Nanoscale ferroelectric/relaxor composites: Origin of large strain in lead-free Bi-based incipient piezoelectric ceramics

La-doped (Bi0.5Na0.41K0.09)TiO3 ceramics were synthesized using a conventional solid-state reaction route. X-ray diffraction and electric-field-induced polarization studies revealed a phase transformation from a ferroelectric to a relaxor with increasing La content. Piezoresponse force microscopy and transmission electron microscopy indicated the coexistence of ferroelectric and relaxor phases at the morphotropic phase boundary (MPB). A normalized strain of 857 pm/V was observed at the MPB at 4 kV/mm, which is comparable to that of other macro ferroelectric/relaxor composite materials. We propose that the inherent nanoscale ferroelectric/relaxor composite in the La-doped (Bi0.5Na0.41K0.09)TiO3 ceramic be the origin of such giant electric-field-induced strain.