  The coexistence of different ferroelectric phases enables the tunability of
the macroscopic properties and extensive applications from piezoelectric
transducers to nonvolatile memories. Here we develop a thermodynamic model to
predict the stability and low-energy orientations of boundaries between
different phases in ferroelectrics. Taking lead zirconate titanate and bismuth
ferrite as two examples, we demonstrate that the low-energy orientations of
interphase boundaries are largely determined by minimizing the electrostatic
and elastic energies. Phase-field simulations are employed to analyze the
competition between the interfacial energy and the electrostatic and elastic
energies. Our simulation results demonstrate that the lowering of crystal
symmetry could occur due to the electrical and mechanical incompatibilities
between the two phases, which can be used to explain the experimentally
observed low-symmetry phases near morphotropic phase boundaries. Our work
provides theoretical foundations for understanding and controlling the
interphase boundaries in ferroelectric materials for multifunctional
applications.
