Oxygen and rare-earth doping of the 90-K superconducting perovskite YBa2Cu3O7−x

Abstract

Structural, magnetic and electronic properties of compounds in the series R${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{x}}$ (R=Nd,Sm,..., and Lu) were studied. Resistivity, Meissner-effect, and shielding measurements have revealed superconductivity among all the rare-earth compounds, except La, Pr, and Tb, with critical temperatures $T_c$ measured at the midpoint of the resistive transition ranging from 87 to 95 K. No depression of $T_c$ was observed upon introduction of most of the rare-earth magnetic ions. Susceptibility measurements down to 1.6 K have shown that an antiferromagnetic ordering (most likely due to dipole-dipole interactions) occurs only for the Gd compound. Changes in oxygen content in these materials drastically affect their physical properties. The importance of the cooling rate during the synthesis of the sample has been correlated to oxygen content. $T_c$’s are optimized by slow cooling. Annealing at 700°C in oxygen pressure of 40 atmospheres slightly increase $T_c$, while annealing under vacuum at 420°C destroys $T_c$ and induces a semiconducting behavior. These changes in oxygen content and $T_c$ are perfectly reversible.