Phase transitions and the piezoelectricity around morphotropic phase boundary in Ba(Zr_0.2Ti_0.8)O₃-x(Ba_0.7Ca_0.3)TiO₃ lead-free solid solution

In this paper, two displacive phase transitions around the morphotropic phase boundary (MPB) in Ba(Zr_0.2Ti_0.8)O₃-x(Ba_0.7Ca_0.3)TiO₃ (BZT-xBCT) ceramics were detected by inspecting two anomalies of the Raman Ti⁴⁺-O^2- longitudinal optical mode (∼725cm^-1). Further, permittivity and X-ray diffraction results demonstrated these two phase transitions originate from tetragonal (T) to rhombohedral (R) through an intermediate orthorhombic (O) phase. Importantly, we found that the maximum piezoelectric response (d₃₃=545pC/N) was achieved at the boundary between the T and O phase, indicating that the giant piezoelectricity of BZT-xBCT may mainly stem from the T-O phase boundary due to easier polarization rotation and larger lattice softening.