Abstract
ABSTRACTThe lattice instability of lead-free potassium sodium niobate, (K0.5Na0.5) NbO3 (KNN50) piezoelectric crystals were studied by Raman scattering in the temperature range between 20 and 550 °C. The KNN50 undergoes the cubic to tetragonal ferroelectric phase transition at TC = 430 °C, and the tetragonal to monoclinic structural phase transition at Ttr = 190 °C. In the monoclinic phase, the lowest frequency mode at 68 cm−1, which is related to the soft mode of KNbO3, is clearly observed at room temperature. On heating from room temperature it shows noticeable softening towards Ttr and becomes overdamped. For further heating, the intense central peak (CP) appears in the tetragonal phase just above Ttr, and the CP intensity shows a peak at TC. The relaxation time determined from the CP width shows the critical slowing down towards TC.
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