Elastic Anomaly of Ba2NaNb5O15

Abstract

The temperature characteristics of elastic, piezoelectric, dielectric, and optical constants of Ba₂NaNb₅O₁₅ have been measured from room temperature up to the Curie point. The shear elastic compliance related to the tetragonal principal axes (_TS₆₆) is found to become large near the transition point from tetragonal to orthorhombic phase. A phenomenological approach is tried, in which the free energy is expanded in power series of the polarization and strain, and it becomes clear that this tetragonal‐orthorhombic transition is little dependent of the behavior of spontaneous polarization but that elastic instability is the main cause of this phase transition. The elastic, piezoelectric, dielectric, and optical constants in the orthorhombic phase are derived from the constants in the tetragonal phase by the biased effect of the spontaneous shear strain to the elastic higher‐order terms in the free energy.