Résumé
The lattice dynamics of ferroelectric relaxors and nanoparicles is still the subject of many theoretical and experimental investigations. In addition to the usual structural techniques, spectroscopic methods sensitive to vibrational modes are widely used to elucidate phase transition mechanisms. Numerous studies have been already performed using inelastic neutron scattering (INS), Raman scattering (RS) and infrared reflectivity (IR). Using all these techniques, it is very difficult to disclose neatly the behavior at very low frequencies in these materials. 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 selection rules that are different from both IR and RS. For example, in the Pm3 ̅m simple cubic perovskite, the polar F1u modes are active both in IR and HRS, while the "silent" F2u mode is active only in HRS. This provides HRS as an experimental tool that can probe polar and non-polar excitations and that gives details about the spectral shape down to frequencies of a few cm−1. In this talk, recent HRS experimental studies reporting the lattice vibrations of prototypical ferroectric relaxors such as PMN and PMT [1-3] and of the well-known quantum paraelectric material strontium titanate (micro- and nano-particles) [4] will be discussed. Special attention will be given to the dynamics of the low-lying vibrations such as the ferroelectric soft modes. These results show that HRS can be a very useful tool for the investigation of the vibrations of ferroelectric materials. [1] A. Al-Zein, B. Hehlen, J. Rouquette, and J. Hlinka, Phys. Rev. B 78, 134113 (2008). [2] A. Al-Zein, J. Hlinka, J. Rouquette, and B. Hehlen, Phys. Rev. Lett. 105, 017601 (2010). [3] A. Al-Zein, J. Hlinka, J. Rouquette, A. Kania, and B. Hehlen, J. Appl. Phys. 109, 124114 (2011). [4] J. M. Kiat, C. Bogicevic, P. Geimener, F. Karolak, N. Guiblin, F. Porcher, A. Al-Zein, B. Hehlen, and R. Haumont, submitted to Phys. Rev. B.