Temperature dependent, large electromechanical strain in Nd-doped BiFeO₃-BaTiO₃ lead-free ceramics

Lead-free piezoceramics with the composition 0.7(Bi_1-xNd_x)FeO₃-0.3BaTiO₃+0.1 wt% MnO₂ (BNxF-BT) were prepared using a conventional solid state route. X-ray diffraction and temperature dependent permittivity measurements indicated a transition from a composition lying at a morphotropic phase boundary (MPB) to a pseudocubic phase as a function of Nd concentration. The highest maximum strain (S_max ∼ 0.2% at 60 kV/cm) and effective piezoelectric coefficient (d₃₃* = 333 pm/V) were obtained at room temperature for the composition BN0.02F-BT. The decrease in remanent polarization (P_r) and Berlincourt d₃₃ with increase in Nd concentration can be attributed to the coexistence of ferroelectric and relaxor phases. In-situ polarisation and strain measurements revealed an increase in P_r and d₃₃* with temperature and a reduction in the coercive field E_C. Presumably this behavior is due to a combination of thermally activated domain wall motion and lowering of the activation energy for a field induced relaxor-ferroelectric transition, as the Curie maximum is approached.