Tetragonal superconducting family (CaxLa1−x) (Ba1.75−xLa0.25+x)Cu3Oy: The effect of cosubstitution on the transition temperature

We have prepared a family (${\mathrm{Ca}}_{\mathit{x}}$ ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$)(${\mathrm{Ba}}_{1.75\mathrm{-}\mathit{x}}$ ${\mathrm{La}}_{0.25+\mathit{x}}$)${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{y}}$ within the 1:2:3 high-temperature-superconductor system in which Ca occupies solely the Y site while La partly occupies the Y site and partly the Ba site. This family is characterized by constant charge (7.25) of the noncopper cations via compensating substitutions. It is tetragonal throughout the single-phase-existence range 0.0≤x≤0.5. Attempts to prepare ${\mathrm{CaLaBaCu}}_3$ ${\mathrm{O}}_7$ were unsuccessful yielding the x=0.5 member as a major phase and impurity phases. We have determined atomic positions and occupancies by Rietveld refinement of x-ray powder-diffraction data, and oxygen stoichiometry by iodometric titration. A double-plateau behavior in transition temperature (${\mathit{T}}_{\mathit{c}}$) vs cation concentration (x), very similar to the ${\mathit{T}}_{\mathit{c}}$ vs δ variation in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$, has been observed.