Order-disorder nature and elastic anomaly of successive phase transition of (K_0.5Na_0.5)NbO₃ proved by broadband Brillouin scattering

ABSTRACT

The importance of the environment-friendly lead-free piezoelectric materials has attracted much attention. The (K_0.5Na_0.5)NbO₃ (KNN) crystals with the perovskite structure undergo the cubic (C) to tetragonal (T) ferroelectric phase transition at T_CT = 430 °C, and the tetragonal to monoclinic (M) structural phase transition at T_TM = 190 °C. The elastic properties and relaxation process of these phase transitions of the KNN crystals have been studied by broadband Brillouin scattering. The softening of the LA and TA velocities was observed in the vicinity of T_CT and T_TM. The power law behavior of the velocity indicates three-dimensional fluctuations. The broadband central peak showed the critical slowing down in the vicinity of the T_CT and T_TM. The slowing down is attributed to the order-disorder nature of the off-center displacement of Nb in KNN.

KEYWORDS:

• KNN
• ferroelectric phase transition
• Brillouin scattering
• acoustic phonon
• relaxation

Acknowledgments

The authors are thankful to T. Ariizumi, J. Zushi, and R. Ohta for the Brillouin scattering measurements and related discussion.

Additional information

Funding

This work was supported in part by the JSPS KAKENHI Grant Number JP17K05030.