Résumé
The hyper-Raman spectroscopy (HRS) of three single crystals of the relaxor PbMg1/3Nb2/3O3 has been performed [1,2]. The temperature dependence of the three F1u-symmetry polar modes was investigated between 20 K and 800 K. In addition, 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 nonpolar F2u 'silent' mode of the parent Oh cubic structure. From a structural point of view, the results reveal that the HRS response is sensitive to the B-site distortions and that oxygen atoms play a minor role in the structural changes, at least between 300 K and 800 K. The advantage of HRS for the measurement of zone-center soft modes is apparent from Fig. 1a. The very good instrumental resolution combined with a very weak second harmonic generation signal at high temperature (narrow band at w=0 in Fig. 1a) allows measurements of polar modes by HRS down to a few cm-1. Figure 1b shows a series of spectra measured between 520 and 750 K. Clearly, the soft- TO-mode is splitted into a doublet over the whole temperature range explored. 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 [3] (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 [4], 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 [5]. It is underdamped at high temperature and becomes overdamped near Td and below. Its frequency w0 as well as its "relaxational" frequency wrel = w0 2/G, (G, full width at half maximum) extrapolate to zero at the Curie- Weiss temperature T0 @ 400 K [6], as shown in Figure 2. Moreover, the static dielectric constant calculated from the Lyddane-Sachs-Teller relation nicely mimics the behaviour of the experimental one [6] (inset of Fig. 2). These observations strongly suggest that this vibration corresponds to the "primary" soft mode of PMN. In addition, the intensity of the soft TO-doublet exhibits a step-like anomaly around Td, a behaviour which might relate to the growth of a local polarization-related order parameter. Similar observations have been made in others relaxor-type compounds, thus suggesting a universal behaviour of the low-frequency dynamics of cubic relaxors.