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. One major interest of this technique is its high sensitivity to all polar vibrations including soft polar modes, and also 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 the local and average structure of ferroelectric relaxors. The hyper-Raman spectroscopy (HRS) of three single crystals of the relaxor PbMg1/3Nb2/3O3 has been performed [1,2]. The relative scattering intensities obtained for 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 F1u-symmetry polar modes was investigated between 20 K and 800 K. The anomalous behaviours observed can be explained in term of local structural changes. At low frequency, HRS reveals a clear splitting of the soft TO-mode. The high frequency component corresponds to previous neutron scattering expectations while the low frequency one was not observed so far [3]. Its frequency decreases to zero at T0 @ 400 K. The results strongly suggest that this vibration is the primary soft mode of PMN. Similar observations have been made in another relaxor-type compound, PbMg1/3Ta2/3O3, thus suggesting a universal behaviour of the low-frequency dynamics of cubic relaxors. [1] B. Hehlen, G. Simon, and J. Hlinka, Phys. Rev. B 75, 052104 (2007) [2] A. Al-Zein, B. Hehlen, J. Rouquette, and J. Hlinka, Phys. Rev. B 78, 134113 (2008) [3] A. Al-Zein, J. Hlinka, J. Rouquette, and B. Hehlen, Phys. Rev. Lett. 105, 017601 (2010)