WAVE-VECTOR DEPENDENCE OF FERROELECTRIC INSTABILITIES IN KNBO3

First principles LAPW linear response calculations of the lattice dyna mics of cubic KNbO3 reveal planar instability in the Brillouin zone. I n real space, these correspond to chains along the [100] directions of Nb atoms coherently displaced along the chain direction. Zone-center calculations in the rhombohedral phase yield stable modes (and no unus ually soft modes) consistent with experiment. The similarity of the fe rroelectric phases in BaTiO3 suggests such chain instabilities also oc cur. It is likely that the ferroelectric phase transitions in these ma terials will depend on the dynamics of such chains.