  In this paper, we reported the structural properties and superconductivity of
Mg(B1-xCx)2 compounds. Powder x-ray diffraction results indicate that the
samples crystallize in a hexagonal AlB2-type structure. Due to the chemical
activity of Mg powders, a small amount of MgO impurity phase was detected by
x-ray diffraction. The lattice parameters decrease slightly with increasing
carbon content. Magnetization measurements indicate the non-stoichiometry of
MgB2 has no influence on the superconducting transition temperature and the
transition temperature width. The addition of carbon results in a decrease of
Tc and an increase in the superconducting transition width, implying the loss
of superconductivity.
