Complete substitution of Sr for Ca in ${\mathrm{Bi}}_2{\mathrm{Sr}}_{2+x}{\mathrm{Ca}}_{1-x}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$

We have systematically substituted Sr for Ca as spacer of the ${\mathrm{CuO}}_2$ planes in ${\mathrm{Bi}}_2{\mathrm{Sr}}_{2+x}{\mathrm{Ca}}_{1-x}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ thin films deposited by molecular beam epitaxy, from $x=0\mathrm{}$ (2212 phase) to $x=1\mathrm{}$ (2302 phase). From dc transport measurements we have found a decrease of $T_c$-zero from 88 K for $x=0\mathrm{}$ down to 55 K for $x=1.\mathrm{}$ The remarkable result reported here is the presence of superconductivity when Ca is completely replaced by Sr. Comparison of structural and transport data shows that the enlargement of the unit cell due to the substitution has no dramatic effect on the onset of superconductivity. This result is compared to previous studies and existing results on bilayer systems.