Low-field strain-driven phase diagram of (Bi1/2Na1/2)TiO3–SrTiO3–LiNbO3 lead-free relaxor ceramics for actuator applications

Abstract

This work presents the design and systematic investigation of a new ternary system, (1 − x − y)(Bi_1/2Na_1/2)TiO₃–xSrTiO₃–yLiNbO₃ (BNST100x/100yLN) ceramics, which was created to address the problems associated with high driving fields and large hysteresis in conventional (Bi_1/2Na_1/2)TiO₃ (BNT)-based ceramics. The synthesis of these ceramics was achieved through a conventional solid-state reaction route, followed by extensive characterization regarding structural evolution, phase transition behavior, dielectric response, ferroelectric properties, and electric-field-induced strain. The addition of LiNbO₃ (LN) greatly accelerates the sintering behavior kinetics allowing high densification at lower temperatures while also causing an evident transition from non-ergodic relaxor (NER) to ergodic relaxor (ER) states. This transition is always confirmed by (i) strong frequency dispersion and T_m shift in dielectric spectra, (ii) progressive slimming of P–E loops with suppressed remanent polarization, and (iii) evolution of bipolar strain from butterfly-shaped to nearly linear behavior. A low-field (≤4 kV mm⁻¹) strain-driven phase diagram was constructed based on multiple quantitative criteria; it shows a systematic shift of the phase boundary toward the BNT-rich region upon LN incorporation. A broad NER-ER coexistence region has been found where an optimal balance between polarization magnitude and relaxor dynamics gives rise to a superior electromechanical response. For example, compositions like BNST24/1LN and BNST24/2LN show big reversible strains (∼0.22–0.24%) with reduced hysteresis under a relatively low electric field of 4 kV mm⁻¹. The enhanced performance is due to defect-mediated polarization dynamics where Li⁺/Nb⁵⁺ co-substitution creates oxygen vacancies and strong random fields that promote very dynamic polar nanoregions (PNRs) and flatten the free energy landscape. This finding shows that LN is acting as an effective phase-boundary engineering component instead of just being a passive dopant; thus, giving a good way toward developing high-performance low-field lead-free piezoelectric actuators.