Enhanced Electrostrain in Polarized Cu-Doped KTN Single Crystals by Reversible Domain Switching

Improving electrostrain in lead-free piezoelectric materials is critical for practical use. This study examines KTN crystals and employs two primary strategies to enhance their electrostrain: (1) Cu²⁺ doping creates a restoring force enabling reversible domain switching. (2) Polarizing Cu:KTN crystals and applying an electric field perpendicular to the polarization direction ensure that all domains contribute to the strain. In 0.36 wt % Cu-doped KTN crystal, a large unipolar strain of 0.32% was achieved under an electric field of 15 kV/cm, among the highest reported for KTN crystals. To reveal the mechanism of electrostrain enhancement, Raman spectroscopy was used to analyze the effects of ion doping on the lattice structure, showing that Cu doping leads to lattice contraction. High-resolution transmission electron microscopy images confirm that this contraction results in significant internal stress and strain. The variation of domain structure with the electric field confirms the existence of reversible domains, and Electron Paramagnetic Resonance spectroscopy demonstrates that the source of the restoring force for reversible domains is polar defect dipoles, which corresponds to large strain and double hysteresis loops. This work aids in obtaining large electrostrain via reversible domain switching in aged acceptor-doped piezoelectric crystals, promoting practical use of lead-free piezoelectrics.