Stress-inducedR−MA−MC−Tsymmetry changes in BiFeO3films

Abstract

Recent reports on epitaxial BiFeO${}_{3}$ films show that the crystal structure changes from nearly rhombohedral (``$R$ like'') to nearly tetragonal (``$T$ like'') at strains exceeding \ensuremath{\approx}\ensuremath{-}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 ${M}_{C}$ 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.