Structural and superconducting properties of Ca-doped MgB2 superconductors

A series of Ca-doped MgB2 superconducting compounds has been synthesized under high pressure and high temperature with a calcium source of Ca3B2N4. Their structure and superconducting properties have been investigated, revealing that the lattice parameters of a and c for the Ca-doped MgB2 are enlarged and the superconducting transition temperature T-c is suppressed from 37.8 K for pure MgB2 to 36.5 K for Mg0.92Ca0.08B2 by the Ca doping. In addition to the main phase of the Ca-doped MgB2, the MgO and CaB6 phases have been observed with the MgO and CaB6 grains being of the size of several hundred nanometres and being embedded randomly in the MgB2 matrix. In comparison to the undoped MgB2, the rapid decrease in J(c) observed in the Ca-doped samples is mainly due to the relatively large grain sizes of the second phases, which result in the poor connectivity between grain boundaries.