Effects of Ca doping and oxygen ordering in Cu-O chains on the superconducting properties of (Ca_xLa_1-x)(Ba_2-x-yLa_x+y)Cu₃ O_7+d prepared by amorphous metal complex method

The role of sintering conditions in synthesis of tetragonal superconducting (Ca_xLa_1-x)(Ba_2-x-yLa_x+y)Cu₃ O_7+d (x = 0.0-0.6, y = 0.25) polycrystalline samples prepared via an amorphous metal complex method was investigated. The series of samples sintered at 910, 930 and 950 °C in O₂ were characterized by XRD, EDS, Raman spectroscopy and iodometric titration to establish the structural and chemical differences responsible for their different superconducting properties determined from AC complex magnetic susceptibility measurements. We concluded that heat treatment below 930 °C was required to preserve the high compositional homogeneity of the initial precursor powder and obtain sharp superconducting transition for the final sample. For such high quality samples, we also found an interesting feature in the behavior of T_c vs. Ca concentration, which has not been reported yet. The T_c decreased with increasing Ca content for x<0.10, and only after that T_c started to increase with increasing Ca content for above 0.10.