Defect engineering on phase structure and temperature stability of KNN ‐based ceramics sintered in different atmospheres

Lead‐free MnO‐doped 0.955K 0.5 Na 0.5 NbO 3 ‐0.045Bi 0.5 Na 0.5 ZrO 3 (Abbreviated as KNN ‐0.045 BNZ ) ceramics have been prepared by the conventional solid‐state sintering method in reducing atmosphere ( p o 2  = 1 × 10 −10  atm) and air. For ceramics sintered in reducing atmosphere, only Mn 2+ ions exist in ceramics who preferentially occupy the cation vacancies in A‐site at x  =   0.2‐0.4, whereas Mn 2+ ions substitute for Zr 4+ ions in B‐site to form defects ( Mn Zr ″ ) at x  >   0.4. For ceramics sintered in air, mixed Mn 2+ , Mn 3+ , and Mn 4+ ions coexist here. The Mn 2+ ions preferentially occupy the cation vacancies in A‐site at x  =   0.2‐0.4 and then Mn 2+ ions substitute for Zr 4+ ions in B‐site at x  >   0.4. Meanwhile, the Mn 3+ ions and Mn 4+ ions substitute for Nb 5+ ions in B‐site to form defects ( Mn Nb ′andMn Nb ″ ) at x  =   0.2‐0.8. The ( V Na ′ − V o · ·,V K ′ − V o · ·, andV Bi ″ ′ − V o · ·) dipolar defects show a positive dipolar defect contribution ( DDC ) to the d 33 ∗ , whereas the dipolar defects ( Mn Zr ″ − V o · ·) show a negative DDC to the d 33 ∗ . The dipolar defects ( Mn Nb ′ ‐ V o · ·andMn Nb ″ − V o · ·) can help improve the temperature stability of d 33 ∗ . The 0.4% MnO‐doped KNN ‐0.045 BNZ ceramics sintered in reducing atmosphere show excellent piezoelectric constant d 33  = 300 pC/N and 0.2% MnO‐doped KNN ‐0.045 BNZ ceramics sintered in air possess optimal piezoelectric constant d 33  = 290 pC/N.