Preparation of ceramics (RCa)Ba2Cu4O8 (R=Y, Ho, Er, Tm and Yb) by conventional solid-state reaction

Bulk superconductors of Ca-doped RBa(2)Cu(4)O(8) (R124, R=Y, Ho, Er, Tm and Yb) were synthesized at 1 atm oxygen pressure. Although little undoped Yb124 material was obtained by a conventional solid-state reaction method, a single-phase sample was obtained upon Ca doping. The solubility limit of Ca increased with decreasing ionic radius of R. The zero-resistant temperature (T-c0) for Ca-doped R124 increased with increasing Ca content. The highest T-c0 (= 85.4 K) was obtained for the 5% Ca-doped Yb124 sample. In addition, T-c0 of undoped Yb124 ceramics was estimated to be about 80 K from the result for Ca-doped samples.