Decoding the effect of defect and domain on piezoelectric properties of K0.5Na0.5NbO3-based single crystals

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The ultra-high piezoelectric activity of K0.5Na0.5NbO3 (KNN)-based single crystals with excellent electromechanical coupling characteristics has attracted great interest. However, the growth of KNN-based single crystals is restricted by their high melting point and harsh equipment conditions. In this work, a large-sized single crystal of (K0.5Na0.5)0.994Bi0.006Nb0.998Cu0.004O3 (KNNBC) was grown using an inexpensive and simple seed-free solid state crystal growth method. The growth mechanism, domain structures, defects, piezoelectric properties, as well as the conduction mechanisms of the KNNBC single crystal were systematically investigated. A significant piezoelectric coefficient (d33) as high as 392 pC/N is obtained in the KNNBC single crystal, which is three times higher than that of ceramics with the same composition, and the single crystal also maintains a high Curie temperature (Tc ∼ 399 °C). The superior piezoelectric properties are believed to stem from the ordered arrangement of large-sized striped domains (∼300 nm) and the sensitivity to polarization rotation due to the flattening of the Landau energy density. Additionally, the spontaneous polarization combined with complex defect dipoles consisting of Cu2+ ions and O2− vacancies effectively modulates both the quality factor (Qm) and the piezoelectric coefficient (d33) of the single crystals. This work broadens the growth strategy of high-performance KNN-based single crystals and highlights the great benefits of the synergistic modulation of defects and domains on the piezoelectric properties of single crystals.