Microstructure basis for strong piezoelectricity in Pb-free Ba(Zr _0.2Ti_0.8)O₃-(Ba_0.7Ca _0.3)TiO₃ ceramics

In this letter, we use transmission electron microscopy to study the microstructure feature of recently reported Pb-free piezoceramic Ba(Zr _0.2Ti_0.8)O₃-x(Ba_0.7Ca _0.3)TiO₃ across its piezoelectricity-optimal morphotropic phase boundary (MPB) by varying composition and temperature, respectively. The domain structure evolutions during such processes show that in MPB regime, the domains become miniaturized down to nanometer size with a domain hierarchy, which coincides with the d₃₃-maximum region. Further convergent beam electron diffraction measurement shows that rhombohedral and tetragonal crystal symmetries coexist among the miniaturized domains. Strong piezoelectricity reported in such a system is due to easy polarization rotation between the coexisting nano-scale tetragonal and rhombohedral domains.