Oxygen-stoichiometry dependence of the electronic structure of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ with δ(0<δ<0.7): Possibility of a highly correlated mixed-valent state

${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ is obtained over a wide range of stoichiometry (0<δ<0.7) by proper thermal treatment in ${\mathrm{O}}_2$. Samples are carefully characterized by x-ray powder diffraction and thermogravimetry. Their superconducting properties are investigated mainly by measuring the susceptibility (${\chi }_{\mathrm{dc}}$) in low fields (Meissner effect). The main part of this paper is devoted to an x-ray photoemission spectroscopy study of core levels (mainly O 1s and Cu 2p) with the aim of elucidating the change of the electronic structure on passing from the semiconducting (δ>0.5) to the superconducting metallic state (δ≃0). On the basis of our experimental results, we argue that the superconducting state is obtained from a mixture of configurations ‖3$d^9$〉 and ‖3$d^9$Lk〉 in the ground state of copper (L stands for a hole in the O-Cu ligand and k for a conduction electron). We find that the ratio of the ‖3$d^9$Lk〉 configuration is directly related to the magnitude of $T_c$. The charge-neutrality rule is approximatively followed, provided that the ${\mathrm{Cu}}^{3+}$ state is identified as a ‖3$d^9$Lk〉 configuration. We discuss also the presence of the 3$d^{10}$ configuration (i.e., ${\mathrm{Cu}}^{+}$) in the ground state. A possible consequence of these ${\mathrm{Cu}}^{+}$ would be to increase the number of formal ${\mathrm{Cu}}^{3+}$ (here 3$d^9$Lk).