Ultrahigh piezoelectric coefficient of a lead-free K_0.5Na_0.5NbO₃-based single crystal fabricated by a simple seed-free solid-state growth method

Lead-free has been a goal that people pursue due to the environmental pollution caused by lead in lead-based materials. K_0.5Na_0.5NbO₃(KNN) is one of the most promising lead-free piezoelectric materials. Theoretically, the piezoelectric properties of single crystals are much better than those of polycrystalline materials. However, it is difficult to fabricate uniform KNN-based crystals by the high-temperature melt method because of volatilization of elements. This method also needs complicated equipment and is expensive. Here, a seed-free, solid-state crystal growth (SFSSCG) method was used to prepare Mn, Li, and Bi co-doped KNN crystals, which show an ultrahigh piezoelectric coefficient of about 1050 pC N^-1, around one order of magnitude higher than that of pure KNN ceramics. Furthermore, the effective piezoelectric coefficient d₃₃∗ of this crystal can reach 2290 pm V^-1 at a unipolar electric field of 10 kV cm^-1. This crystal also shows a high Curie temperature and good ferroelectricity. It is expected that the lead-free KNN-based single crystals fabricated by this simple method show promise for high performance ferroelectric and piezoelectric applications in industry and are environment-friendly.