Elastic Anomaly and Elastic Heterogeneity in Ferroelectric Materials

Ferroelectric materials have been extensively applied to various devices due to technologically important functionalities such as piezoelectric, pyroelectric, electro-optic, nonlinear optic, elastooptic effects, photocatalytic properties, and high-energy storage density. These functionalities are closely related to ferroelectric phase transitions and heterogeneity. This chapter reviews the elastic anomaly of ferroelectric phase transitions and elastic heterogeneity caused by equivalent orientation states of spontaneous polarizations in ferroelectric materials. At first, elastic moduli for isotropic materials and elastic constants for crystals with point symmetry are introduced under various external conditions. Brillouin scattering spectroscopy, piezo-resonance method, and scanning acoustic microscope (SAM) are introduced as experimental methods to measure elastic properties. The elastic anomaly of ferroelectric phase transitions studied by measurements of Brillouin scattering is discussed. Measurements of SAM are used to discuss the elastic heterogeneity caused by ferroelectric and ferroelastic domains.