  The effect of Mg and C contents on TC in MgCNi3, and structure and
superconductivity of MgCNi3-xCox were studied. It is found that the excess of
Mg and C in initial material mixture is favorable to improve TC and obtain
single-phase samples. For preparing MgCNi3 superconductor, the optimum
composition of starting materials is MgC1.45Ni3 with 20wt.% excess of Mg of the
stoichiometric composition. In MgCNi3-xCox system, a continuous solid solution
is formed, lattice parameter decreases slightly and TC decreases obviously with
increasing x. A suppression of superconductivity is observed due to the
substitution of Co (Mn) for Ni. The suppression effect is smaller for the
substitution of Co than that of Mn.
