A structural phase boundary due to oxygen octahedral tilt–untilt transition in Bi0.5Na0.5TiO3-based piezoelectric ceramics

The investigation of fine structural evolutions around the morphotropic phase boundary (MPB) is important to understand the origin of the high piezoelectricity in ferroelectric ceramics. Here, we report a new phase boundary associated with oxygen octahedral tilt–untilt transition by exploring average and local structural evolutions in the vicinity of MPB in Bi0.5Na0.5TiO3–PbTiO3 (BNT–PT) ceramics. It is found that the local structural change of the oxygen octahedral tilt–untilt transition is decoupled with the average structural change of the rhombohedral-to-tetragonal phase transition at MPB in BNT–PT. The slight octahedral tilting distortion at MPB is responsible for the large high-field-induced electrostrain and small low-field-induced piezoelectricity in BNT–PT. The crucial difference of MPB between BNT–PT and the classical piezoelectric Pb(Zr,Ti)O3 is discussed from their local structural evolutions of octahedral tilting to the performance changes at MPB. It helps us to figure out the origin of large high-field-induced electrostrain but small piezoelectricity in BNT-based ceramics.