Origin of composition-insensitive electrostrictive coefficient and continuous decrease of domain wall density in (1-x)NaNbO₃-xBaTiO₃ lead-free ferroelectrics

Perovskite ferroelectric ceramics with giant electrostrictive coefficient Q₃₃ have attracted much attention due to their high strain response with ultralow hysteresis. In this work, the electrostrictive properties as well as domain morphology of the lead-free (1−x)NaNbO₃-xBaTiO₃ (NN-xBT) (0.10 ≤ x ≤ 0.27) system were investigated systematically. A composition-insensitive Q₃₃ of 0.0406 m⁴/C² is identified in the NN-xBT ceramics (0.15 ≤ x ≤ 0.27). An ab initio computation indicates that transverse Q₁₂ would play an important role in understanding of the composition-insensitive Q₃₃. Furthermore, distinctively banding domains and inhomogeneous distribution of nanodomains were observed in NN-xBT by piezoresponse force microscopy. A continuous decrease of domain wall density was identified with respect to the increase of x. This study would not only deepen our understanding of the origin of composition-insensitive Q₃₃ in NN-BT and other related systems, but also provide a way to enhance the Q₃₃, i.e., by forming the solid solutions with end members, which possess high Q₁₂.