Origin and Dynamics of Morphotropic Phase Boundary in Multiferroic Films

Materials with morphotropic phase boundary (MPB) compositions attract people's interests because of their huge electromechanical response. The ambiguity of crystal structure, phases and physics behind presents a challenge to interpret the origin of abnormally high piezoelectric coefficient. Recently, epitaxial strain can be used to stabilize the MPB in BiFeO$_{3}$. [1] However, to better understand the origin of MPB in multiferroic perovskite, it is essential to probe into the strain/polarization coupling mechanism. In this abstract, origin and dynamics of the phase transition in BiFeO$_{3}$ near MPB have been investigated by exploring the ferroelectric domains. Careful analysis of domain configurations across the phase boundary reveals the rotation of ferroelectric vectors from phase to phase, a new strain/polarization coupling state in nature. The phase transition dynamics are assisted by the re-orientation of ferroelectric polarizations. Phase-field simulation further proves the origin of this MPB is the strain-driven rotation and re-orientation of ferroelectric vectors. This investigation provides new insight into the study of high electromechanical response in MPB-like crystal and helps engineer other lead free-ferroelectrics. Reference: 1. R. J. Zeches, et al., Science 326, 977 (2009).