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Résumé:

In the past few years, we have tried to employ the Hyper'Raman Scattering technique in the investigations of the prototype relaxor crystal ' lead magnoniobate, PMN. This nonlinear technique is usually applicable only in the paralecric phase of ferroelectric substances, but is is often quite conveninent as the Hyper'Raman selection rules allways allow scattering by polar modes. In case of realoxors, investigations can be pursued well below the Burns temperature. Recently, the technique has been sucessfully applied to the investigation of the soft polar mode. We shall demonstrate that the lowest'frequency polar mode observed is the \"primary\" polar soft mode of PMN, responsible for the Curie'Weiss behavior of its dielectric permittivity above the Burns temperature.