Dielectric, ferroelectric and piezoelectric behaviors of thulium-doped KNN ceramics fabricated by microwave sintering

(K,Na)NbO 3  −  x at.% Tm 2 O 3 (KNNT x ) piezoelectric ceramics with submicron grain size were successfully prepared by solid-state method combined with microwave sintering. With the increase of content of Tm 3+ , KNNT x ceramics gradually transformed from orthorhombic (O) phase to the coexistence of O and tetragonal (T) phase at room temperature, and the phase transition temperature of O–T phase shifted to low temperature region. The leakage behaviors, dielectric, ferroelectric, and piezoelectric properties of KNNT x ceramics depend on the structure evolution, cationic vacancy and grain refinement caused by Tm 3+ doping, and the relevant mechanisms have been illustrated. The substitution of Tm 3+ at A sites increases the critical driving force for the rapid growth of grains and obviously refine the grain of KNNT x ceramics, resulting in its deteriorated ferroelectricity and enhanced piezoelectricity. Especially the piezoelectric constant of KNNT x ceramics increased from 109.8 to 163.6 pC/N. These findings are of great significance for developing excellent lead-free piezoelectric materials.