Increase of the superconducting Tc, irreversibility fields, and critical currents in tetragonal YBaSrCu3-xMoxO7+d

Recently, we have investigated how the superconducting transition temperature Tc, critical currents jc, and irreversibility fields Birr, of compounds with multiple CuO2 planes depend on physical properties and structural features of the blocking layer. To modify conductivity of the CuO chains in the blocking layer and decrease anisotropy of the YBa2Cu3Oy compound, substitutions of Mo for Cu and Sr for Ba were done. Synthesis of the YBaSrCu3-xMoxO7+d compound was optimized for the highest Tc and jc. Materials prepared in air at 960oC and annealed in 250 atm. O2 at around 650oC are tetragonal for x≥0.1. The Mo substitution on the Cu chain site increases the oxygen content above 7, and increases Tc from 81 K for x=0 to about 86 K for x=0.05--0.10. The ratio of nominal Mo content to the amount of excess oxygen is very close to 2:3 for all compositions (0.05≤x≤0.2) and may result from the presence of randomly distributed dimers of corner shared MoO6 octahedra. The critical currents for the substituted samples reveal the second peak effect arising from an increased pinning force likely due to the formation of the dimers of MoO6 octahedra in the CuO chains. The irreversibility lines show good pinning characteristics when compared to Y123