A candidate for lead-free ultrahigh-temperature piezoelectrics: The excellent electro-mechanical properties of Aurivillius oxides, Ca _1-5xLi_2xNd_2x□_xBi ₂Nb_2-2xSc_xW_xO_9-1.5x

Modified ultrahigh-temperature piezoelectric ceramics, Ca _1-5xLi_2xNd_2x□_xBi ₂Nb_2-2xSc_xW_xO_9-1.5x (CBNO-LiNd-ScW-x, x = 0, 0.01, 0.02, 0.03, 0.04; □ represented A-site vacancies), were synthesized by a traditional solid-state reaction process. XRD Rietveld refinement, TEM, HRTEM and Raman spectroscopy were used to characterize the crystal structures and domain structures of the prepared ceramics. The lithium/neodymium (Li/Nd) and scandium/tungsten (Sc/W) co-substitution as well as the existence of small amounts of A-site vacancies significantly enhanced the ferroelectric and electro-mechanical properties of the CBNO-LiNd-ScW-x ceramics. The Curie temperature (T_c) decreased from 941°C to 832°C as the x values increased in CBNO-LiNd-ScW-x, because of an enhancement in the cation disordering between the A site and the bismuth oxide layer. The CBNO-LiNd-ScW-0.01 ceramic with an ultrahigh T_c of 908°C had a high piezoelectric activity (d₃₃) of 20.6 pC N ^-1, a large field-induced strain (S₃₃) of 0.27 × 10^-3 and a high remnant polarization (P_r) of 6.1 μC cm^-2, due to the perfect growth in the ferroelectric domains and the contribution from the reversal of the defect dipoles (V _Ca′′-V_O^•• and Sc _Nb′′-V_O^••); interestingly, it still has a high d₃₃ of 19.5 pC N^-1 after up to 800°C thermal annealing. In addition, the CBNO-LiNd-ScW-0.01 ceramic had a relatively high resistivity of >10⁶ Ω cm at 600°C and >10⁵ Ω cm at 700°C.