The precursor dynamics of a cubic-tetragonal ferroelectric phase transition of potassium tantalate niobate ͑KTa 1−x Nb x O 3 with x = 0.32͒ crystals is studied by Brillouin scattering. The appearance of the central peak ͑CP͒ and marked softening of the elastic constant c 11 are clearly observed above the Curie temperature, T C-T , owing to the interaction between the LA mode and local polarization fluctuations of polar nanoregions ͑PNRs͒. The relaxation time determined by the CP width clearly shows a critical slowing down above T C-T , indicating an order-disorder feature of the ferroelectric phase transition. The size of a dynamic PNR is evaluated, and it increases toward T C-T .

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