Microstructure, depolarization temperature, and piezoelectric properties of (Bi_0.5Na_0.4K_0.1)Ti_0.98M_0.02O_3-δ (M³⁺ = Al³⁺, Fe³⁺) lead-free ceramics

(Bi0.5Na0.4K0.1)Ti0.98Al0.02O3-δ (BNKT-A) and (Bi0.5Na0.4K0.1)Ti0.98Fe0.02O3-δ (BNKT-F) ceramics were prepared via a conventional solid-state reaction method. Microstructure and piezoelectric properties of the two ceramics were comparatively studied. Both the samples exhibit single-phase structures. There is no obvious change in microstructure between the two samples. The depolarization temperatures were estimated from dielectric spectra, change of piezoelectric constant with annealing temperature, and thermally stimulated depolarization current. Compared with BNKT-A, BNKT-F exhibits higher piezoelectric constant and higher depolarization temperature. The Rayleigh analysis exhibits that BNKT-F has higher extrinsic contribution from irreversible domain wall translation process compared with BNKT-A.