Controllable gradient piezoelectric properties in ferroelectric single crystals

Functional gradient materials (FGMs) possess gradient-varying properties, which make them important in applications for connecting different materials and inhomogeneous environments. Ferroelectric single crystals have multiple excellent physical properties, but it is difficult to design gradient properties during the crystal growth. Here, a method is reported to achieve gradient piezoelectric properties in the tetragonal Mn&Fe-doped KTa_1−xNb_xO₃ (Mn&Fe: KTN) crystals by alternating current poling and internal strain design. Furthermore, opposite piezoelectric coefficients are obtained in the direction perpendicular to the applied electric field, with a gradient variation from −221 to 227 pC/N. This phenomenon has been revealed to result from the co-regulating effect of flexoelectric field and alternating current electric field on defect dipoles and domain structures. This study contributes to the fabrication of functional gradient piezoelectric single crystals and expands the application scenarios of FGMs.