Defect structure and superconducting properties of ${\mathrm{La}}_{1.8}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{Ca}}_{1.2\mathrm{-}\mathit{x}}$${\mathrm{Cu}}_2$${\mathrm{O}}_{6\mathrm{-}\mathrm{\delta }}$

We have studied the relationship between structural defects and superconductivity in ${\mathrm{La}}_{1.8}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{Ca}}_{1.2\mathrm{-}\mathit{x}}$${\mathrm{Cu}}_2$${\mathrm{O}}_{6\mathrm{-}\mathrm{\delta }}$ (0≤x≤0.8). The samples were prepared by synthesis under different oxygen pressures, P(${\mathrm{O}}_2$)=50, 250, and 400 atm. Six of the seven samples were found to be superconducting with 22≤${\mathit{T}}_{\mathit{c}}$≤58 K. The structural properties were determined by neutron powder diffraction. The inter-${\mathrm{CuO}}_2$-plane spacing, ${\mathit{d}}_{\mathrm{Cu}\mathrm{-}\mathrm{Cu}}$, increases as Ca on the M(1) site between these planes is replaced by the larger Sr and La ions. The metal-site ordering is influenced by the oxygen pressure during synthesis. We have used the inter-${\mathrm{CuO}}_2$-plane spacing, neutron-diffraction measurements of the scattering from the metal sites, and chemical constraints to determine the occupancies of La, Sr, and Ca at the M(1) site. For the same overall composition, higher oxygen pressure leads to a larger fraction of La on the M(1) site. Oxygen occupancy of the vacant O(3) site in the M(1) plane increases sharply when ${\mathit{d}}_{\mathrm{Cu}\mathrm{-}\mathrm{Cu}}$ exceeds 3.5 Å. The superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ decreases systematically as the occupancy of O(3) increases for samples that would otherwise be expected to be superconducting.