Critical Role of Order-Disorder Behavior in Perovskite Ferroelectric KNbO₃

The evolution of local atomic configuration and macroscopic electrical properties of KNbO₃ is investigated in detail using ab initio molecular dynamics simulations within the framework of density functional theory. By analyzing the local off-center displacements of K and Nb atoms, the presence of the crossover of displacive and order-disorder mechanisms in the NbO₆ octahedron due to orbital hybridization is found, whereas the K is a completely displacive type due to the nonoriented ionic K-O bond. Besides, the order-disorder behavior of Nb atoms is enhanced with the increase in temperature, especially undergoing the phase transition. The predicted high dielectric constant of KNbO₃ confirms the key role of the Nb's order-disorder behavior. We discover that the anomalous increase in dielectric constant in the vicinity of the phase boundary is not only from the transformation of the polarized direction but also from the enhancement of order-disorder contribution in the nonpolarized direction. The high dielectric constant with large spontaneous polarization boosts the piezoelectricity at the orthorhombic-tetragonal phase boundary of 500 K, giving rise to the nearly 100% increase than that of 300 K. A rebonding model is developed to illustrate the origin of high piezoelectricity around the tetragonal ferroelectric phase boundary, and the significance of introducing a tetragonal structure is emphasized for developing high piezoelectricity in the inorganic perovskite ferroelectrics.