Types and configurations of domain walls in ferroelectric Bi₄Ti₃O₁₂ single crystals

In the ferroelectric Bi₄Ti₃O₁₂, the major component P_s(a) of spontaneous polarization P_s lies along the a axis; the component P_s(c) along the c axis is small. The two switchable components are expected to make up various types of domain walls (DWs). According to group-theory analysis, 11 permissible types of DWs are predicted to exist theoretically, and they include five types of ferroelectric DWs, one type of anti-phase boundary (APB) and five types of APB-combined ferroelectric DWs. The five types of ferroelectric DWs are P_s(a)-90° DWs, P_s(a)-180° DWs, P_s(c)-180° DWs, P_s(a)-90°/P_s(c)-180° DWs and P_s-180° DWs. In Bi₄Ti₃O₁₂ single crystals, just the five types of ferroelectric DWs were observed using transmission electron microscopy, with no trace of APBs or APB-combined ferroelectric DWs seen. The P_s(a)-90° domains are lamellar and do not have even thickness. Both the P_s(a)-90° DWs and P_s(a)-90°/P_s(c)-180° DWs lie mainly on the (110) plane, but often fold to the (001) plane. The P_s(a)-180° domains are predominantly flaky. Both the P_s(a)-180° DWs and P_s-180° DWs lie mainly on the (001) plane. The P_s(c)-180° DWs observed are irregularly curved.