Cu-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. 
T. Wu, R. Zhou, M. Hirata, I. Vinograd, H. Mayaffre, R. Liang, W.N.   Hardy, D.A. Bonn, T. Loew, J. Porras, D. Haug, C.T. Li...
cond-mat.supr-con