A New Phase Boundary of Oxygen Octahedral Tilt-Untilt Transition in Bi 0.5Na 0.5TiO 3-PbTiO 3 Piezoelectric Ceramics

The investigation on the fine structure evolutions around morphotropic phase boundary (MPB) is important to understand the origin of high piezoelectricity in ferroelectric ceramics. Here, we discovered a new boundary associated with oxygen octahedron tilt-untilt phase transition by exploring the average and local structure evolutions in the vicinity of the MPB in (Bi_0.5Na_0.5)TiO₃-PbTiO₃ (BNT-PT) ceramics. It is found that the local structure transition due to oxygen octahedron rotation is decoupled with the phase transition from rhombohedral to tetragonal phase at MPB in the BNT-PT. Such a local tilt-untilt transition is responsible to the large high-field-induced electrostrain at MPB of the BNT-PT. The crucial difference in MPB between the BNT-PT and the classical piezoelectric Pb(Zr_1-xTi_x)O₃ is discussed by linking the structure evolutions to the performance at the MPBs. This work helps us to figure out the origin of the unique large high-field-induced electrostrain in BNT-based ceramics, and may be helpful to the design of high-performance piezoelectric ceramics in the future.