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. Intensity measurement s were carried out at two energies below the Co edge. The difference d ata could then be expressed in terms of the local structure around a C o atom. The short range order parameters indicate that the Co and Cu a toms are nearly randomly distributed on the Cul sites. The first neigh bor Cul-inplane Co-Co distances are significantly shortened, indicatin g that the Co atoms are displaced from their average positions. The pr esent study shows that the Cu-O2 structural coherence is altered on a scale smaller than the superconducting coherence length. As far as the superconductivity is concerned the Cu2 valence remains one of the mos t important parameters in determining the superconducting properties o f the Co-doped 123 compounds.