Phenomenological Theory of BaTiO3-Based Piezoelectrics

Compositions in the pseudo-ternary system formed by BaZrO3, BaTiO3, and CaTiO3 (BZT-xBCT) have attracted intense interest as lead-free alternatives to conventional lead zirconate titanate piezoceramics. However, the intrinsic origins of the high piezoelectric response in BZT-xBCT remain controversial. In this work, a Landau-Devonshire phenomenological theory for selected isopleths in the BZT-xBZT system was developed. The theory was constructed in multiple stages. First, the parameters of a Landau polynomial for BaTiO3 were determined from dielectric and high-accuracy heat capacity data. The validity of polynomial parameterization was verified by computing electric field-temperature and pressure-temperature phase diagrams and comparing predicted properties with experimental data for the spontaneous polarization, lattice parameters, dielectric permittivity, and the electrocaloric effect. Superior agreement was found when contrasted with other polynomials reported in the literature. Second, an analytical method was applied for reproducing self-consistently the phase diagrams and properties of the pseudo-binary solid solutions BaTiO3-BaZrO3 and BaTiO3-CaTiO3. Finally, the phase diagrams and electromechanical properties along isopleths in the BZT-xBCT systems were predicted using a linear mixing approximation. The results were used to explore phenomena most often invoked to rationalize the piezoelectric response, including vanishing crystallographic anisotropy of polarization and coincidence of convergence and tricritical points. Factors controlling the piezoelectric response in polymorphic systems were compared with those in morphotropic systems both with and without the presence of a monoclinic phase