Structure evolution and electric-field-induced reversible transition in perovskite Na(Nb_1−xTa_x)O₃ ceramics

Na(Nb_1−xTa_x)O₃ binary solid-solution ceramics with high quality were fabricated by conventional solid-state sintering routes for improving the electric(E)-field-induced irreversible polarization and transition behaviors of NaNbO₃. The studied results confirm that this binary solid-solution ceramics exhibit orthorhombic Pbcm space group companying with reduced unit-cell volume at x ≤ 0.4, and orthorhombic Pbnm space group at x = 0.5. As the Ta⁵⁺ content increases in the binary solid-solutions, the E-field-induced irreversible antiferroelectric → ferroelectric (AFE → FE) transition becomes reversible at x ≥ 0.2, giving rise to double-polarization hysteresis; the key E-fields triggering both irreversible and reversible transitions (E_F) increase in general. In particular, the E-field-induced FE phase at x = 0.15 is unstable upon unloading E-field to zero, which can return to AFE phase with time lapse. At x = 0.5, the Curie temperature (T_C) of AFE shifts to below room temperature, but E-field-induced reversible transition is still observed, which results in a nonlinear polarization with the lowest hysteresis and contributes to the largest energy-storage density. This transition is not due to the AFE ↔ FE transition but rather to the order ↔ disorder behavior of polar clusters or/and nanoregions within nonpolar Pbnm structure matrix.