High-temperature resistance of the YBa2Cu3O6+x tetragonal phase.

Abstract

We have studied the resistance behavior of tetragonal $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ (YBCO with $xl0.5$) at high temperatures. Resistance measurements were performed following different thermal procedures in flowing Ar atmosphere. Measurements taken during slow heating exhibit a maximum in the temperature dependence of resistance ($R$ vs $T$). The temperature at which this maximum occurs systematically shifts towards lower temperatures as the sweeping rate decreases. In the limit of vanishing sweeping rate, i.e., when the steady state resistance is measured after long (\ensuremath{\sim} ${10}^{3}$ min) isothermal heat treatment, the $R$ vs $T$ curve exhibits a maximum at \ensuremath{\sim}650 \ifmmode^\circ\else\textdegree\fi{}C. Resistance measurements taken during slow cooling (0.1-0.5 \ifmmode^\circ\else\textdegree\fi{}C/min) generally depend on the thermal cycle. Weight measurements were used to evaluate the effects of the change of oxygen stoichiometry in different experiments. The comparison of resistance and weight measurements shows that bulk oxygen diffusion occurs in a time scale much shorter (\ensuremath{\sim} 30 min at 650 \ifmmode^\circ\else\textdegree\fi{}C) than resistance relaxation (\ensuremath{\sim} ${10}^{3}$ min at 650 \ifmmode^\circ\else\textdegree\fi{}C). We propose that the long relaxation time observed in resistivity measurements, similarly to other time dependent phenomena reported in the literature, is related to ordering (or disordering) of oxygen defects and we conclude that the resistance maximum at 650 \ifmmode^\circ\else\textdegree\fi{}C mainly reflects different oxygen configurations achieved above 500 \ifmmode^\circ\else\textdegree\fi{}C.