Phase-Composition- Dependent Piezoelectric and Electromechanical Strain Properties in (Bi1/2Na1/2)TiO3-Ba(Ni1/2Nb1/2)O3 Lead-Free Ceramics

New lead-free perovskite solid solution ceramics of (1-x)(Bi1/2Na1/2)TiO3-xBa(Ni1/2Nb1/2)O-3[(1-x)BNT-xBNN,x=0.02-0.06) were prepared and their dielectric, ferroelectric, piezoelectric, and electromechanical properties were investigated as a function of the BNN content. The X-ray diffraction results indicated that the addition of BNN has induced a morphotropic phase transformation from rhombohedral to pseudocubic symmetry approximately at x=0.045, accompanying an evolution of dielectric relaxor behavior as characterized by enhanced dielectric diffuseness and frequency dispersion. In the proximity of the ferroelectric rhombohedral and pseudocubic phase coexistence zone, the x=0.045 ceramics exhibited optimal piezoelectric and electromechanical coupling properties of d(33)similar to 121pC/N and k(p)similar to 0.27 owing to decreased energy barriers for polarization switching. However, further addition of BNN could cause a decrease in freezing temperatures of polar nanoregions till the coexistence of nonergodic and ergodic relaxor phases occurred near room temperature, especially for the x=0.05 sample which has negligible negative strains and thus show the maximum electrostrain of 0.3% under an external electric field of 7kV/mm, but almost vanished piezoelectric properties. This was attributed to the fact that the induced long-range ferroelectric order could reversibly switch back to its original ergodic state upon removal of external electric fields.