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Résumé:

Hyper-Raman spectroscopy has been performed in PbMg1/3Nb2/3O3 (PMN) from 800K to 50K, with particular attention to the low frequency region, down to about 5 cm-1 [1]. The high temperature spectra exhibit two resolved peaks at low frequency confirming the doublet structure of the soft mode of PMN, as previously anticipated [2,3]. An even more pronounced doublet structure has also been observed in PbMg1/3Ta2/3O3 (PMT) emphasizing therefore a common property of cubic relaxors. In PMN the lowest frequency component becomes overdamped at the onset of the Burns temperature Td but very interestingly, it hardens again on cooling down to 50 K. The relaxational frequency 02/ mimics the behavior of the inverse dielectric constant over the full temperature range explored: it extrapolates to the Curie temperature T0  390 K in the high temperature region, and close to the freezing temperature Tf  220K in the low temperature region. In between, the dynamics is controlled by a strongly overdamped soft excitation defining thereby a relaxational regime of the polarization fluctuations, characteristic of the relaxor’s state.