Résumé
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.