POLARIZATION DEPENDENCE OF BORN EFFECTIVE CHARGE AND DIELECTRIC-CONSTANT IN KNBO3

The Born effective charge Z and dielectric tensor epsilon(infinity) o f KNbO3 are found to be very sensitive to the atomic geometry, changin g by as much as 27% between the paraelectric cubic and ferroelectric t etragonal and rhombohedral phases. Subtracting the bare ionic contribu tion reveals changes of the dynamic component of Z as large as 50%, f or atomic displacements that are typically only a few percent of the l attice constant. Z, epsilon(infinity), and all phonon frequencies at the Brillouin zone center were calculated using the ab initio lineariz ed augmented plane-wave linear response method with respect to the ref erence cubic, experimental tetragonal, and theoretically determined rh ombohedral ground-state structures. The ground-state rhombohedral stru cture of KNbO, was determined by minimizing the forces on the relaxed atoms. In contrast to the cubic structure, all zone-center phonon mode s of the rhombohedral structure are stable, and their frequencies are in good agreement with experiment. In the tetragonal phase, one of the soft zone-center modes in the cubic phase is stablized. In view of th e small atomic displacements involved in the ferroelectric transitions , it is evident that not only the soft-mode frequencies but also the B orn effective charge and dielectric constants are very sensitive to th e atomic geometry.