Because of the presence of OH and H_2O 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 d33 values of KN nanowires increased to 104.5, 137.1 and 146.0 pmV~(-1) for the orthorhombic, tetragonal and MPB KN nanowires, respectively, after the KN nanowires were poled along the [100] direction by application of a DC voltage of 10 V.
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