Beschreibung:
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Bulk superconductors of Ca-doped RBa<jats:sub>2</jats:sub>Cu<jats:sub>4</jats:sub>O<jats:sub>8</jats:sub> (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 (<jats:italic>T</jats:italic>
<jats:sub> c0</jats:sub>) for Ca-doped R124 increased with increasing Ca content. The highest <jats:italic>T</jats:italic>
<jats:sub> c0</jats:sub> (=85.4 K) was obtained for the 5% Ca-doped Yb124 sample. In addition, <jats:italic>T</jats:italic>
<jats:sub> c0</jats:sub> of undoped Yb124 ceramics was estimated to be about 80 K from the result for Ca-doped samples.
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