Composition-dependent superconductivity in ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$

We report the dependence of the superconducting critical temperature on x for a series of ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ 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 transition temperatures as measured by resistivity and the dc Meissner effect. Relatively high onset temperatures are observed, and we infer the presence of another, possibly metastable, material with a critical temperature substantially higher than that of bulk ${\mathrm{K}}_2$ ${\mathrm{NiF}}_4$ structure-type ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$.