Giant LO-TO splittings in perovskite ferroelectrics

We perform a first-principles investigation of the role of Coulomb interactions in eight AB${\mathrm{O}}_{3}$ cubic perovskite compounds. The predicted spontaneous polarization and the LO and TO phonon frequencies are found to be in good agreement with experiment. Anomalously large dynamical effective charges give rise to very strong mixing of the mode eigenvectors on going from the TO to the LO case, resulting in a ``giant LO-TO splitting'' in the sense that the soft TO mode is most closely related to the hardest LO modes. The results help explain the extreme sensitivity of these compounds to electrostatic boundary conditions.