Evidence for incipient O dimerization and absence of Cu+3 in temperature-dependent XPS of highTc superconducting oxides

From temperature-dependent X-ray photoemission (XPS) spectra between 300 and 10 K of Sr doped La₂CuO₄ and “123” compounds (YBa₂Cu₃O_7-d) we deduce the absence of the Cu3d⁸(Cu³⁺) configuration in the ground state of these Mott insulators. The metallic character of these samples and involving those Cu atoms which are relevant for the superconducting properties arises from a(d⁹p⁶)+ß(d¹⁰p⁵) valence fluctuations, while the superconducting transition is indicated to be closely related to an incipient oxygen dimerization which is best detected upon cooling far belowT_c.