STUDY OF OXYGEN ORDERING IN HTC SUPERCONDUCTORS BY MAGNETIC-RESONANCEOF DIFFERENT NUCLEI

NMR of Tm-169 and NQR/NMR of (63)/(CU)-C-65 was applied to investigate the order of oxygen in the HTc superconductors of the type RBa(2)Cu(3 )O(x). From the spectra of three different lattice sites, one for Tm a nd two for Cu, it is evident, that three ordered structures exist: tet ra for x = 6.0, ortho-II for x = 6.5 and ortho-I for x = 7.0. No other ordered structures could be detected. A coexistence of ortho-I and -I I is evident in a wide region between 6.6 and 6.8. The existence of ex tended domains of ortho-II is essential for the onset of superconducti vity. In the tetragonal region below 6.3 we find mainly insulated oxyg en ions in the Cu(1) plane; in supercon ducting samples of the 60 K pl ateau oxygen is ordered in chains of a mean length of about 12 oxygen ions. In the region of the 90 K plateau the chains become very long an d the oxygen vacancies are clustered in empty fragments of chains.