Résumé
High resolution hyper-Raman spectroscopy (HRS) of single crystals of the relaxor PbMg1/3Nb2/3O3 has been performed. The temperature dependence of the three F1u-symmetry polar modes is investigated between 20 K and 800 K. The soft mode is clearly observed above 520 K (Fig. 1) [1]. It is splitted into a doublet up to the highest temperature investigated. The high frequency component (A1) appears to be underdamped even hundred of degrees below the Burns temperature Td 620 K in PMN. Its frequency nicely matches with previous high resolution neutron scattering experiments (Fig. 2) but is too high to explain the behaviour of the dielectric constant of PMN. This is the reason why the search of a lower-frequency vibration was subject to numerous experimental studies, but still, a clear understanding of the soft mode dynamics of relaxors was lacking. This vibration, labelled E1 in Figure 1, is clearly observed in HRS. It is underdamped at high temperature and becomes overdamped near Td and below. Its frequency 0 as well as its "relaxational" frequency rel = 02/, (, full width at half maximum) extrapolate to zero at the Curie-Weiss temperature T0 400 K (Fig. 2). Moreover, the static dielectric constant calculated from the Lyddane-Sachs-Teller relation nicely mimics the behaviour of that obtains from dielectric measurements. These observations strongly suggest that this vibration corresponds to the "primary" soft mode of PMN. Similar observations have been made in others relaxor-type compounds, thus suggesting a universal behaviour of the low-frequency dynamics of cubic relaxors.