Enhancement of Nonlinear Optical and Piezoelectric Properties in Ferroelectrics from Symmetry-Breaking Strains*

In the pursuit of lead-free piezoelectrics, recent studies have used temperature, pressure, applied voltage, and other methods to enhance piezoelectric properties in ferroelectrics. Here, we investigate changes in symmetry, polarization, and domain structure near phase boundaries in BaTiO3 crystals and BaTiO3-xBaSnO3 thin films in order to understand how property enhancement depends on external stimuli and replicate the property enhancements in versatile thin films. We use second harmonic generation (SHG) polarimetry to detect changes in point group symmetry and polarization direction, supplemented by scanning SHG microscopy, which reveals the domain structure and allows polarimetry of specific domains. We observe lower-symmetry monoclinic phases with SHG and piezoelectric property enhancements of up to 4 times near temperature phase boundaries and with applied voltage in BaTiO3 crystals. In addition, we observe similar symmetry lowering near a compositional phase boundary of BaTiO3-xBaSnO3 thin films, indicating the potential for similar property enhancements in the thin film system which is more easily integrable into a variety of devices.