Synthesis and properties of the YBa₂Cu₄O₈ superconductor

A new simple synthesis route was used to produce the '124' superconductor YBa₂Cu₄O₈. The method utilizes YBa₂Cu₃O₇ powder ('123') as a precursor, which is then converted to the 124 superconductor by reaction with a stoichiometric amount of CuO through normal grinding and sintering, without the need for high oxygen pressure processing. Sintered bars of the 124 superconductor exhibited a T_c of ∼ 75 K in resistivity and AC magnetic susceptibility measurements. Transport critical current density was measured to be ∼ 150 A/cm² at 60 K, and showed a strong field dependence. This behavior, in combination with a relatively high J_c (magn.) of 4 × 10⁴ A/cm² at 60 K and H = 0.9 T, is indicative of Josephson weak links at grain boundaries, which is similarly observed in the 123 phase. It is also noted that the intragrain J_c in the twin-free 124 superconductor is about the same as that in the twinned 123 superconductor at ∼ 15 degrees below their respective T_c.