Abstract
The resistance of the nonstoichiometric compounds YBa2Cu3O6.8 (single crystals, ceramics) and YBa2Cu3O6.8 (Ce) (single crystal) to low-temperature decomposition (at 200°C) in air and an argon atmosphere is studied by X-ray diffraction and electron microscopy. At the first stage, the compounds are found to undergo oxygen separation into two phases, namely, oxygen-rich and oxygen-lean phases as compared to the initial state, that have different lattice parameters. After 20-to 35-h annealing, the disordering of the heavy Y, Ba, Ba, Y atoms along the c axis in ceramic pellets begins mainly because of the action of elastic stresses. This process occurs via the formation of the packets of numerous stacking faults on (001) planes. The disordering of the ceramic matrix ends in the formation of a CsCl-type cubic phase upon 100-h annealing, which is accompanied by a significant decrease in the diamagnetic response and an increase in Tc from 75 K in the initial state to 90 K. The retained superconductivity and the increase in Tc are caused by the presence of a large number of oxygenrich ortho-phase particles in a nonsuperconducting matrix; these particles are correlated with each other and form a multicoupled system of superconducting filaments.
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