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

Because of the presence of OH− and H2O in the KN unit cell, tetragonal KNbO3 (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 Nb5+-doped SrTiO3 substrate, and their d33 values were measured for the first time. A tetragonal KN nanowire exhibited a d33 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 d33 value of 40.0 pm V−1. The d33 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.