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

In ferroelectric perovskites, the strong spontaneous polarization is due to giant values of the Born effective charges, which are much larger than predicted on the basis of the nominal ionicity: we investigate the physical origin of such charges, which by definition measure the current traversing the sample during polarization reversal. Our first-principles calculations for KNbO3 identify unambiguously the dominant mechanism with the presence of covalent bonding. The giant charges are reduced to their nominal values in a computational experiment performed on a fake material, where covalence has been artificially suppressed.