  A morphotropic phase boundary driven by epitaxial strain has been observed in
a lead-free multiferroic BiFeO3 thin films and the strain-driven phase
transitions were widely reported to be iso-symmetric Cc-Cc ones by recent
works. In this paper, we suggest that the tetragonal-like BiFeO3 phase
identified in epitaxial films on (001) LaAlO3 single crystal substrates is
monoclinic MC. This MC phase is different from MA type monoclinic phase
reported in BiFeO3 films grown on low mismatch substrates, such as SrTiO3. This
is confirmed not only by synchrotron x-ray studies but also by piezoresponse
force microscopy measurements. The polarization vectors of the tetragonal-like
phase lie in the (100) plane, not the (110) plane as previously reported. A
phenomenological analysis was proposed to explain the formation of MC Phase.
Such a low symmetry MC phase, with its linkage to MA phase and the multiphase
coexistence open an avenue for large piezoelectric response in BiFeO3 films and
shed light on a complete understanding towards possible polarization rotation
paths and enhanced multiferroicity in BiFeO3 films mediated by epitaxial
strain. This work may also aid the understanding of developing new lead-free
strain-driven morphotropic phase boundary in other ferroic systems.

Materials Science