LOCAL ORDER IN YBA2CU3-YCOYO6-X-RAY SCATTERING(2X STUDIED BY ANOMALOUS DIFFUSE)

The local order in a single crystal of YBa2Cu2.53Co0.47O7.13 has been studied with anomalous diffuse x-ray scattering. (For such a Co concen tration the compound is nonsuperconducting.) Intensity measurements we re carried out at two energies below the Co edge. The difference data could then be expressed in terms of the local structure around a Co at om. The short-range order parameters (alpha's) indicate that the Co an d Cu atoms are nearly randomly distributed on the Cu l sites. The esti mated size of the Co-free ''domains'' is 5-7 angstrom. The first neigh bor in-plane Co-Co distances are significantly shortened, indicating t hat the Co atoms are displaced from their average positions. The data also show a significant decrease of the Co-O1 distance, leading to an increase of the Cu2-O1 distance. The lengthening of the Cu2-O1 distanc e implies a lowering of the Cu2 formal valence. The Co substitution al so affects the in-plane Cu2 positions. The present study shows that th e Cu-O2 structural coherence is altered on a scale smaller than the su perconducting coherence length. As far as the superconductivity is con cerned, the Cu2 valence remains one of the most important parameters i n determining the superconducting properties of the Co-doped 123 compo unds. On the other hand, there is some evidence that in order for supe rconductivity to occur in this and other doped cuprate compounds, the size of dopant-free regions in the basal plane may have to exceed the superconducting coherence length.