Madelung energy of the valence-skipping compound BaBiO3

Several elements show valence skip fluctuation, for instance, Tl forms the compounds in valence states +1 and +3 and Bi forms in +3 and +5 states. This kind of fluctuation gives rise to a negative effective attractive interaction and the Kondo effect. In the compounds of valence-skipping elements, the carrier doping will induce superconductivity with high critical temperature. For example, Ba1-xKxBiO3 shows high T-c which is unlikely from the conventional electron-phonon mechanism. The reason for missing of some valence states in such valence skip compounds remains a mystery. We have performed the evaluation of the Madelung potential for BaBiO3 and have shown that the charge-ordered state is stabilized if we take into account the polarization of the oxygen charge. We argue that the effective Coulomb interaction energy U may be negative evaluating the local excitation energy.