Piezoelectric properties of metastable (Li, Na)NbO3 ceramics

Piezoelectric properties of a metastable state in lithium sodium niobate (Li_0.12Na_0.88)NbO₃ ceramics were investigated. (Li_xNa_1-x)NbO₃ system has been widely studied and the compounds around x=0.1 have attracted much attention as materials for high frequency piezoelectric applications, because of the large electro-mechanical coupling coefficient and low dielectric permittivity. However, the temperature stability of resonance frequency is not sufficient for these applications, because of the thermal hysteresis of resonance frequency around 100°C. It is considered that the thermal hysteresis is caused by the first-order phase transition between rhombohedral and monoclinic phase. We found that the high temperature monoclinic phase became stable at room temperature after a heat treatment in (Li_0.12Na_0.88)NbO₃ (LNN) ceramics. The thermal hysteresis was not observed in the metastable LNN. The electromechanical coupling coefficients of metastable LNN became about two third of that before the heat treatment, and mechanical quality factor became about six times larger. The metastable state was affected by the heat treatment condition. Cooling from tetragonal state, which is observed above monoclinic phase, is probably needed to obtain the meatstable state.