Acceptor-doping of lead-free (Ba0.82Ca0.18)(Zr0.08Ti0.92)O3 with Fe induces piezoelectric hardening

Abstract

The growing need for electroactive components in modern electronics, autonomous vehicles, and miniaturized medical equipment requires new sustainable materials with improved piezoelectric performance. The lead-free perovskite system (Ba,Ca)(Zr,Ti)O₃ has already been demonstrated to be an excellent alternative to Pb(Zr,Ti)O₃ for soft-type piezoelectric applications, but its adaptability to hard-type piezoelectric requirements remains unclear. Here, we demonstrate hardening of (Ba,Ca)(Zr,Ti)O₃ via targeted defect formation using Fe-acceptor doping. We reveal the underlying defect equations and provide experimental evidence for the formation of defect complexes using electron paramagnetic resonance. We describe the influence of Fe-doping on the crystal structure, phase transition behavior, and electromechanical properties, and found outstanding electrostrictive coefficients, up to 0.063 m² C⁻⁴, in highly doped relaxor-like (Ba,Ca)(Zr,Ti)O₃. Aging experiments on poled compositions with <1% Fe revealed that piezoelectric hardening through defect complexes occurs in BCZT only after prolonged aging. This resulted in an increase of the figure of merit for resonance applications by over 280% and considerably improved high-power behavior, exceeding the stability of commercial Pb(Zr,Ti)O₃ materials. Our results demonstrate the large potential of (Ba,Ca)(Zr,Ti)O₃ for resonant piezoelectric applications and simultaneously give insights into the defect chemistry of acceptor-doped (Ba,Ca)(Zr,Ti)O₃.