Mechanical properties and the anomalous trade-off between ferroelectricity and electrostrain performance via Sm3+ A-/B-site substitution in KNN-based ceramics

Abstract

Utilising the amphoteric doping behaviour of rare-earth ions to regulate defect chemistry is an effective strategy for enhancing the electrical properties of KNN-based lead-free piezoelectric ceramics. In this work, (1−x)K_0.5Na_0.5Nb_0.98Ta_0.02O₃-xSm₂O₃ (KNNT-xSm) ceramics were prepared using a conventional solid-state reaction. The effects of varying x on phase composition, average grain size, oxygen-vacancy concentration, and defect dipoles were systematically investigated, and the correlations of these properties with dielectric dispersion, ferroelectric relaxation, and electrostrain behaviour were explored. Key factors influencing Vickers hardness were also explored. The results showed that Sm³⁺ ions may modulate oxygen vacancy concentration and microstructure, thereby affecting the overall material performance. At x = 1.05 mol%, the ceramics exhibited excellent properties, including a remanent polarisation of 52.64 μC/cm², piezoelectric coefficient of 251.79 p.m./V, and Vickers hardness of 5.58 GPa. An anomalous trade-off between remanent polarisation and piezoelectric coefficient was observed and is suggested to be associated with changes in oxygen-vacancy concentration and defect dipoles arising from the site-occupancy transition of Sm³⁺ ions. Furthermore, the Vickers hardness of KNNT-xSm ceramics appears to be primarily governed by lattice energy and microstructure. This study provides valuable insights into the optimisation of rare-earth-doped KNN-based piezoceramics.