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 BiMeO 3 dopants and therefore the piezoelectric and high-field strain response in BaTiO 3 -BiFeO 3 based ceramics. Compositions in the series 0.05Bi(Me)O 3 -0.25BaTiO 3 -0.7BiFeO 3 (BMe-BT-BF, Me: Y, Sc 1/2 Y 1/2 , Mg 2/3 Nb 1/3 , Sc, Zn 2/3 Nb 1/3 , Zn 1/2 Ti 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 2/3 Nb 1/3 )O 3 and BiScO 3 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 (d 33 ∼145 pC/N). All other BiMeO 3 additions exhibited either secondary phases (Y) and/or promoted a two-phase perovskite matrix (Zn, Ga and Al). The promising initial properties of BiScO 3 doped compositions prompted further studies on 0.05BiScO 3 -(0.95-x)BaTiO 3 -(x)BiFeO 3 (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 d 33 * (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.