Oxygen in-diffusion processes in tetragonal ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{x}}$ oxide

In situ resistance measurements have been used to investigate the oxygen in diffusion in tetragonal ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{x}}$ oxide. The oxygen content has been measured by nuclear reaction and by weighing; x-ray diffraction has been used to determine the crystalline structure. The polycrystalline bulk material with x=0.65 has been heated in oxygen to 200–370 °C. As a function of time the resistance curves continuously decrease and present two slopes: fast at the beginning and slow later. At first the material is tetragonal and nonsuperconducting. Complementary weight measurements indicate a significant increase only after a delay, and the oxygen uptake is associated with the slow variation of resistance. The initial slopes of the isothermal resistance versus time curves follow, in an Arrhenius plot, a straight line, suggesting a process with an activation energy of 0.40±0.05 eV. The sharp decrease in resistivity can be attributed to the presence of a thin continuous conductive skin around the grains or, more likely, to a disorder-order transformation activated by the presence of oxygen and involving a large part of the material. A process having an activation energy of 0.9±0.1 eV is responsible for the slow variation of the resistance. This is in agreement with published data obtained for oxygen in diffusion in similar conditions.