Flexopiezoelectricity at ferroelastic WO3 twin walls

Abstract

Ferroelastic twin wall is a contact surface of two bulk crystals with different orientations, at which intriguing properties emerge, such as polarity, conductivity, ion accumulation, and magnetization pinning effect absent in the bulk. Although twin walls with a nanoscale width occupy a small volume in a crystal, emergent properties at twin walls can dominate the whole properties of a crystal. Therefore, finding a new functionality emerging at twin walls is important not only for nanodevice applications but also to reveal emerging physical properties. In this thesis, flexopiezoelectricty is introduced as a new phenomenon at twin walls. Despite piezoelectric response has been expected to exist at twin walls, the phenomenon has not been directly observed and explained due to the lack of appropriate materials. We have successfully synthesized WO3 epitaxial films with a hierarchical ferroelastic twin structure. We analyze the strain gradient at twin walls and explore the existence of flexopiezoelectricity. Flexopiezoelectricity is a new phenomenon described by a trilinear free energy density of polarization, strain, and strain gradient. It is the origin of the piezoelectric response at a twin wall, where a large strain gradient exists, in a centrosymmetric material. Successful characterization of flexopiezoelectric symmetry and coefficient along with control of twin wall orientation provide a useful pathway into twin wall electromechanics.