Composition dependence of T sub c in superconducting La sub (2-x) Sr sub x CuO sub (4-delta).

We report the dependence of the superconducting critical temperature on the strontium concentration for a series of La sub (2-x) Sr sub x CuO sub (4-delta) samples prepared under identical conditions. A strong peak in the fraction of the sample which is superconducting occurs near x=0.15, the composition which also gives the highest midpoint for the resistive transition, the highest temperature for which the resistivity is zero, and the highest temperature for the onset of bulk superconductivity as determined by the dc Meissner effect. The onset of the resistive transition varies less systematically, and is less reproducible from sample to sample. The presence of another, possibly metastable, material with a critical temperature substantially higher than that of bulk K sub 2 NiF sub 4 structure type La sub (2-x) Sr sub x CuO sub (4-delta) is therefore inferred.