Triple-point-type morphotropic phase boundary based large piezoelectric Pb-free material-Ba(Ti _0.8Hf _0.2)O ₃-(Ba _0.7Ca _0.3)TiO ₃

We report a large piezoelectric Pb-free system-Ba(Ti _0.8Hf _0.2)O ₃-(Ba _0.7Ca _0.3)TiO ₃, designed upon a triple-point morphotropic phase boundary (TMPB) idea. The system shows anomalies of both ac (dielectric and piezoelectric) and dc (P-E hysteresis) properties around TMPB, especially the d ₃₃ achieving ∼550 pC/N at room temperature. Moreover, the detected non-zero thermal hysteresis around triple point shows a first order transition nature and non-isotropic polarization state, as well as verified by our theoretical Landau-type deduction, thus being considered as an important factor to influence the piezoelectricity along TMPB. Our work may stimulate the study on triple-point-related critical phenomena in other ferroic systems.