Using the configuration-interaction approach, we show that various core-level spectroscopic observations on the high-$T_c$ superconducting oxides can be consistently described in terms of mixing between $d^9$ and $d^{10}$ configurations, with a negligible amount of the $d^8$ (${\mathrm{Cu}}^{3+}$) state. From our analysis of the existing experimental data, we provide evidence of a wide and continuous valence transition in these systems as a function of temperature. The changes in the hybridization strengths deduced here are indicative of structural modifications of the square-planar ${\mathrm{CuO}}_4$ clusters with decreasing temperature.

• Received 8 January 1988

DOI:https://doi.org/10.1103/PhysRevB.37.7948