G. P. Kostikova

Valence States of Lead and Bismuth Atoms in the High-Temperature Superconductor BaPb1-xBixO3

X-ray photoelectron spectra of valence bands and core levels of BaPb0.8Bi0.2O3, PbO, PbO2, BaPbO3, BaBiO3, NaBiO3, and Bi2O3 were studied. Comparison of the electron binding energies of the Pb 4f7/2 or Bi 4f7/2 core levels for all the oxides studied showed that the high-temperature oxide superconductor BaPb0.8Bi0.2O3 contains simultaneously two different valence forms of lead atoms (PbIV and PbII) and two different valence forms of bismuth atoms (BiV and BiIII). Parameters of the X-ray photoelectron spectra of the valence bands do not contradict the conclusion on heterovalent states of lead and bismuth atoms in BaPb0.8Bi0.2O3.

Role of core and valence atomic orbitals in the formation of chemical bonds

The general principles of the electronic structure of molecules and complexes are discussed. An analysis of photoelectron and x-ray photoelectron spectra reveals that in most cases, the interaction of the valence AOs results in the formation of delocalized MOs, which are populated by electrons, but are not bonding according to their energies (molecules with small atomic cores are exceptions). These MOs do not make a positive contribution to the exothermic effect of chemical bonding. The delocalization of the valence electrons in such MOs reduces their charge density in the region of the atomic cores and lowers the energy of the inner “atoms-in-molecules” levels as a result of the reduction of the internal shielding; the ESCA shifts of the inner electronic levels confirm this stabilization. In accordance with the virial theorem, the lowering of the energy of the core levels of the central atoms (with simultaneous, generally small changes in the energy of the core levels of the ligands and despite the significant destabilization of the delocalized MOs in comparison to the valence orbitals of the corresponding free atoms) makes up the main contribution to the energy of chemical bonds.