Valence-skipping compounds as positive-U electronic systems

We find that arguments favoring negative-U behavior of intra-atomic origin, based upon valence skipping for compounds of column III, IV, and V metals, do not stand up to closer examination. It is suggested that real negative-U behavior (disproportionation in compounds such as TlBr2, Pb2O3, and BaBiO3) can, and does, only arise through relaxation of neighbors associated with changes in bond populations. Even then, very high dielectric constants are required to reduce the positive-U contribution, and disproportionation is not expected with lighter metals. Analogous displacements of water molecules may produce disproportionation in aqueous solutions of TlBr2. We find that Tl impurities in PbTe also disproportionate, through atomic displacements with transfer of electrons between bonds, which could explain, as well as would an atomic-electronic origin, an apparent "charge-Kondo effect" behavior, and enhanced superconductivity, found by Matsushita, Blume, Geballe, and Fisher.