Polarization dependence of Born effective charge and dielectric constant in KNbO3

The Born effective charge Z* and dielectric tensor epsilon(infinity) of KNbO3 are found to be very sensitive to the atomic geometry, changing by as much as 27% between the paraelectric cubic and ferroelectric tetragonal and rhombohedral phases. Subtracting the bare ionic contribution reveals changes of the dynamic component of Z* as large as 50%, for atomic displacements that are typically only a few percent of the lattice constant. Z*, epsilon(infinity), and all phonon frequencies at the Brillouin zone center were calculated using the ab initio linearized augmented plane-wave linear response method with respect to the reference cubic, experimental tetragonal, and theoretically determined rhombohedral ground-state structures. The ground-state rhombohedral structure of KNbO, was determined by minimizing the forces on the relaxed atoms. In contrast to the cubic structure, all zone-center phonon modes 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 the small atomic displacements involved in the ferroelectric transitions, it is evident that not only the soft-mode frequencies but also the Born effective charge and dielectric constants are very sensitive to the atomic geometry.