Reverse Monte Carlo study of apical Cu-O bond distortions in YBa ₂Cu ₃O _6.93

A combination of neutron total scattering measurement and reverse Monte Carlo (RMC) refinement is applied to the study of apical Cu-O bond distortions in the high-T _c superconductor YBa ₂Cu ₃O _6.93. 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 Q _max 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 EXAFS 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.