Optimising dopants and properties in BiMeO3 (Me = Al,Ga, Sc,Y, Mg2/3Nb1/3, Zn2/3Nb1/3, Zn1/2Ti1/2) lead-free BaTiO3-BiFeO3 based ceramics for actuator applications

A crystallochemical framework is proposed based on electronegativity difference (e_n) and tolerance factor (t) to optimise the BiMeO₃ dopants and therefore the piezoelectric and high-field strain response in BaTiO₃-BiFeO₃ based ceramics. Compositions in the series 0.05Bi(Me)O₃-0.25BaTiO₃-0.7BiFeO₃ (BMe-BT-BF, Me: Y, Sc_1/2Y_1/2, Mg_2/3Nb_1/3, Sc, Zn_2/3Nb_1/3, Zn_1/2Ti_1/2, Ga, and Al) were fabricated using solid state synthesis and furnace cooled. Scanning electron microscopy and X-ray diffraction revealed that only Bi(Mg_2/3Nb_1/3)O₃ and BiScO₃ dopants, which lie in a narrow range of e_n vs. t, form homogeneous ceramics, free from secondary phases reflected in their superior piezoelectric coefficients (d₃₃ ～145 pC/N). All other BiMeO₃ additions exhibited either secondary phases (Y) and/or promoted a two-phase perovskite matrix (Zn, Ga and Al). The promising initial properties of BiScO₃ doped compositions prompted further studies on 0.05BiScO₃-(0.95-x)BaTiO₃-(x)BiFeO₃ (BS-BT-BF, x?=?0.55, 0.60, 0.625, 0.65, and 0.70) ceramics. As x increased the structure changed from predominantly pseudocubic to rhombohedral, resulting in a transition from a relaxor-like to ferroelectric response. The largest d₃₃^* (465?pm/V) was achieved for x?=?0.625 under 5?kV/mm at the crossover from relaxor to ferroelectric behaviour. BS-BT-BF with x?=?0.625 showed >0.3% strain under 6?kV/mm up to 175?°C, demonstrating its potential for actuator applications.