The effect of microstructure on piezoelectric properties and temperature stability for MnO doped KNN-based ceramics sintered in different atmospheres

Lead-free 0.4% MnO doped 0.955K0.5Na0.5NbO3-0.045Bi0.5Na0.5ZrO3 (Abbreviated as 0.955KNN-0.045BNZ) ceramics were prepared by a conventional solid-state sintering method in both ambient air and reducing atmosphere (H2/N2/H2O, oxygen partial pressure po2  = 1 × 10−10 atm). All samples show a polymorphic phase boundary (PPB) with rhombohedral (R)/tetragonal (T) phase. The R phase fraction of ceramics sintered in reducing atmosphere is higher than that of ceramics sintered in ambient air. The valence state of almost all of the Mn ions are +2 for the ceramics sintered in reducing atmosphere. Mixed valence states (+2, +3, +4) of Mn ions coexist in the 0.4% MnO doped 0.955KNN-0.045BNZ ceramic sintered in air. The formation of dipolar defect (MnNb′-Vo·· and MnNb″-Vo··) in poled ceramics sintered in air is due to the substitution of Nb5+ ions with Mn3+ ions and Mn4+ ions. The ceramics sintered in air show strong dielectric diffusion behavior under electric field (35 kV/cm) due to the existence of (∼200 nm) sub-micrometer domains with high activation energy. Nanoscale domains (∼90 nm) in ceramics sintered in reducing atmosphere is the origin of its high piezoelectric properties.