Influence of doping effect on structure and superconducting properties of MgB_2

Mg 1- x M x B 2 (M=Li or deficiency) compounds were prepared and their structure and superconducting properties were studied. The results show that nearly single phased Mg 1- x Li x B 2 samples have been obtained while x ≤0.3. With increasing x , lattice parameters a decreases and c shows no obvious change, and the superconducting transition temperature T c of Mg 1- x Li x B 2 decreases. Moreover, loss of superconductivity occurs for sample with x =0.5. For Mg 1- x B 2 nearly single phased samples can be obtained at x =0. MgB 2 coexists with MgB 4 phase and some minor impurity phases while 0 x 0.5. With the increase of x , a and c of MgB 2 decreases and increases, respectively, and T c decreases. The obvious difference in the superconductivity of Mg 1- x Li x B 2 and Mg 1- x B 2 suggests that Li indeed dopes into the structure of MgB 2.