VALENCE STATE FOR BISMUTH IN THE SUPERCONDUCTING BISMUTH CUPRATES

X-ray-absorption near-edge spectra (XANES) of high-Tc bismuth superconductors (2:2:0:1, 2:2:1:2, and 2:2:2:3) have been recorded at the Bi LIII, LI, and Cu K edges. With respect to the reference Bi2O3 [Bi(iii) valence state], edge shifts towards low energies have been found in the spectra of all the superconducting cuprates at the Bi LIII edge. This has been interpreted in terms of a reduced bismuth valency smaller than Bi (iii). This result is a characteristic of the superconducting intergrowths, since other layer compounds with Bi cations in similar environments, Bi2SrNb2O9 and Bi2Sr2CaFe2O9, do not present any shift with respect to Bi2O3. The copper formal valency has been deduced either from charge-balance considerations assuming a well-known oxygen stoichiometry or edge shifts at the copper K edge. High copper valencies have been found, especially in the 2:2:0:1 compound, but again, as was observed before in thallium and lead-substituted thallium cuprates, critical temperatures increase when the copper valency decreases, i.e., when the number of holes per copper in an oxygen-like p band decreases, provided this number stands above a critical value estimated previously to be 0.17. © 1990 The American Physical Society.