Phase transition, ferroelectric and piezoelectric properties of B-site complex cations (Fe_0.5Nb_0.5)⁴⁺-modified Ba_0.70Ca_0.30TiO₃ ceramics

Ba_0.70Ca_0.30Ti_1-x (Nb_0.5Fe_0.5)_xO₃ lead-free ceramics (abbreviated as BCTNFx, x = 0–0.12) were prepared by solid-state reaction method. The influence of B-site (Fe_0.5Nb_0.5)⁴⁺ substitution on phase structure, microstructure, dielectric, ferroelectric and piezoelectric properties of Ba_0.70Ca_0.30TiO₃ ceramics were systematically studied. The BCTNFx ceramics consist of diphasic tetragonal (T) and orthorhombic (O) phases at x = 0–0.04, pseudocubic (PC) and O phases at x = 0.05–0.08, cubic (C) and O phases at 0.08 < x ≤ 0.12. Increase of (Fe_0.5Nb_0.5)⁴⁺ doping improves sinterability, depresses the ferroelectric - paraelectric phase transition temperature (T_m) monotonically from 128 °C at x = 0 to -86 °C at x = 0.12, induces enhanced relaxor behavior and weakens piezoelectric properties. The room temperature relative dielectric constant (ε_r) and dielectric loss tangent (tanδ) increase with increasing x and reach the maximum values of 4869 and 0.06 respectively at x = 0.06. Whereas the average grain size (abbreviated as AGS), ferroelectric properties and electric field induced strain increase significantly by introducing 2.0 mol% B-site complex cations (Fe_0.5Nb_0.5)⁴⁺ into Ba_0.70Ca_0.30TiO₃ ceramics. The BCTNF0.02 ceramics achieve optimal electrical properties with ε_r = 1268, tan δ = 0.03, T_m = 102 °C, P_max = 12.8 μC/cm², P_r = 6.1 μC/cm², E_C = 6.0 kV/cm, d₃₃ = 117 pC/N, k_p% = 0.18%, Q_m = 236, bipolar S_max% = 0.20%, unipolar S_max% = 0.16% and d₃₃ ^∗ = 314 pm/V. The variation of electrical properties of BCTNFx ceramics is attributed to the combined action of grain size effect, phase transition, weakness of octahedral distortion, clamping effects of the internal bias electric field E_i on domain wall motion induced by B-site cations (Fe_0.5Nb_0.5)⁴⁺ displacement.