Superconductivity at 40 K in the oxygen-defect perovskites la _2-xSr_xCuO_4-y

Structural, magnetic, and electronic properties of compounds in the series La//2// minus //xSr//xCuO//4// minus //y for 0. 05 less than equivalent to x less than equivalent to 1. 1, with DELTA x equals 0. 025, were studied. Resistance, susceptibility, Meissner, and shielding measurements have revealed superconductivity among several members of the series. For x equals 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 equals 0. 15 sample in oxygen at 500 degree 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.