Large Piezoelectricity Induced by Internal Stress in (K,Na)NbO 3 Single Crystals.

Achieving a high piezoelectric response and excellent stability is essential for practical applications of ferroelectric materials. Herein, large piezoelectricity of d 33 = 167 pC/N and k t = 0.52 is found in a K 0.7 Na 0.3 NbO 3 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.