Large Piezoelectricity Induced by Internal Stress in (K,Na)NbO₃ Single Crystals

Achieving a high piezoelectric response and excellent stability is essential for practical applications of ferroelectric materials. Herein, large piezoelectricity of d₃₃ = 167 pC/N and k_t = 0.52 is found in a K_0.7Na_0.3NbO₃ lead-free ferroelectric single crystal without poling, which is comparable to the artificially poled KNN crystals. The large piezoelectricity is maintained up to 196 °C, showing excellent thermal stability. It was demonstrated that the high piezoelectricity is associated with strong self-polarization in the crystals. The strong internal stress formed during crystal growth gives a preferred spontaneous polarization orientation, resulting in a net macro total polarization. In addition, the internal stress also pins domain wall motions and provides a "restoring force" for the domain switching. This work provides a strategy for designing and optimizing the piezoelectric performance of ferroelectric materials.