180° head-to-head flat domain walls in single crystal BiFeO3

We investigate flux-grown BiFeO3 crystals using transmission electron microscopy (TEM). This material has an intriguing ferroelectric structure of domain walls with a period of approximately 100 nm, alternating between flat and sawtooth morphologies. We show that all domain walls are of 180° type and that the flat walls, lying on (112) planes, are reconstructed with an excess of Fe and a deficiency of Bi. This reconstruction is similar to that observed in several previous studies of deposited layers of BiFeO3. The negative charge of the reconstructed layer induces head-to-head polarisation in the surrounding material and a rigid-body shift of one domain relative to the other. These characteristics pin the flat 180° domain walls and determine the domain structure of the material. Sawtooth 180° domain walls provide the necessary reversal of polarisation between flat walls. The high density of immobile domain walls suppresses the ferroelectric properties of the material, highlighting the need for careful control of growth conditions.