Precursor structures in ferroelectrics from first-principles calculations

Abstract

Based on an analysis of the wavevector dependence of lattice instabilities in KNbO3, the existence of a real-space chain-like instability was deduced[1] To further study this instability, we have constructed an ab initio effective Hamiltonian for KNbO3 that reproduces the low-energy Born-Oppenheimer surface. Classical molecular dynamics (MD) simulations yield the observed sequence of ferroelectric phase transitions. The transition temperatures are lower than experiment, especially for the higher temperature transitions. Simulations of the static structure factor reveal features consistent with chain-like correlations and qualitatively reproduce diffuse X-ray scattering measurements. The calculated dynamic structure factor reveals anharmonically stabilized TO vibrational modes that soften along the entire Γ − X line in the Brillouin zone, as the phase transition is approached from above. A real-space analysis of the intercell correlations of the soft-mode coordinate, unambiguously establishes the long-lived dynamic character of these chain-like structures.