Various cubic-based polymorphic phase boundary structures in (1-y)(Na_0.5K_0.5)(Nb_1-xSb_x)-yCaTiO₃ ceramics and their piezoelectric properties

CuO-added (1-y)(Na_0.5K_0.5)(Nb_1-xSb_x)-yCaTiO₃ ceramics with 0.0 ≤ x ≤ 0.1 and 0.04 ≤ y ≤ 0.055 were sintered at 970 °C. Various cubic-based polymorphic-phase-boundary (PPB) structures were formed: orthorhombic-tetragonal-cubic, and tetragonal-cubic PPB structures. The crystal structure of the specimens near Curie temperature is a mixture of Pm3m cubic and P4mm tetragonal structures and these ceramics showed ferroelectric properties. A schematic phase diagram of these ceramics has been suggested. The piezoelectric properties of these specimens were influenced by their crystal structure. All the specimens are expected to have good fatigue properties. In particular, the specimen with a tetragonal-cubic PPB structure (x = 0.04 and y = 0.045) has a large d₃₃ of 338 pC/N and a high strain of 0.17% at 4.5 kV/mm. This specimen has good temperature stability; a d₃₃ value of 300 pC/N and strain of approximately 0.12% were obtained at 175 °C, indicating that this specimen is a good candidate for use as a piezoelectric multilayer actuator.