Résumé
Lattice vibrations of PbMg1/3Nb2/3O3 relaxor and re- lated materials have been already subject of numerous theoretical and experimental studies, but despite of these efforts, so far none of the traditional techniques was capa- ble to disclose a comprehensive and broadly acceptable picture of the dynamical processes associated with the order parameter dynamics in such systems. For com- putational methods starting from first-principles, this is a difficult task since the characteristic time and length- scales of the structural fluctuations, which are believed to be responsible for the relaxor behavior, are beyond the capabilities of the standard brut-force methods. Perhaps the most definite observation is the very existence of a soft dynamics in the system. The "royal" experi- mental lattice dynamics technique, the inelastic neutron scattering, has brought the first demonstration that the A-site cation vibration modes are showing at least partially the classical softening and hardening behavior at high and low temperatures, and these observations were later fully supported also by the "most proper" technique for studies of ferroelectric soft modes, the far-infrared spectroscopy. Nevertheless, there is a more than a hundred K broad range of the intermediate temperatures, where the situation is far from being clear. Among others, it was obscured by the continuing discussions about the so called waterfall effect and various spurious phenomena. But the analysis is mainly difficult because the soft mode response cannot be well modeled by a single damped harmonic oscillator response: the mode seems to be split and/or coupled to other excitations of vibrational and/or relaxational nature (central peaks). Moreover, for both far-infrared and inelastic neutron scattering techniques it is technically very di±culties to disclose neatly the be- havior at very low frequencies in these materials. At the same time, several rather fine spectral features could be identified in Brillouin and Raman spectra, and very detailed temperature-dependent experiments could reveal anomalies linked to the characteristic "critical" temperatures established independently from other techniques. Unfortunately, there is no consensus about the assignment of these features and precise meaning of these temperatures. In fact, even the mechanism responsible for activation of the Raman scattering is a matter of controversy. In order to broaden the perspective for the considerations about this subject, we shall offer an overview of our recent studies of PMN lattice dynamics by the hyper-Raman scattering technique, which, according to our opinion, provides rather interesting new insights in the relaxor soft mode problem. Hyper-Raman scattering (HRS) spectroscopy is based on a non-linear optical effect where two incident pho- tons produce one scattered photon after interaction with a phonon or other excitation. One of the interest of this technique is that its selection rules are different from RS and IR ones. For example, in the Pm3m simple cubic perovskite, the polar F1u modes are active both in IR and HRS, while the "silent" F2u mode is active only in HRS. Here we mostly exploit the fact that hyper-Raman technique combines advantage of infrared spectroscopy to probe the polar modes irrespectively on the structural °uctuation in the structure (in contrast with the the Raman spectroscopy), that the same modes can be inves- tigated in several nonequivalent geometries (similarly to neutron scattering), and, at the same time, this technique allows to probe the spectral profiles with a sufficient de- tail down to frequencies of a few cm-1. The key results to be discussed are the following: 1. there is a remarkable correspondence between the LO band intensities (!) in the HRS and Im[1/epsilon] spectra 2. experiments reveal enhancement of the intensity of the LO modes in the forbidden backscattering geometry, 3. the pronounced mode observed in cross-polarized spectra near 250 cm-1 is assigned as F2u mode, 4. soft mode is splitting is clearly observed and the soft mode spectra could be followed well below the Burns temperature, 5. temperature dependence of soft mode intensity re- veals freezing of the relaxor order parameter in a very similar way as the refractive index data does it in the original works of Burns and coworkers. Although all the observations are not fully understood, we are convinced that the HRS scattering has a potential to become extremely useful tool for studying polar modes in lead-based relaxor materials.