ROLE OF COVALENT BONDING IN THE POLARIZATION OF PEROVSKITE OXIDES - THE CASE OF KNBO3

In ferroelectric perovskites, the strong spontaneous polarization is d ue to giant values of the Born effective charges, which are much large r than predicted on the basis of the nominal ionicity: we investigate the physical origin of such charges, which by definition measure the c urrent traversing the sample during polarization reversal. Our first-p rinciples calculations for KNbO3 identify unambiguously the dominant m echanism with the presence of covalent bonding. The giant charges are reduced to their nominal values in a computational experiment performe d on a fake material, where covalence has been artificially suppressed .