Structural dependence of the piezoelectric properties of KNbO3 nanowires synthesized by the hydrothermal method

Abstract Because of the presence of OH − and H 2 O in the KN unit cell, tetragonal KNbO 3 (KN) nanowires were formed when the synthesis was carried out at 120 °C for 48 h. However, when the fabrication was conducted at high temperatures (⩾150 °C) or at 120 °C for a long period of time (⩾72 h), orthorhombic KN nanowires were formed. Moreover, the KN nanowires synthesized at 120 °C for 60 h showed a morphotropic phase boundary (MPB) structure in which both tetragonal and orthorhombic structures coexisted. Tetragonal, orthorhombic and MPB KN nanowires were also grown on the Nb 5+ -doped SrTiO 3 substrate, and their d 33 values were measured for the first time. A tetragonal KN nanowire exhibited a d 33 value of 23.5 pm V −1 , which is larger than that of the orthorhombic KN nanowire (11.6 pm V −1 ), probably because of the softening effect of the metal vacancies. The MPB KN nanowires exhibited a larger d 33 value of 40.0 pm V −1 . The d 33 values of KN nanowires increased to 104.5, 137.1 and 146.0 pm V −1 for the orthorhombic, tetragonal and MPB KN nanowires, respectively, after the KN nanowires were poled along the [1 0 0] direction by application of a DC voltage of 10 V.