Oxygen ordering, phase separation and the 60-K and 90-K plateaus in YBa₂Cu₃O_x

Abstract

A PLOT of the superconducting transition temperature (T_c) against oxygen content (x) for YBa₂Cu₃O_x exhibits two 'plateaus' when oxygen is removed from the material at low temperatures. T_c remains nearly constant at ∼60K for x= 6.6–6.7 and nearly constant at ∼90 K for x= 6.8–7.0. It is now common to assume that there are two distinct superconducting phases in YBa₂Cu₃O_x, the '60-K' and '90-K' phases, and that the two plateaus correspond to single-phase regions of the respective phases^1,2. YBa₂Cu₃O_x samples prepared at low temperatures contain a variety of ordered oxygen superstructures^3–5. Several theoretical studies have tried to predict the phase equilibria between these ordered structures^6,7 and to explain how oxygen ordering leads to the Tc plateaus^8,9, but no clear understanding of the interrelationships amongst these factors has yet emerged. This is partly because the techniques used previously to prepare the low-temperature samples do not control or monitor all of the key processing variables, particularly the oxygen partial pressure. Here we report on the ordered oxygen structures and superconducting properties of YBa₂Cu₃O_x samples prepared in precisely controlled oxygen environments using a solid-state ionic technique. We find no evidence for phase separ-ation between structures that have widely different oxygen content, but we do see electron diffraction evidence for phase separation between distinct phases that differ only slightly in oxygen content, and these regions of phase separation coincide with the T_c plateaus. These results show that the commonly held view that the two plateaus correspond to single-phase regions of respective 60-K and 90-K phases is incorrect: the changes in superconducting properties with oxygen content in YBa₂Cu₃O_x cannot be explained on this basis.