Phase transition, ferroelectric and piezoelectric properties of B-site complex cations (Fe0.5Nb0.5)4+-modified Ba0.70Ca0.30TiO3 ceramics

Ba 0.70 Ca 0.30 Ti 1- x (Nb 0.5 Fe 0.5 ) x O 3 lead-free ceramics (abbreviated as BCTNF x , x  = 0–0.12) were prepared by solid-state reaction method. The influence of B-site (Fe 0.5 Nb 0.5 ) 4+ substitution on phase structure, microstructure, dielectric , ferroelectric and piezoelectric properties of Ba 0.70 Ca 0.30 TiO 3 ceramics were systematically studied. The BCTNF x 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 0.5 Nb 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 (tan δ ) 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 0.5 Nb 0.5 ) 4+ into Ba 0.70 Ca 0.30 TiO 3 ceramics. The BCTNF0.02 ceramics achieve optimal electrical properties with ε r ${\epsilon }_r$  = 1268, tan δ $\delta$  = 0.03, T m  = 102 °C, P max  = 12.8 μC/cm 2 , P r  = 6.1 μC/cm 2 , E C  = 6.0 kV/cm, d 33  = 117 pC/N, k p % = 0.18%, Q m  = 236, bipolar S max % = 0.20%, unipolar S max % = 0.16% and d 33 ∗ $d_{33}^{\ast }$  = 314 pm/V. The variation of electrical properties of BCTNF x 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 0.5 Nb 0.5 ) 4+ displacement.