Stress-induced R-MA-MC-T symmetry changes in BiFeO3 films

TL;DR

This study demonstrates that strain alone can induce a sequence of structural phase transitions in BiFeO3 films, mimicking the behavior near morphotropic phase boundaries in lead-based perovskites, potentially enhancing piezoelectric properties.

Contribution

It is the first to show strain-induced R-MA-MC-T phase transitions in a single-component system, paralleling MPB phenomena in solid solutions.

Findings

Stress induces R-MA-MC-T phase sequence in BiFeO3.

Substrate strain causes symmetry lowering similar to MPB effects.

Enhanced c/a ratios observed in MC and T phases.

Abstract

The recent discovery of a stress-induced structural phase transition in the single-component perovskite BiFeO3 has revived interest in this lead-free ferroelectric. The coexistence of different phases may lead to large piezoelectric coefficients, a property typically associated with complex solid solutions of lead-based perovskites. Here we report combined experimental and computational results showing that the stress-induced phase transitions in BiFeO3 follow the path of rhombohedral(R)-to-monoclinic(MA)-to-monoclinic(MC)-to-tetragonal(T), where both MC and T show highly enhanced c/a ratios. This R-MA-MC-T sequence is otherwise observed only near morphotropic phase boundaries (MPBs) in lead-based perovskites (i.e. near a compositionally induced phase instability), where it is controlled by electric field, temperature, or composition. Our results represent the first time that this evolution has been induced in a single component system using strain alone, and show that substrate- imposed symmetry lowering results in a similar phase instability as the proximity to a MPB in solid solutions.