Electric polarization modulation through continuous phase regulation of the KNbO₃ nanocrystals at room temperature

Phase variation always endows noncentrosymmetrical materials with different polarization properties. Here, we prepared KNbO₃ nanocrystals with three different phases using the traditional hydrothermal method. By simply changing the KOH concentration, the tetragonal (T-KNbO₃), monoclinic (M-KNbO₃) and orthorhombic (O-KNbO₃) phases were obtained. The anisotropy growth mechanism driven by the OH^- concentration plays a dominant role in manipulating the morphology and phase. Due to the large off-center ratio of O-KNbO₃ and T-KNbO₃, they show strong spontaneous polarization along the [010] and [001] directions, respectively. However, a further decrease in lattice symmetry endows the M-KNbO₃ crystal with two perpendicular spontaneous polarization axes along the [100] and [001] directions, as confirmed by theoretical calculations and second-harmonic generation characterizations. This work provides a facile strategy for modulating the nonlinear performance of perovskite oxide materials and devices.