the Burns temperature Td ≈ 620 K. The splitting of the LO2-mode strongly increases below Td, a behaviour which likely relates to the growth of the nano-domains. The soft TO-mode is also clearly observed, with a frequency decreasing to zero near Td. Same behaviours have been observed in another relaxor-type compound, PbMg1/3Ta2/3O3. Finally, a weak but clear transition-like anomaly in the temperature dependence of the lowest frequency LO-mode (LO1) is observed near the Curie temperature Tc = 210 K. These experimental observations will be compared to Raman and neutron scattering literature data, and confronted to the proposed scenarios for the evolution of the local structure of PMN with temperature.

The concentration of the fourfold and threefold ring modes of normal and densified silica has been quantitatively extracted using time-domain Raman spectroscopy. Their frequency and width are precisely determined even when they are partly masked by a broad background in spontaneous Raman spectroscopy [1,2]. Experiments performed in GeO2 also reveal the existence of a weak vibrational mode ascribed to oxygen motion in three-membered rings [3]. In permanently densified silicas, the variation of the Raman intensity upon densification is very different for bending and stretching modes. For the former we find a Raman coupling-to-light coefficient CB µ w2. This allows to derive a simple law relating the frequency of the bending modes with the Si-O-Si bond-angle in the network and in the fourfold and threefold rings [4]. The model has been applied to densified silicas and sodo-silicates glasses, and the results compare well to numerical simulations. This simple Raman-spectra analysis could become part of the routine characterization of the local and medium range structure of silica-based glasses.

Relative Raman scattering intensities are obtained in three samples of vitreous silica of increasing density. The variation of the intensity upon densification is very different for bending and stretching modes. For the former we find a Raman coupling-to-light coefficient C-B proportional to omega(2). A comparative intensity and frequency dependence of the Raman spectral lines in the three glasses is performed. Provided the Raman spectra are normalized by C-B, there exists a simple relation between the Si-O-Si bond angle and the frequency of all O-bending motions, including those of fourfold (n = 4) and threefold ( n = 3) rings. For 20% densification we find a reduction of similar to 5.7 degrees of the maximum of the network angle distribution, a value in very close agreement with previous NMR experiments. The threefold and fourfold rings are weakly perturbed by the densification, with a bond angle reduction of similar to 0.5 degrees for the former.