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Separation of the two Cu sites in YBa2Cu3O6.6

Site selectivity is produced by the variable Cu(1) and Cu(2) site contributions for different Bragg reflections. Table i lists the calculated values of the crystallographic weights for each site, tex2html_wrap_inline2012 and tex2html_wrap_inline2014, for the eight different (00L) Bragg reflections that were measured. The weights were calculated using the atomic coordinates reported by Cava, et al. [29]. Note that there is considerable variation in the crystallographic weights for these eight (00L) Bragg reflections. All of these reflections were easily accessible with the 2-circle spectrometer and the (001) film orientation. Also note that the calculated contributions of the two sites do not depend on the lattice constant, but only on the relative positions of the sites within the unit cell.

The Cu weights for the different Bragg reflections listed in Table i have two primary features. First, both sites contribute to all the Bragg reflections. Since a single Bragg reflection cannot be used to determine the local structure of a single Cu site, the contributions to several Bragg reflections are needed to resolve the individual sites. Second, the sign of the Cu(2) weight relative to the Cu(1) weight changes for different Bragg reflections. These sign changes increase the site separation sensitivity, and make it easy to assign near-edge features to a particular site. The assignment of the near-edge features based on these sign changes is discussed in Section 8.1.