New observation of a phase boundary in oxygen deficient YBa2Cu3O7-δ single crystals

We have measured the superconducting transition temperature Tc of oxygen deficient single crystals of YBa2Cu3Ox, where x=7-δ in the range 6.5<x<6.9, after the crystals were subjected to a series of quenching and aging experiments. A change in the nature of the equilibrium state was observed at temperatures above a concentration dependent threshold temperature T*. We associate T* with the onset of a transition from a vacancy ordered low temperature (ortho II and/or ortho III) phase to a high temperature ortho I phase. The nature of the vacancy ordering below T* varies with x. We believe that, upon heating above T*, a significant concentration of chain fragments develops in normally vacant chains of the ordered structure. When the crystals were quenched from temperatures above T*, and subsequently permitted to anneal at 51degC, two relaxation processes were found. The two processes have relaxation times differing by about two orders of magnitude. The faster relaxation process is associated with changes in vacancy short-range disorder. The slower process is attributed to restoration of long-range chain vacancy order as misplaced fragments anneal out. T* is determined for different oxygen concentrations and compared to theoretical phase diagram calculations. Electron diffraction measurements on well annealed single crystals with stoichiometries x=6.50 and 6.65 show that different chain ordered phases (primarily ortho II and ortho III, respectively) exist at each of these compositions. Chain vacancy order is also observed at temperatures above T*. (orig.)