High piezoelectric properties of 0.82(Bi 0.5 Na 0.5 )TiO 3 –0.18(Bi 0.5 K 0.5 )TiO 3 lead-free ceramics modified by (Mn 1/3 Nb 2/3 ) 4+ complex ions

The complex ions (Mn1/3Nb2/3)4+ doped 0.82BNT–0.18BKT (BNKT-xMN) ceramics were prepared by conventional solid-state sintering. The effects of the MN content on the structural and electrical properties of the BNKT-xMN ceramics were investigated. The grain size decreases sharply after doping MN. With the increase of the MN content, the phase structure changes from the rhombohedral and tetragonal phase to the tetragonal phase, then to the pseudo-cubic phase. The ferroelectric phase transforms to the relaxor phase. At critical phase (x = 0.03), the maximum positive bipolar strain and unipolar strain are 0.38 and 0.386%, respectively. The corresponding $$d_{33}^{*}$$ and d33 are 767 pm V–1 and 158 pC N–1, respectively. Meanwhile, the dielectric constant gradually decreases with the increase of the MN content, which flattens the permittivity curves. The large piezoelectric responses are closely associated with the reversible relaxor-ferroelectric phase transformation.