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Page 2 of 10 Ge et al. Microstructures 2023;3:2023026 https://dx.doi.org/10.20517/microstructures.2023.13

[5,6]
magnetic order in multiferroic devices .

[4]
BiFeO has a rhombohedral structure (space group R3c) below its Curie temperature T = 1,100 K , with a
3
c
pseudocubic lattice parameter of 3.965 Å and rhombohedral angle 89.4° (pseudocubic indexing is used
throughout this paper). Its spontaneous ferroelectric polarisation (P ) has a magnitude P approximately
s
s
100 μC cm along [111] and mostly arises from the displacement of Bi ions relative to their surrounding
-2
[7]
FeO cages . Oxygen octahedra are tilted antiphase around the three-fold [111] axis by 10°-14° (a a a in
- - -
6
Glazer notation ). The formation of domains in BiFeO , similar to other ferroelectric materials, occurs to
[8]
3
minimise the total energy, comprised of electrostatic, depolarisation, and elastic components . Depending
[9]
on the angle ϕ between P of adjacent domains, the meeting of these domains at domain walls can result in
s
either purely ferroelectric (ϕ = 180°) or ferroelectric-ferroelastic (ϕ = 71° or 109°). The orientation or habit
plane of domain walls has been of great interest as it provides insight into the competing energy
components [10-13] and determines functional properties [14,15] .
The batch of flux-grown single-crystals of BiFeO investigated in this study have been subject to several
3
previous investigations [16,17] , all of which have revealed a dense array of parallel domain walls seen in
piezoresponse force microscopy (PFM) and conventional transmission electron microscopy (TEM) as
alternating sawtooth and flat bands of contrast [Figure 1]. The complex microdomain structure in these
crystals is extremely stable, exhibiting no change upon observation, even in the thinnest specimens.
Furthermore, no observable switching (or rapid back-switching into the pristine domain pattern) was
found, even with 200 V applied to the surface with an AFM tip. The inability to access the functional
properties of these high-quality single crystals is concerning and may have implications for the exploitation
of BiFeO in practical applications. Therefore, it is important to understand the nature and origin of this
3
domain structure.

The first study of these crystals showed the domain walls to be either 180°- or 109°-type and due to the
[16]
high predicted energy of 180°-type domain walls, it was proposed that they were probably 109°-type. A
second study of the same batch of crystals using negative C high-resolution TEM imaging found a variety
[17]
s
of domain wall types, including 71°-, 109°-, and 180°-type. Another recent study of similar material
[18]
confirms the sawtooth walls to be 180°-type but proposes that flat domain walls are 109°-type. In this article,
[19]
along with its companion publication , we revisit the domain structure in this same batch of crystals using
a combination of PFM, conventional TEM, convergent beam electron diffraction (CBED), atomic
resolution scanning TEM (STEM), and electron energy loss spectroscopy (EELS). The high stability of the
domains ensured there was no influence of specimen preparation on the structure. We find that there are
only 180° domains in the crystal, i.e., all domain walls are 180°-type. The difficulties experienced in previous
work may potentially be attributed to projection effects when the three-dimensional domain structure is
observed in an electron-transparent foil. To overcome this, we use a focused ion beam (FIB) technique to
prepare multiple sections with different orientations from the same region of the crystal. We describe the
[19]
sawtooth domains in a companion article ; the focus of this article is on the flat 180° domain walls.
MATERIALS AND METHODS
[16]
BiFeO single crystals were obtained from the same batch used in the studies of Berger et al. and
3
[17]
Jia et al. , who described growth conditions in detail. In brief, crystals were precipitated from a
Fe O /Bi O /B O flux cooled very slowly from 1,170 K to 875 K. Much of the growth took place below the
2
3
2
3
2
3
paraelectric-ferroelectric phase transition at 1,098 K. The crystals were compact and had sizes of half to a
few millimetres, with a top surface of four (hhl) facets, only a few degrees away from (001). For this study,
several well-formed single crystals that had no contact with the crucible sidewalls were selected.