Anomalous Scattering Study of the Bi Distribution in the 2212 Superconductor: Implications for Cu Valency

The distribution of the bismuth atoms over the cation sites in the 2212 Bi-Sr-Ca-Cu-O superconductor has been determined by anomalous scattering synchrotron crystallography. The analysis of reflection pairs measured at wavelengths of 0.9243 and 0.9600 angstrom shows a delocalization of the bismuth atoms over the calcium and strontium sites. The ``mixed'' plane between the CuO$_{2}$ layers contains 6.0(1.4) percent bismuth (where the number in brackets represents the statistical standard deviation derived from the least-squares refinement of the data), and a much smaller amount of strontium than often assumed. The strontium deficiency is charge-compensated by the creation of electron holes in the CuO$_{2}$ layer. The result supports the view that neither extra oxygen nor overlap of the bismuth 6p and copper 3d bands is needed to account for the holes, which are an essential feature of the superconductivity mechanism.