Understanding the grain size dependence of functionalities in lead-free (Ba,Ca)(Zr,Ti)O₃

Grain size effects in ferroelectric ceramics have long been exploited to tailor their functional properties. However, the underlying mechanism for the grain size effects is not yet fully understood. Here, we study the grain size dependence of domain wall activities in a lead-free piezoceramics system, (Ba,Ca)(Zr,Ti)O₃, with grain sizes in the range of 4 − 22 μm. The dielectric permittivity is highest at intermediate grain sizes (∼12 μm) where there are moderate lattice distortion and the most active domain wall motion under low voltages. Despite the larger fraction of switched domains in the material with larger distortion (∼22 μm), as revealed by in situ electric-field synchrotron X-ray diffraction, time-resolved characterizations demonstrate an easier and faster domain wall dynamics in the sample with moderate lattice distortion. Our analysis and phase-field simulations show that the grain size dependence of the domain wall dynamics of polycrystalline ferroelectrics is dictated by the interaction between an external electric field and the grain size-related change in intergranular stress, and this interaction is most effective in stimulating the movement of non-180° domain wall at intermediate grain sizes during the initial phase of polarization reversal. Our results provide a new fundamental understanding to guide the future design of materials for improving functionalities.