Effect of Cd Doping on Structure and Superconductivity in Mg0.5Cd0.5B2

The effect of Cd doping on structure and superconductivity in Mg_0.5Cd_0.5B₂ fabricated by a solid-state reaction at ambient pressure has been investigated. The resulting changes in crystal structure, superconducting transition temperature T_c and critical current density J_c are characterized by X-ray diffraction, dc magnetization, electrical resistance, and magnetic measurements. It reveals that Cd does not occupy the atomic Mg sites in the MgB₂ crystal structure, but merely reacts with Mg and forms a MgCd₃ phase. It is striking to note that although the nonsuperconducting phase MgCd₃ is as high as about 67 vol.% in Mg_0.5Cd_0.5B₂, the T_c of the doped sample drops only by about 1 K. Most important, a surprising improvement of J_c of 5.0 × 10⁵ A/cm² (5 K, 0 T) has been achieved in Mg_0.5Cd_0.5B₂. It is suggested that the improvement in J_c in Mg_0.5Cd_0.5B₂ is primarily due to pinning effects induced by MgCd₃. Also, it is thought that MgCd₃ may fill up gaps among grains in MgB₂ and produce better grain linkage, which may be another source of improvement in J_c in Mg_0.5Cd_0.5B₂.