Study of intrinsic / extrinsic piezoelectric contributions in La-doped BiFeO3 - PbTiO3 ceramics using the Rayleigh method

BiFeO₃ -PbTiO₃ (BFPT) based materials have received interest as high temperature piezoelectrics due to an extremely high Tc of 632degC, and a spontaneous strain >18%; this high strain makes these materials extremely difficult to pole. Doping with small concentrations of La allows materials to be prepared with a d₃₃ > 100 pm V^-1 whilst maintaining a T_c > 450degC. Presented here are results from a Rayleigh analysis of these materials which shows that the extrinsic contribution to the piezoelectric effect is extremely low, providing Rayleigh coefficients (alpha_d) between 4.2 and 6.3 times 10^-18 m²V^-2, considerably lower than that observed in PZT. The huge deviation from observations in PZT and BiFeO₃ -PbTiO₃ outline considerable domain wall locking in BFPT, which has a detrimental effect on the piezoelectric activity in this system.