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

Recent reports on epitaxial BiFeO3 films show that the crystal structure changes from nearly rhombohedral ("R like") to nearly tetragonal ("T like") at strains exceeding approximate to-4.5%, with the T-like structure being characterized by a highly enhanced c/a ratio. While both the R-like and the T-like phases are monoclinic, our detailed x-ray diffraction results reveal a symmetry change from M-A and M-C type, respectively, at this R-like-to-T-like transition. Therefore, the ferroelectric polarization is confined to different (pseudocubic) planes in the two phases. By applying additional strain or by modifying the unit-cell volume of the film by substituting Ba for Bi, the monoclinic distortion in the T-like MC phase is reduced, i.e., the system approaches a true tetragonal symmetry. Therefore, in going from bulk to highly strained films, a phase sequence of rhombohedral (R)-to-monoclinic (R-like M-A)-to-monoclinic (T-like M-C)-to-tetragonal (T) is observed. This sequence is otherwise seen only near morphotropic phase boundaries in lead-based solid-solution perovskites (i.e., near a compositionally induced phase instability), where it can be controlled by electric field, temperature, or composition. Our results now show that this evolution can occur in a lead-free, stoichiometric material and can be induced by stress alone.