HIGH-TEMPERATURE RESISTANCE OF THE YBA2CU3O6+X TETRAGONAL PHASE

We have studied the resistance behavior of tetragonal YBa2Cu3O6+x (YBC O with x<0.5) at high temperatures. Resistance measurements were perfo rmed following different thermal procedures in flowing Ar atmosphere. Measurements taken during slow heating exhibit a maximum in the temper ature dependence of resistance (R vs T). The temperature at which this maximum occurs systematically shifts towards lower temperatures as th e sweeping rate decreases. In the limit of vanishing sweeping rate, i. e., when the steady state resistance is measured after long (similar t o 10(3) min) isothermal heat treatment, the R vs T curve exhibits a ma ximum at similar to 650 degrees C. Resistance measurements taken durin g slow cooling (0.1-0.5 degrees C/min) generally depend on the thermal cycle. Weight measurements were used to evaluate the effects of the c hange of oxygen stoichiometry in different experiments. The comparison of resistance and weight measurements shows that bulk oxygen diffusio n occurs in a time scale much shorter (similar to 30 min at 650 degree s C) than resistance relaxation (similar to 10(3) min at 650 degrees C ). We propose that the long relaxation time observed in resistivity me asurements, similarly to other time dependent phenomena reported in th e literature, is related to ordering (or disordering) of oxygen defect s and we conclude that the resistance maximum at 650 degrees C mainly reflects different oxygen configurations achieved above 500 degrees C.