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 (en) and tolerance factor (t) to optimise the BiMeO3 dopants and therefore the piezoelectric and electrostrictive response in BaTiO3-BiFeO3 based ceramics. Compositions in the series 0.05Bi(Me)O3-0.25BaTiO3-0.7BiFeO3 (BMe-BT-BF, Me: Y, Sc1/2Y1/2, Mg2/3Nb1/3, Sc, Zn2/3Nb1/3, Zn1/2Ti1/2, Ga, and Al) were fabricated using solid state synthesis and furnace cooled. Scanning electron microscopy and X-ray diffraction revealed that only Bi(Mg2/3Nb1/3)O3 and BiScO3 dopants, which lie in a narrow range of en 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 175ºC, demonstrating its potential for actuator applications.