In situ observation of the motion of ferroelectric domain walls in Bi₄Ti₃O₁₂ single crystals

Five types of ferroelectric domain walls (DWs) are present in Bi₄Ti₃O₁₂ single crystals (Ye et al., 2015). Here their motion was investigated in situ using transmission electron microscopy and optical microscopy. The motion of P_(a)-90° DWs, P_(a)-180° DWs and P_(c)-180° DWs was observed through electron beam poling in a transmission electron microscope. The growth of new P_s(a)-180° nanodomains was frequently seen and they tended to nucleate at preexisting P_s(a)-90° DWs. Irregularly curved P_(c)-180° DWs exhibit the highest mobility, while migration over a short range occurs occasionally for faceted P_s(a)-90° DWs. In addition, the motion of P_s(a)-90° DWs and the growth/annihilation of new needle-like P_s(a)-90° domains in a 20 µm-thick crystal were observed under an external electric field on an optical microscope. Most of the new needle-like P_s(a)-90° domains nucleate at preexisting P_s(a)-90° DWs and the former are much smaller than the latter. This is very similar to the situation for P_s(a)-180° domain switching induced by electron beam poling in a transmission electron microscope. Our observations suggest the energy hierarchy for different domains of P_s(c)-180° ≤ P_s(a)-180° ≤ P_s(a)-90° ≤ new needle-like P_s(a)-90° in ferroelectric Bi₄Ti₃O₁₂.