Enhancement of piezoelectric response based on oxygen vacancy migration behavior in (K, Na)NbO₃ crystals

Piezoelectric properties of materials are strongly influenced by atomic-scale defects. Proper design and modulation of oxygen vacancy (V_O^{⋅ ⋅}) through external fields are crucial for the comprehensive optimization of piezoelectric materials. In this study, we investigate the effects of poling on V_O^{⋅ ⋅} in potassium sodium niobate (KNN) crystals, and design corresponding strategies to enhance the piezoelectric performance. We confirm the migration of V_O^{⋅ ⋅} toward the negative surface under an electric field, which is then retained after the removal of the field, resulting in a V_O^{⋅ ⋅}-rich negative surface. Based on this, we grind away the negative surface to reduce V_O^{⋅ ⋅} and weaken the shielding effect, significantly improving the small-signal d ₃₃ (from 276 to 338 pC/N) and electric field-induced strain performance (from 0.038% to 0.081%). Furthermore, the decreased V_O^{⋅ ⋅} enhances the switching capability of domains, resulting in an additional transformation of the polarization orientations from [1 1¯0] to [1¯01], compared to that of the original crystal. These findings contribute to fully exploring the application potential of KNN crystals. The V_O^{⋅ ⋅}-rich surface of poled KNN crystals may offer promising applications in piezoelectric catalysis. Our results provide valuable insights for the modulation of piezoelectric properties.