  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.
