Unusual Superconductivity in the Homologous Series (Cu$_{0.75}$Mo$_{0.25})$Sr$_{2}$(Ce,Y)$_{s}$Cu$_{2}$O$_{5+2s+\delta }$

The structures and bulk superconductivity ($>$30{\%} Meissner volume fraction) of the first four members of the high-$T_{c}$ series (Cu$_{0.75}$Mo$_{0.25})$Sr$_{2}$(Ce,Y)$_{s}$Cu$_{2}$O$_{5+2s+\delta }$have been successfully determined. Partial Mo substitution for Cu in the square-chains enhances $T_{c}$ to 87 K (for $s $= 1) and leads to significant oxygen loading capabilities well beyond the levels achieved in typical YSr$_{2}$Cu$_{3}$O$_{6+\delta }$, YBa$_{2}$Cu$_{3}$O$_{6+\delta }$, and other similar cuprates. Higher members of the series have their adjacent superconducting CuO$_{2}$ layers separated by increasingly thicker fluorite-like (Ce,Y)$_{2}$O$_{2}$ insulating blocks. Insertion of two or more of these blocks must drastically affect the CuO$_{2}$ interlayer coupling and causes $T_{c}$ to immediately drop and saturate at $\sim $57 K ($s$ = 2-4). The infinite chains of Cu-centered squares in YBCO change to alternate chains of mixed Cu squares and Cu and Mo octahedra. Neutron diffraction confirms the formation of reservoir blocks with a new structure and stoichiometry and of a surprisingly large Cu oxidation state of $\sim $2.5+, suggesting the possibility of an unusual superconducting pairing mechanism.