Theoretical calculations, based on newly obtained experimental geometries in strained BiFeO3 thin films, predict an almost barrierless transition between co-existing phases. This facile transition provides insight into the origin of the high electromechanical responses found in coexisting phases in this Pb-free material.
Epitaxial strained BiFeO3 thin films show an unusually high piezoelectric response in the compressively strained region. The film is not structurally uniform, and it has long been thought that interplay between two co-existing phases (T′ and S′) drives the high electromechanical response. In this work, X‑ray diffraction identified crystal parameters of the previously uncharacterized S′ phase. The knowledge of these structures allowed theory to map the transition between the two phases. Surprisingly, although S′ has features similar to those of the rhombohedral ground-state, it is very close in energy to the tetragonal-like T′, and there exists almost no barrier along the path between the phases. This facile transformation may lead to the high piezoresponse and may inform design of Pb-free piezoelectrics with high electromechanical responses.
H. Dixit, C. Beekman, C. M. Schlepütz, W. Siemons, Y. Yang, N. Senabulya, R. Clarke, M. Chi, H. M. Christen, and V. R. Cooper, “Understanding strain-induced phase transformations in BiFeO3 thin films,” Adv. Sci. 2, 1500041 (2015). DOI: 10.1002/advs.201500041