Valence state for bismuth in the superconducting bismuth cuprates

Abstract

X-ray-absorption near-edge spectra (XANES) of high-$T_c$ bismuth superconductors (2:2:0:1, 2:2:1:2, and 2:2:2:3) have been recorded at the Bi $L_{\mathrm{III}}$, $L_{\mathrm{I}}$, and Cu K edges. With respect to the reference ${\mathrm{Bi}}_2$ ${\mathrm{O}}_3$ [Bi(i i i) valence state], edge shifts towards low energies have been found in the spectra of all the superconducting cuprates at the Bi $L_{\mathrm{III}}$ edge. This has been interpreted in terms of a reduced bismuth valency smaller than Bi (i i i). This result is a characteristic of the superconducting intergrowths, since other layer compounds with Bi cations in similar environments, ${\mathrm{Bi}}_2$ ${\mathrm{SrNb}}_2$ ${\mathrm{O}}_9$ and ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaFe}}_2$ ${\mathrm{O}}_9$, do not present any shift with respect to ${\mathrm{Bi}}_2$ ${\mathrm{O}}_3$. 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.