Superconductivity at 40 K in the Oxygen-Defect Perovskites La2-xSrxCuO4-y.

Structural, magnetic, and electronic properties of compounds in the series La2-xSrx CuO4-y for 0.05 </= x </= 1.1, with deltax = 0.025, were studied. Resistance, susceptibility, Meissner, and shielding measurements have revealed superconductivity among several members of the series. For x = 0.15, the transition temperature T(c), measured at the midpoint of the resistive transition, is a maximum at 39.3 K with a width of 2 K. At other compositions the transition is broader and occurs at a lower temperature. Annealing the x = 0.15 sample in oxygen at 500 degrees C increases T(c) to 40.3 K, while annealing at the same temperature under vacuum suppresses the superconductivity. These changes in oxygen content and T(c) are reversible.