Synergistic Defect and Phase Engineering in KNN-Based Ceramics Enable Giant Electro-Strain with Superior Symmetry at Low Sintering Temperatures

Fabricating potassium sodium niobate (KNN)-based materials that simultaneously exhibit high electro-strain and high bipolar strain symmetry at reduced sintering temperatures (Tsinter) is challenging. This study demonstrates that the synergistic effect of Ti ion doping and a reducing atmosphere significantly enhances lattice activity, thereby lowering Tsinter by nearly 160 °C. Reducing Tsinter is more effective than B-site Ti substitution in suppressing oxygen vacancy concentration, which consequently diminishes the defect polarization (PD) and weakens the internal bias field (Ei). Furthermore, the 3% Ti dopant promotes the formation of small-sized nanodomains via constructing an R/O/T multiphase coexistence. This strategy synergistically tailors both defect and multiphase engineering, resulting in nanodomains and a weak PD. The combined effect of these features flattens and symmetrizes the Landau energy profile, yielding a high electro-strain coefficient (d33* = 554 pm/V) and a superior bipolar strain symmetry (β = 0.61%). The developed KNN-based materials show great potential for cofiring with low-cost internal electrodes, enabling the production of economical multilayer devices with a large electro-strain.