PRECURSOR STRUCTURES IN FERROELECTRICS FROM FIRST-PRINCIPLES CALCULATIONS

Based on an analysis of the wavevector dependence of lattice instabili ties in KNbO3, the existence of a real-space chain-like instability wa s deduced ([1]). To further study this instability, we have constructe d an ab initio effective Hamiltonian for KNbO3 that reproduces the low -energy Born-Oppenheimer surface. Classical molecular dynamics (MD) si mulations yield the observed sequence of ferroelectric phase transitio ns. The transition temperatures are lower than experiment, especially for the higher temperature transitions. Simulations of the static stru cture factor reveal features consistent with chain-like correlations a nd qualitatively reproduce diffuse X-ray scattering measurements. The calculated dynamic structure factor reveals anharmonically stabilized TO vibrational modes that soften along the entire Gamma-X line in the Brillouin zone, as the phase transition is approached from above. A re al-space analysis of the intercell correlations of the soft-mode coord inate, unambiguously establishes the long-lived dynamic character of t hese chain-like structures.