Large Piezoelectric Effect in Pb-Free Ceramics

We report a non-Pb piezoelectric ceramic system $\mathrm{Ba}({\mathrm{Ti}}_{0.8}{\mathrm{Zr}}_{0.2}){\mathrm{O}}_{3}\mathrm{\text{\ensuremath{-}}}({\mathrm{Ba}}_{0.7}{\mathrm{Ca}}_{0.3}){\mathrm{TiO}}_{3}$ which shows a surprisingly high piezoelectric coefficient of ${d}_{33}\ensuremath{\sim}620\text{ }\text{ }\mathrm{pC}/\mathrm{N}$ at optimal composition. Its phase diagram shows a morphortropic phase boundary (MPB) starting from a tricritical triple point of a cubic paraelectric phase ($C$), ferroelectric rhombohedral ($R$), and tetragonal ($T$) phases. The high piezoelectricity of the MPB compositions stems from the composition proximity of the MPB to the tricritical triple point, which leads to a nearly vanishing polarization anisotropy and thus facilitates polarization rotation between $⟨001⟩T$ and $⟨111⟩R$ states. We predict that the single-crystal form of the MPB composition of the present system may reach a giant ${d}_{33}=1500--2000\text{ }\text{ }\mathrm{pC}/\mathrm{N}$. Our work may provide a new recipe for designing highly piezoelectric materials (both Pb-free and Pb-containing) by searching MPBs starting from a TCP.