Aurivillius phase CaBi₂Nb₂O₉ (CBNO) ceramic with an ultrahigh Curie temperature (T_C) of ~934 °C shows huge potential in high-temperature piezoelectric applications. However, low piezoelectricity and poor electric insulation prevent its applications in high-temperature sensing. Here, we propose an effective multi-field coupling strategy to synergistically optimize piezoelectric property, electrical conduction behavior and temperature stability of CBNO ceramic. The constructed lattice stress and electric fields induced by introducing Li/Pr and Bi/Sc doping have great impacts on the lattice structure, microstructure, domain structure and defect chemistry. Therefore, a significant increase in piezoelectric activity (d₃₃) is resulted from the enhancement of polarization, the improvement of breakdown electric field and the production of nanoscale domains. In especial, the existence of pseudo-tetragonal phase boundary is helpful for the enhanced d₃₃. In the designed Ca_1–3x (Li_0.5Pr_0.5)_xBi_2+2xNb_2–xSc_xO₉ system, a high d₃₃ of ~18.2 pC/N accompanied by an ultrahigh T_C of ~938 °C is achieved in the x = 0.02 ceramic. This combined with high electrical resistivity (ρ~1.72 MΩ·cm at 600 °C) and nearly stable d₃₃ (up to 800 °C) indicates that it is a very promising piezoelectric material for high-temperature (up to 600 °C or higher) sensing applications.

Off-stoichiometry of perovskite structural Bi_0.5Na_0.5TiO₃ (BNT) ferroelectrics can give rise to considerable oxide-ion conductivity. The inherent structural characteristics are urgent to be resolved due to its particular sensitivity of the conduction mechanism to the nominal composition and synthesis process. Herein, a thorough study of the temperature-dependent neutron, X-ray diffraction and Raman spectrum is carried out on a series of equivalently substituted A-site deficient non-stoichiometric and pristine BNT. Phase transition and defect association are systemically investigated in these dominated rhombohedral phases at room temperature, associated with well saturated ferroelectric states. Significant structural evolution identified by Rietveld refinements and the origin of the electrical performance are clarified at elevated temperatures, focusing on the subtle distortions of ionic displacements, oxygen octahedral tilts and local chemical environments for oxygen vacancies. The ion migration ability mediated by oxygen vacancies that are not energetically favorable in BNT mainly depends on the external substitutional disorder, and is strongly affected by the dopant concentration. Together with the lone pair substitution concept, superior oxide ionic conductivity is achieved, and an alternative strategy is provided in designing BNT based oxide ion conductors.