A combination of neutron total scattering measäurement and reverse Monte Carlo (RMC) refinement is applied to the study of apical Cu—O bond distortions in the high-Tc superconductor YBa2Cu3O6.93. We show that the average structure is not consistent with a split-site model for the corresponding Cu and O positions, but that the local structure nevertheless reveals the existence of two separate apical Cu—O bond lengths. Using G(r) data obtained from a variety of Qmax values we show that this result is independent of the data treatment methodology. We also find that the resulting ‘short’ and ‘long’ Cu—O bond lengths agree well with the results of previous äEXAFS studies. The existence of bimodal apical Cu—O bond distributions in the context of a single-site average structure model is interpreted in terms of correlated displacements of the Cu and O atoms. We investigate the possibility of clustering of short apical Cu—O bonds within our RMC configurations.
© by Oldenbourg Wissenschaftsverlag, OX1 3QR Oxford, Germany