Copper NMR and NQR in YBa2Cu3Ox and GdBa2Cu3Ox with x between 6 and 7. A Study of the Oxygen Ordering

The NQR and NMR spectra of copper in polycrystalline samples of YBa2Cu3Ox and GdBa2Cu3Ox with different oxygen concentration x have been measured at 4.2 and 1.2 K. The different influence of Gd spins on the Cu(1) and Cu(2) sites was used to separate the contributions of both sites in the spectra. The quantitative analysis of the spectra in the region 6.3 < x < 6.75 allows the following conclusions about the ordering of the oxygen ions in the chains of the Cu(1) planes and about the magnetic state of the Cu(2) ions to be drawn, At about x = 6.5 oxygen orders in the ortho-II structure with every second chain filled. For higher x the coexistence of ortho-II and oxygen depleted ortho-I (all chains filled to about 80 %) gives the best description of the results. The average length of the Cu-O chains, n, is about 10 oxygen atoms per chain in the region of the 60 K plateau and decreases rapidly at lower x. The superconductivity disappears when n < 6. No signal of the Cu(2) sites is found in samples with T-c below the 60 K plateau due to the slowing down of the fluctuation rate of the electron spins of the Cu(2) ions. Magnetic order appears only in samples which do not become superconducting.