Discovery of a universal morphotropic phase boundary behavior in rare-earth substituted BiFeO$_{3}$ using thin film composition spreads

Epitaxial thin film composition spreads can be used to perform systematic substitution and chemical modification studies where changes in structural and physical properties of materials can be continuously tracked. I will discuss one particular example where we study substitution of various trivalent RE$^{ }$ions into the A-site of BiFeO$_{3}$ (BFO) using a series of composition spreads. We had earlier reported on the transition of the rhombohedral ferroelectric structure of the undoped BFO to an orthorhombic phase in Sm-substituted BFO [S. Fujino et al., Appl.Phys.Lett. 92, 202904 (2008)]. At the structural phase boundary, electromechanical properties including the piezoelectric coefficient d$_{33}$ and the dielectric constant are substantially enhanced. The value of d$_{33}$ at the boundary can be as high as 110 pm/V, nearly double that of values typically reported for undoped BFO thin films. We show that there is a universal behavior with all RE dopants and that the structural transition accompanied by enhanced dielectric and piezoelectric properties can be universally described by one control parameter, the A-site average ionic radius. This work is performed in collaboration with D. Kan, L. P\'{a}lov\'{a}, V. Anbusathaiah, C.-J. Cheng, S. Fujino, V. Nagarajan, and K. M. Rabe.