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K 0.5 Na 0.5 NbO 3 (KNN)-based lead-free piezoelectric single crystals have been prepared by the Seed-Free, Solid-State Crystal Growth (SFSSCG) method, and the effects of MnO 2 doping on the structural and electrical properties were systematically investigated. The results showed that the MnO 2 -doping can promote the growth of K 0.5 Na 0.5 NbO 3 -BiAlO 3 (KNN-BA) single crystals and enhance its electrical properties. Furthermore, the size of the (1 - x)(KNN-BA)- x MnO 2 crystals reached the maximum value of about 18 × 17 × 2 mm3 at x = 0.005. Also, the color of the crystals did gradually change, from light yellow to dark brown, due to the addition of MnO 2. The XRD and refinement results showed that the addition of MnO 2 , in the range of 0 ≤ x ≤ 0.009, did not change the crystalline structure of the perovskite crystals. A two-phase perovskite crystalline structure was present in all crystals: an orthorhombic and a tetragonal phase. The content of the orthorhombic phase decreased, while the content of the tetragonal phase increased, with an increase in the MnO 2 content. In addition, the domains in the crystals were mainly composed of 180° ones and had a parallel lamellar structure. Also, the smallest average domain width was obtained for crystals with x = 0.003. These crystals had the largest piezoelectric constant of 568 pC/N and an excellent overall performance of d 33 = 568 pC/N, P r = 32.2 μC/cm2, Q m = 55.3, k t = 0.36, k p = 0.39, T C = 418 °C, ε r = 430, and tan δ = 0.015. • Compared with BiAlO 3 ceramics, single crystal exhibits more excellent electrical properties. • Using the Seed-Free, Solid-State Crystal Growth(SFSSCG) method, the process is simpler. • MnO 2 dopant promotes the growth of single crystal grains and the improvement of electrical properties. • The prepared KNN-BiAlO 3 single crystal has high piezoelectric properties. [ABSTRACT FROM AUTHOR]

K 0.5 Na 0.5 NbO 3 (KNN)-based lead-free piezoelectric single crystals have been prepared by the Seed-Free, Solid-State Crystal Growth (SFSSCG) method, and the effects of MnO 2 doping on the structural and electrical properties were systematically investigated. The results showed that the MnO 2 -doping can promote the growth of K 0.5 Na 0.5 NbO 3 -BiAlO 3 (KNN-BA) single crystals and enhance its electrical properties. Furthermore, the size of the (1 - x)(KNN-BA)- x MnO 2 crystals reached the maximum value of about 18 × 17 × 2 mm3 at x = 0.005. Also, the color of the crystals did gradually change, from light yellow to dark brown, due to the addition of MnO 2. The XRD and refinement results showed that the addition of MnO 2 , in the range of 0 ≤ x ≤ 0.009, did not change the crystalline structure of the perovskite crystals. A two-phase perovskite crystalline structure was present in all crystals: an orthorhombic and a tetragonal phase. The content of the orthorhombic phase decreased, while the content of the tetragonal phase increased, with an increase in the MnO 2 content. In addition, the domains in the crystals were mainly composed of 180° ones and had a parallel lamellar structure. Also, the smallest average domain width was obtained for crystals with x = 0.003. These crystals had the largest piezoelectric constant of 568 pC/N and an excellent overall performance of d 33 = 568 pC/N, P r = 32.2 μC/cm2, Q m = 55.3, k t = 0.36, k p = 0.39, T C = 418 °C, ε r = 430, and tan δ = 0.015. • Compared with BiAlO 3 ceramics, single crystal exhibits more excellent electrical properties. • Using the Seed-Free, Solid-State Crystal Growth(SFSSCG) method, the process is simpler. • MnO 2 dopant promotes the growth of single crystal grains and the improvement of electrical properties. • The prepared KNN-BiAlO 3 single crystal has high piezoelectric properties. [ABSTRACT FROM AUTHOR]