Local oxygen ordering in superconducting YBa₂Cu₃O_6.4 observed by neutron diffraction

THE superconducting transition temperature, Tc, in the YBa2Cu3O6+x system is known to depend not only on the oxygen stoichiometry1, but also on the degree of oxygen order2,3. Observations of oxygen ordering structures by electron microscopy4,5 have led to the suggestion that the '60-K plateau' near x = 0.5 in the curve of Tc versus x arises from an ordered structure (the 'ortho-II phase') in which Cu–O and Cu chains alternate in the basal plane6.

Until now, however, the ortho-II phase has been seen only by electron diffraction, except for one observation at an oxygen stoichiometry of x = 6.7 by X-ray diffraction7. Compared with true bulk methods such as X-ray and neutron diffraction, electron diffraction suffers from the disadvantage that only a very small portion of the sample is probed.

Here we report the observation, by neutron diffraction, of local ortho-II ordering in an YBa2Cu3O6.4 crystal (Tc = 38 K). The observed correlation lengths along the principal axes are 24b (along the chains), 10a and 2c. Our observation of the existence of the ortho-II phase for an oxygen stoichiometry close to the minimum for which superconductivity is observed (x≈0.4) supports existing theoretical models8,9, in particular one8 that relates Tc to the presence of two types of ordered domain.