Observation and identification of a 107 K superconducting family in the CaBa₂Ca₂Cu₃O_9-δ system

A new family of C-free high-temperature superconducting oxides in the Ba-Ca-Cu-O system showing a T_c up to 107 K has been synthesized under high pressure. Structural analyses, based on X-ray and electron diffractions and high resolution electron microscopy, have revealed that these oxides can be represented by CaBa₂Ca₂Cu₃O_9-δ [Ca-1223-Ba] with a tetragonal subcell that has lattice constants α approximately 3.85 angstroms and c approximately 16.5 angstroms and is isostructural to Tl-1223-Ba with the Tl sites occupied mainly by Ca. These oxides have the same active block of [(CuO₂)(Ca)(CuO₂)(Ca)(CuO₂)] but with cation mixing and substitution of Ca by Cu within the charge reservoir block [(BaO)(CaO)(BaO)]. This results in the observed variations in composition and superstructure but has only minor influence on T_c.