Polymerized Complex Synthesis and Post-_O2 HIP Treatment of _YBa_2-xSr_xCu₄O_y Superconductors

Superconducting YBa2-xSrxCu4Oy [(Sr/Ba)-124] ceramics with x=0.0, 0.2, 0.4 and 0.6 have been synthesized by a combined process of the polymerized complex method and high oxygen pressure technique. Substitution of larger Ba ions by smaller Sr ions induces compression of lattices, which reflects decrease in lattice parameters, a, b, and c, with increasing Sr content x. Complex magnetic susceptibility measurements have revealed pure samples of (Sr/Ba)-124 with onsets of the superconducting transitions at 81.3 - 81.8 K and with their transition widths ĢTc (10-90%) = 2 - 3.5 K. Of particular interest is the invariance of Tc(onset) by Sr-doping in 124 in marked contrast with the corresponding 123 system where Tc(onset) is monotonically decreasing with increasing Sr content at a rate of about 2 K/(10% Sr). Raman spectroscopic measurements have shown that the vibrational mode for the apex oxygen, O(4), does not shift at all with increasing Sr content. This suggests that the charge balance between the CuO2 plane and the CuO double chain remains almost unchanged, which may in turn predict a constant Tc in the (Sr/Ba)- 124 system as was indeed observed in this study.