Dramatic influence of Dy³⁺ doping on strain and domain structure in lead-free piezoelectric 0.935(Na_1/2Bi_1/2)TiO₃-0.065BaTiO₃ ceramics

An electric-field induced giant strain response and doping level dependent domain structural variations have been studied in the dysprosium (Dy³⁺)-modified 0.935(Na_1/2Bi_1/2)TiO₃-0.065BaTiO₃(xDy: NBBT) ceramics with the doping levels of 0%, 0.5%, 1%, and 2%. X-ray diffraction and Raman spectroscopy analyses not only demonstrates the change in ionic configurations induced by Dy³⁺ doping, but also shows the local crystal symmetry for x ≥ 0.5% doping levels to deviate from the idealized cubic structure. Piezoresponse force microscopy measurement exhibits the presence of an intermediate phase with orthorhombic symmetry at the critical Dy³⁺ doping level of 2%. Moreover, at this doping level, a giant recoverable nonlinear strain of ∼0.44% can be observed with high normalized strain (S_max/E_max) of 728 pm/V. At the same applied field, the strain exhibits a 175% increase than that of NBBT ceramic. Such a large strain stems from the varying coherence lengths of polar nanoregions (PNRs) and an unusual reversible 90° domain switching caused by the symmetry conforming property of point defects, where the restoring force is provided by unswitchable defects. The mechanism reveals a new possibility to achieve large electric-field strain effect for a wide range of ferroelectric systems, which can lead to applications in novel "on-off" actuators.