APPLICATION OF THE SELECTIVE ATOM DIFFRACTION METHOD TO THE CATION DISTRIBUTION IN HIGH-TC BISMUTH CUPRATES

The bismuth distribution over the cation planes in the 2212 BiSrCaCuO superconductor has been determined with synchrotron radiation anomalous scattering at the Bi absorption edge. The expressions used are based on the differential structure factor, which is expressed directly in terms of the parameters of the anomalous scatterers. Analysis of 154 pairs of reflections at wavelengths of 0.9243 and 0.9600 angstrom lead to a 5(1)% bismuth occupancy of the Sr sites and a 6(1)% occupancy of the Ca sites in the 2212 BiSrCaCuO phase. Subsequent refinement of the conventional X-ray data on the same sample indicate a deficiency of Sr in the SrO planes. The results correspond to the stoichiometry Bi2.15Sr1.92Ca0.75Cu2O8.1, and an average valency of 2.21(4) for the Cu atoms, and indicate that cation vacancies significantly contribute to the hole concentration in the copper-oxygen planes. Experiments on a Pb-doped sample of the Ca-containing 2201 phase at 0.9 x 513 and 0.9624 angstrom show the lead atoms to substitute for bismuth. No evidence is found for preferential location of Pb at the Sr sites.