Valence skip behavior in BaBiO 3 and TlS

Abstract

Several elements show valence skip fluctuation, for example, Bi forms the compounds in +3 and +5 valence states, and Tl forms in +1 and +3 valence states. If the average valence is Bi 4+ or Tl 2+ in a compound, the system spontaneously forms the charge density wave (CDW) state like (Bi 3+, Bi 5+) or (Tl 1+, Tl 3+). This means that the effective electron–electron interaction is attractive, contrary to the well-known on-site Coulomb repulsive interaction. In the compounds of valence skip elements, it is expected that some carrier doping can induce superconductivity with high- T c. For example, (Ba, K)BiO 3 shows high- T c that is unlikely from the conventional electron–phonon mechanism. Tl-doped PbTe is also considered as the superconducting compound that Tl valence fluctuates and enhances T c. We estimated the formation energy of the CDW of BaBiO 3 and TlS by evaluating the Madelung potentials, in order to show that the CDW state is more stable than the uniform charge state, and the valence skip is actually realized.