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

Structural, magnetic and electronic properties of compounds in the series RBa2Cu3O7-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 Tc measured at the midpoint of the resistive transition ranging from 87 to 95 K. No depression of Tc 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. Tcs are optimized by slow cooling. Annealing at 700°C in oxygen pressure of 40 atmospheres slightly increase Tc, while annealing under vacuum at 420°C destroys Tc and induces a semiconducting behavior. These changes in oxygen content and Tc are perfectly reversible.