X-RAY-EMISSION SPECTRA AND ELECTRONIC-STRUCTURE OF HIGH-T(C) SUPERCONDUCTORS AND BINARY OXIDES

It is shown that the OK. X-ray emission spectra of high-T(c) superconductors may be presented as superpositions of the contributions from O-Cu, O-M3+ and O-M2+ bonds, in the ratio determined by the formula content of the compound. It is established by X-ray emission spectroscopy that O-Cu and O-M3+ bonds are practically identical to those in the initial oxides, whereas the O-M2+ contribution is shifted by 0.5-1.0 eV towards higher energies, that indicates the increase of the covalent contribution in the O-M2+ interaction. The sensitivity of the O-M2+ bonds to the interatomic interaction is related to an active role played by the M2+ alcaline-earth elements in the transfer of the electron density from M-cations into CuO2 planes. The correlation is found between n(M2+)/N(M(total)) and T(c).