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Structure-electromechanical property correlations in rare-earth ͑RE͒-substituted ͑001͒ BiFeO 3 ͑BFO͒ epitaxial thin films have been investigated. Quantitative piezoelectric coefficient ͑d 33 ͒ and dielectric constant ͑ 33 ͒ measurements, in conjunction with selected area electron diffraction, reveal that the enhancement in d 33 and 33 at the morphotropic phase boundary ͑MPB͒ of the RE-substituted films ͑RE= Dy 3+ , Gd 3+ , and Sm 3+ ͒ is correlated with the presence of a competing intermediate antipolar phase with the rhombohedral ferroelectric and nonpolar orthorhombic phase. This leads to a complex nanoscale phase coexistence at the MPB. Extending the studies to RE =La 3+ case, we find the nanoscale phase coexistence to be less pronounced. This explains the lack of increase in d 33 for the La 3+-substituted BFO films, in contrast to the Dy 3+ , Gd 3+ , and Sm 3+-substituted films.

Detailed structural investigations on the substitution-induced structural phase transition from the rhombohedral phase to an orthorhombic phase in (Bi,RE)FeO 3 epitaxial thin films (RE ¼ La, Sm, and Dy) grown on (100) SrTiO 3 substrates are presented. X ray diffraction reveals that the unit cell dimensions of the orthorhombic phase are strongly dependent on the type of the RE dopant. For RE ¼ La 3þ ion, which has an ionic size comparable to the Bi 3þ ion, the unit cell is found to be a 0 Â a 0 Â 2a 0 , where a 0 is the pseudo-cubic lattice parameter. This is in contrast with the H2a 0 Â H2a 0 Â 2a 0 unit cell for the case of smaller ionic radius RE (¼ Sm and Dy) elements. While clear double-hysteresis loops in the polarization versus electric-field curves due to fieldinduced transitions are observed for smaller ionic radius RE (¼ Sm and Dy), no signature of the double hysteresis loops is observed for the RE ¼ La case across the structural boundary. We have also performed systematic tracking of the structural phases as functions of the RE composition and temperature, based on which we propose a phase diagram. This work reveals that the ionic size of the RE element plays a critical role in the evolution of the structural and functional properties of RE-substituted BiFeO 3 thin film materials systems. V