63Cu-NMR study of oxygen disorder in ortho-II YBa2Cu3Oy.

We show that 63Cu-NMR spectra place strong constraints on both the nature and the concentration of oxygen defects in ortho-II YBa2Cu3Oy. Systematic deviation from ideal ortho-II order is revealed by the presence of inequivalent Cu sites in either full or empty chains. The results can be explained by two kinds of defects: oxygen clustering into additional chains, or fragments thereof, most likely present at all concentrations (6.4<y<6.6), and oxygen vacancies randomly distributed in the full chains for y<6.50 only. Furthermore, the remarkable reproducibility of the spectra in different samples with optimal ortho-II order (y≃6.55) shows that chain-oxygen disorder, known to limit electronic coherence, is ineluctable because it is inherent to these compounds. [ABSTRACT FROM AUTHOR]