Growth and orientation dependence of electrical properties of 0.92Na0.5Bi0.5TiO3-0.08 K0.5Bi0.5TiO3 lead-free piezoelectric single crystal

A 0.92Na(0.5)Bi(0.5)TiO(3)-0.08 K0.5Bi0.5TiO3 (0.92NBT-0.08KBT) lead-free piezoelectric single crystal with dimensions of Phi 35 x 10 mm was successfully grown by the top-seeded solution growth method. The effective segregation coefficient for K was calculated to be 0.27. An x-ray powder diffraction measurement showed that the as-grown crystal possesses a rhombohedral perovskite structure at room temperature. The room-temperature dielectric constants for poled < 001 >, < 110 >, and < 111 > oriented crystal samples are found to be 683, 567, and 435 at 1 kHz, respectively. The (T-m, epsilon(m)) values for < 001 >, < 110 >, and < 111 > oriented crystals are (316 degrees C, 4318), (317 degrees C, 4160), and (318 degrees C, 4348) at 1 kHz, which indicate that the dielectric parameters of the as-grown crystals show weaker anisotropy. The curves, epsilon(T), for the three crystallographic orientations show two anomalies at about 170 and 320 degrees C, respectively, relating to the ferroelectric-antiferroelectric phase and the antiferroelectric-paraelectric phase. There is a thermal hysteresis, Delta T approximate to 35 degrees C for the ferroelectric-antiferroelectric phase transformation between heating and cooling. The antiferroelectric phase, the thermal hysteresis, and the dielectric relaxor behavior around 170 degrees C can be attributed to the formation of an intermediate orthorhombic modulated phase at 170-320 degrees C. For the < 001 >, < 110 >, and < 111 > crystal samples, the room-temperature piezoelectric constants, d(33), reach 175, 130, and 70 pC/N, respectively. The remanent polarization for the < 001 >, < 110 >, and < 111 > crystal samples are 8.1, 10.8, and 13.5 mu C/cm(2), respectively, and the ratio is 1:1.33: 1.67, close to 1:root 2:root 3. The diffusive factors, a, are found to be 1.94, 1.91, and 1.50 for the < 001 >, < 110 >, and < 111 > oriented crystal samples, which indicate that the antiferroelectric-paraelectric phase transition of the as-grown 0.92NBT-0.08KBT crystal is a strong diffuse one. The electromechanical coupling coefficients for the < 001 >, < 110 >, and < 111 > oriented 0.92NBT-0.08KBT crystals at room temperature are 52, 50, and 48%, respectively. The values of k(t) for the three main crystallographic orientations change slightly with the increase of temperature from 15 to 140 degrees C, demonstrating a relatively stable thermal electromechanical coupling property. (C) 2011 American Institute of Physics. [doi:10.1063/1.3601116]