Structure–property relationship of metastable monoclinic potassium niobate (KNbO₃) nanowires during phase transitions

Fundamentals in materials science are often based on understanding the relationships between the structures and properties of materials. Recently, our group reported on potassium niobate (KNbO₃) nanowires with a metastable monoclinic phase that has never been observed before in bulk materials. In this paper, we report on the unique phase transition behaviors of these monoclinic nanowires and describe how their nonlinear optical properties are correlated with structural changes. The metastable monoclinic nanowires exhibit phase transition behaviors that are distinct from those of thermodynamically stable orthorhombic nanowires. The monoclinic nanowires undergo two phase transitions: monoclinic to tetragonal at 120 °C and tetragonal to cubic at 380 °C. In contrast, orthorhombic nanowires undergo phase transitions from orthorhombic to tetragonal and from tetragonal to cubic at 130 °C and 330 °C, respectively. The monoclinic and orthorhombic nanowires were found to exhibit clearly distinguishable variations in the second harmonic generation (SHG) intensity with variations in temperature. We show that the tetragonal structures derived from the monoclinic and orthorhombic phases are clearly different. The tetragonal phase derived from the monoclinic phase has higher spontaneous polarization and exhibits higher SHG intensity than the other tetragonal phase. Our analysis can be extended to explain the size-dependent properties of KNbO₃. We calculated the spontaneous polarizations of the various phases based on the structural data and show how to use them to explain the above-mentioned structure–property relationships. The results of this study have important implications for both fundamental understanding and technological applications of metastable nanomaterials.