Local stabilisation of polar order at charged antiphase boundaries in antiferroelectric (Bi0.85Nd0.15)(Ti0.1Fe0.9)O3

Abstract

Observation of an unusual, negatively-charged antiphase boundary in (Bi0.85Nd0.15)(Ti0.1Fe0.9)O3 is reported. Aberration corrected scanning transmission electron microscopy is used to establish the full three dimensional structure of this boundary including O-ion positions to ∼±10 pm. The charged antiphase boundary stabilises tetragonally distorted regions with a strong polar ordering to either side of the boundary, with a characteristic length scale determined by the excess charge trapped at the boundary. Far away from the boundary the crystal relaxes into the well-known Nd-stabilised antiferroelectric phase

The atomic structure and chemistry of Fe-rich steps on antiphase boundaries in Ti-doped Bi0.9Nd0.15FeO3

“…Moreover, it suggests that the ions in the step are very similarly octahedrally coordinated at the steps as in the perovskite matrix. Seeing as the surroundings of the antiphase boundaries are already known to exist in a structure rather similar to the T-phase, 4 it should be no surprise that Fe L 2,3 spectra, rather similar to these previous reports, are seen in this case. This suggests that the coordination of the Fe in the steps is either in tetragonally distorted or undistorted octahedra, rather than in rhombohedrally distorted octahedra.…”

“…The strong Ti segregation to the terraces is consistent with results reported previously. 4 The notable feature of the steps, however, is that they contain almost no Ti whilst being rich in Fe and O. This indicates that the pairs of B-site atoms in steps are very different from the pairs of B-sites atoms in terraces, even if the interatomic distances are similar.…”

“…Atom positions in the beam direction were therefore chosen on the assumption that these are the same as those determined for the equivalent positions in the terraces previously published. 4 This was considered a reasonable assumption since there is always a continuous boundary between the terraces and the steps. The resulting model is shown in Fig.…”

“…Their chemical structure was reconstructed at 10 pm precision in 3 dimensions and it was shown that the core had excess negative charge which resulted in a strong polarisation of the surrounding perovskite and the stabilisation of a polar phase a few unit cells each side of the APB. 4 In this previous study, however, we restricted the discussion to planar sections of the boundaries on the (001) plane. In the present work, we show that such antiphase boundaries can deviate strongly from the {001} planes through the formation of stepped structures.…”

“…We determine quantitatively the structure and chemistry of the steps, and propose a three-dimensional model consistent with the already known terrace structure. 4 Atomic resolution high angle annular dark field (HAADF) images of a novel type of APB, taken from different areas by aberration corrected scanning transmission electron microscopy (STEM), are shown in Fig. 1.…”