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 Nb ⁵⁺-doped SrTiO₃ substrate, and their d₃₃ 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 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.