  Fast-cooling after sintering or annealing of BiFeO3-BaTiO3 mixed oxide
ceramics yields core-shell structures that give excellent functional
properties, but their precise phase assemblage and nanostructure remains an
open question. By comparing conventional electron energy loss spectroscopy
(EELS) with scanning precession electron diffraction (SPED) mapping using a
direct electron detector, we correlate chemical composition with the presence
or absence of octahedral tilting and with changes in lattice parameters. This
reveals that some grains have a 3-phase assemblage of a BaTiO3-rich pseudocubic
shell; a BiFeO3-rich outer core with octahedral tilting consistent with an R3c
structure; and an inner core richer in Ba and even poorer in Ti, which seems to
show a pseudocubic structure of slightly smaller lattice parameter than the
shell region. This last structure has not been previously identified in these
materials, but the composition and structure fit with previous studies. These
inner cores are likely to be non-polar and play no part in the ferroelectric
properties. Nevertheless, the combination of EELS and SPED clearly provides a
novel way to examine heterogeneous microstructures with high spatial
resolution, thus revealing the presence of phases that may be too subtle to
detect with more conventional techniques.
