Investigation of nanodomain pattern and piezoelectric behavior of mixed phases in epitaxial BiFeO3 films

Mixed phases of rhombohedral and tetragonal ferroelectric variants have been recently observed to coexist through epitaxial constraints in bismuth ferrite films. They form stripe-like domain patterns with orientations closely parallel to the substrate edges and exhibit unusually large piezoelectric response in the stripe areas. Here a model is developed for explaining the peculiar domain orientations as well as the enhancement of piezoelectricity in the mixed phase. The former is determined by the relative magnitudes of elastic and depolarization energy, whereas the latter is ascribed to the strain-driven softening in dielectric stiffness. The predictions show the similar trend as those observed in recent experiments.