Nanoscale structural and chemical properties of antipolar clusters in Sm-doped BiFeO₃ ferroelectric epitaxial thin films

The local atomic structure and nanoscale chemistry of an antipolar phase in Bi_0.9Sm_0.1FeO₃ epitaxial thin films are examined by an array of transmission electron microscopy (TEM) coupled with electron diffraction and electron energy-loss spectroscopy methods. The observations are tied to macroscopic properties of the films, namely, polarization-electric field hysteresis loops, dielectric constant-electric field hysteresis loops, and the dielectric loss. At room temperature, the local Sm deficiency was determined to destabilize the long-range ferroelectric state, resulting in the formation of local antipolar clusters with the appearance of PbZrO₃-like antiparallel cation displacements, which give rise to ¹/₄{011} and ¹/₄{211} reflections as well as ¹/₂{321}, because of in-phase oxygen octahedral tilts. Aberration-corrected TEM analysis reveals that the antipolar structure is actually a lamellar of highly dense ferroelectric domains with alternating polarizations. With increasing temperature, a phase transition was observed at 150 °C, which is attributed to the reduction of the antiparallel displacements, giving way to cell-doubling structural transition.