Physical and structural origins of morphotropic phase boundaries (MPBs) in ferroics remain elusive despite decades of study. The leading competing theories employ either low-symmetry bridging phases or adaptive phases with nanoscale textures to describe different subsets of the macroscopic data, while the decisive atomic-scale information has so far been missing. Here we report direct atomically resolved mapping of polarization and structure order parameter fields in a Sm-doped BiFeO(3) system and their evolution as the system approaches a MPB. We further show that both the experimental phase diagram and the observed phase evolution can be explained by taking into account the flexoelectric interaction, which renders the effective domain wall energy negative, thus stabilizing modulated phases in the vicinity of the MPB. Our study highlights the importance of local order-parameter mapping at the atomic scale and establishes a hitherto unobserved physical origin of spatially modulated phases existing in the vicinity of the MPB.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

We have investigated heteroepitaxial films of Sm-doped BiFeO3 with a Sm-concentration near a morphotropic phase boundary. Our high-resolution synchrotron x-ray diffraction, carried out in a temperature range of 25 to 700 °C, reveals substantial phase coexistence as one changes temperature to crossover from a low-temperature PbZrO3-like phase to a high-temperature orthorhombic phase. We also examine changes due to strain for films exhibiting anisotropic misfit between film and substrate. Additionally, thicker films exhibit a substantial volume collapse associated with the structural transition that is suppressed in thinner films.

We have investigated structural phase transitions across a ferroelectric-to-antiferroelectric phase boundary in epitaxial (001) oriented Bi(1-x)SmxFeO3 thin films. For the Sm3+ concentration of 0.1 < x < 0.14, we observe short-range antiparallel cation displacements, verified by the appearance of localized 14{011} weak reflections in the selected area electron diffraction patterns. At the critical composition of x=0.14, the system adopts a complex nanoscale domain mixture with appearance of 14{011}, 12{011}, 12{010}, and 12{111} reflections and an incommensurate phase bridging the rhombohedral and orthorhombic phases. For compositions 0.14 < x < 0.2, orientational twin domains coupled with antiphase oxygen octahedral tilts, identified by 12{hkl} weak superstructure are observed. The above systematic changes in the microstructure as a function of Sm3+ doping are linked to the macroscopic functional properties.

We investigate the microstructural evolution in a ferroelectric to antiferroelectric phase transition at the morphotropic phase boundary in the Bi(1-x)SmxFeO3 system. Continuous Sm3+ substitution on the A-site induces short-range anti-parallel cation displacements as verified by the appearance of localized 1/4(110) weak spots in selected area electron diffraction patterns for 0.1<x<0.14 samples, and thus onset of antiferroelectricity. Kinetic Monte Carlo simulations confirm that increasing the strength of the anti-parallel interactions (i.e. increasing x) induces a ferroelectric to antiferroelectric transition. For 0.14<x<0.2 antiphase oxygen octahedra tilts induce complete antiferroelectricity.