Temperature and composition dependence of the tetragonal-orthorhombic distortion in ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$

We have mapped the temperature and composition dependence of the tetragonal-orthorhombic phase-transition temperature $T_d$ in the high-temperature superconductor ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ using high-resolution powder x-ray diffraction and measurements of the thermal expansion. The maximum volume fraction of ideal diamagnetism occurs in the composition range where $T_d$ is approaching the superconducting transition. In the composition range x>0.20 where no orthorhombic distortion is observed above 12 K, no bulk superconductivity is observed in ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$.