Lattice dynamics of ferroelectrics using the lapw linear response method: Application to KNbO3

We have applied our recently developed LAPW linear response method to the cubic perovskite KNbO3 to calculate the phonon dispersion curves and to study the wave vector dependence of the ferroelectric instability. These are first principles calculations, performed within the framework of the local density functional approximation (LDA), and the results are as accurate as conventional total energy supercell studies. The density functional linear response approach, however, permits investigation of phonons of arbitrary wave vector. Our principle finding is that the cubic structure is unstable for wave vectors over wide regions of the Brillouin zone (BZ) and suggests chain-like structures of distorted Nb-O octahedra that are uncorrelated in the perpendicular directions. This has implications regarding whether the ferroelectric phase transition is driven by a soft-mode instability or is of order/disorder type.