Successive phase transition of lead-free ferroelectric sodium potassium niobate crystals studied by Raman scattering

The 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 degrees C. The KNN50 undergoes the cubic to tetragonal ferroelectric phase transition at T-C = 430 degrees C, and the tetragonal to monoclinic structural phase transition at T-tr = 190 degrees C. In the monoclinic phase, the lowest frequency mode at 68cm(-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 T-tr and becomes overdamped. For further heating, the intense central peak (CP) appears in the tetragonal phase just above T-tr, and the CP intensity shows a peak at T-C. The relaxation time determined from the CP width shows the critical slowing down towards T-C.