Structural distortions in post-transition metal oxides are often explained in terms of the influence of $sp$ hybrid “lone pairs.” Evidence is presented here showing that this model must be revised. The electronic structures of prototypically distorted $\alpha$-PbO and $\alpha \mathrm{\text{−}}{\mathrm{Bi}}_2{\mathrm{O}}_3$ have been measured by high-resolution x-ray photoemission and soft x-ray emission spectroscopies. In contrast with the expectations of the lone pair model, a high density of metal $6s$ states is observed at the bottom of the valence band. The measurements are consistent with the results of density functional theory calculations.

• Received 15 September 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.157403