Piezoelectric ceramics with compositions at the morphotropic phase boundary in the BiFeO₃-PbZrO₃-PbTiO₃ ternary system

Since ceramics in the PbZrO₃-PbTiO₃ binary system display excellent piezoelectric properties and those in BiFeO ₃-PbTiO₃ exhibit high Curie temperatures, morphotropic phase boundary (MPB) compositions in the BiFeO₃-PbZrO ₃-PbTiO₃ ternary solid solution system are investigated for the development of piezoelectric ceramics for high temperature applications. It is found that the MPB compositions in the ternary system deviate away from the mixture of two binary MPB compositions 0.70BiFeO₃-0.30PbTiO ₃ and 0.52PbZrO₃-0.48PbTiO₃. With decreasing amount of BiFeO₃ in the MPB compositions in the ternary system, the Curie temperature T_C is observed to decrease while the piezoelectric coefficient d₃₃ is found to increase. Accompanied with this trend are the decrease in the c/a ratio of the tetragonal phase, the increase in the dielectric constant and the decrease in the loss tangent of the ceramics at room temperature. It is further noticed that the compositions in the rhombohedral-rich side of MPB exhibit slightly better piezoelectric properties. An example of such compositions is 0.511BiFeO₃-0.326PbZrO ₃-0.163PbTiO₃, with T_C of 431°C, d ₃₃ of 101 pC/N, and the electromechanical coupling factor k _p of 0.50.