Optimized synthesis and properties of chemically stabilized CuSr2YCu2O7

Synthesis of the CuSr2YCu2O7 compounds with Re, W and Mo substituted for Cu has been optimized for the highest T-c. Materials prepared in air at 990 degrees C and annealed at 650-700 degrees C in 220 atm. O-2 are tetragonal with the transition elements substituting on the Cu-''chain'' site, have oxygen content similar to 7.3, and are superconducting with T-c similar to 70 K. The irreversibility lines show rather poor pinning characteristics when compared to Y123. Superconducting T-c values were increased up to 77 K by additional substitution of small amounts of Ca for Y. Based on a.c. susceptibility and d.c. magnetization measurements, Ca substitution improves superconducting properties making chemically stabilized CuSr2YCu2O7 materials interesting for high field applications below 30 K.