Electronic states of valence-skipping compounds

Abstract

Valence skip means that an element leaps a particular valence state in compounds. For example, Bi forms the compounds in +3 and +5 valence states, and T1 forms in +1 and +3 valence states. In BaBiO3, the average valence of Bi is +4, but the system spontaneously brakes its crystal symmetry and forms the charge density wave (CDW) state like (Bi3+,Bi5+). This means that the effective electron-electron interaction is attractive, contrary to the well-known on-site Coulomb repulsive interaction. In the valence skip compounds, it is expected that some carrier doping can induce superconductivity with high-Tc. For example, (Ba,K) BiO3 shows high-T c that is not plausible merely from the convectional electron-phonon mechanism. T1-doped PbTe is also considered as the superconducting compound that T1 valence fluctuates and enhances Tc. We estimated the formation energy of the CDW in BaBiO3, T1S and T1Se by evaluating the Madelung energy, in order to show the CDW state is more stable than the uniform charge state, and the valence skip is actually realized. © 2008 IOP Publishing Ltd.