Optimizing the electromechanical response in morphotropic BiFeO₃

Morphotropic phase boundaries (MPBs) in ferroelectrics are highly desired phase transition regions, where crystal structure becomes extremely susceptible to external stimuli leading to very large electromechanical responses with applications in high-performance piezoelectric actuators and sensors. Since the recent discovery of a strain-driven MPB in chemically simple oxides, especially in thin films of BiFeO₃ (BFO), much of the research emphasis has been on the physical structural MPBs which spatially separate intimately mixed phases in corrugated nanoscale regions. Very large electromechanical responses and other coupled novel functionalities, i.e. magnetism and electronic conductivity have been observed at such physical structural MPBs. In this report, we investigate and identify a region in the thickness-dependent phase diagram of the morphotropic BFO thin films at which a superior piezoelectric performance is achieved. In this region, an electrically recoverable strain approximately 40% higher than the previously reported values for the morphotropic BFO is observed. This enhanced electromechanical coupling has been attributed to the degeneracy of the mixed-phase states and concurrent increased mechanical softening.