Nanomechanics of Ferroelectric Thin Films and Heterostructures

Abstract

The focus of this chapter is to provide basic concepts of how externally applied stresses and strains alter the ferroelectric properties of a material and how to quantitatively evaluate these effects on the phase stability, domain structure, and material properties of ferroelectrics using the phase field method. The chapter starts with a brief introduction of ferroelectrics and the Landau–Devonshire description of ferroelectric transitions and ferroelectric phases in a homogeneous ferroelectric single crystal. Due to the fact that ferroelectric transitions involve changes in crystal structure in conjunction with domain formation, strains and stresses can be produced inside of the material if a ferroelectric transition occurs under physical confinement. These stresses and strains, in turn, affect the domain structure and material ferroelectric properties. Therefore, ferroelectric and mechanical properties are coupled to each other. The ferroelectric–mechanical coupling can be used to engineer the ferroelectric material properties by designing the phase and structure of the material. The following section details the calculations of the stresses, strains, and elastic energy of a thin film containing a single domain, a twinned domain, as well as complicated multidomains that are constrained by the underlying substrate. Furthermore, a phase field model for predicting ferroelectric stable phases and domain structure in a thin film is presented. This phase field model is then used to demonstrate how substrate constraint and temperature be used to obtain interesting ferroelectric domain structures in BaTiO3 films.