Structure-property relationships in the lead-free piezoceramic system K₀.₅Bi₀.₅TiO₃ - BiMg₀.₅Ti₀.₅O₃

Distinctive structure-property relationships are revealed in the relaxor ferroelectric ceramic solid solution, (1-x)K ₀.₅ Bi ₀.₅ TiO₃ - xBiMg₀.₅ Ti ₀.₅ O₃ : 0.02 < x < 0.08. The constructed phase diagram and results of in-situ synchrotron X-ray diffraction provide explanations for temperature and electric field dependent anomalies in dielectric, ferroelectric and electromechanical properties. At room temperature a mixed phase tetragonal and pseudocubic phase field occurs for compositions 0 > x ≤ 0.07. As temperature rises to ≥150 °C, the ferroelectric tetragonal relaxor phase changes to a pseudocubic ergodic relaxor phase; this change in length scale of polar order is responsible for an inflection in relative permittivity - temperature plots. The transition is reversed by a sufficient electric field, thereby explaining the constricted form of polarisation-electric field loops measured at >150 °C. It is also responsible for a change in slope of the strain-electric field (S-E) plots which are relatively linear in the ferroelectric regime i.e. at temperatures up to 150 °C, giving unipolar strains of 0.11% at 20 °C and 0.14% at 150 °C (50 kV/cm field). The additional contribution from the effect of the field-induced pseudocubic to tetragonal transition, generates strains of ∼0.2% at 185 °C. Unusual for a piezoelectric solid solution, the maximum strains and charge coefficients (d ₃₃ = 150 pC/N, 20 °C) do not coincide with a morphotropic or polymorphic phase boundary.