  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, Ba$_{1-x}$K$_x$BiO$_3$ shows high $T_c$ which is unlikely from the
conventional electron-phonon mechanism. The reason for the missing of some
valence states in such valence skip compounds remains a mystery. We have
performed the evaluation of the Madelung potential for BaBiO$_3$, and have
shown for the first time that 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.

Strongly Correlated Electrons;Superconductivity