Stoichiometric carbon substitution in MgB2

Carbon has been substituted into the MgB2 lattice, forming the series Mg(B1-xCx)2, by heat treatments in sealed Ta tubes starting with elemental constituents. The superconducting transition temperature, TC, was measured by diamagnetic susceptibility. The superconducting transitions are sharp and the x-ray diffraction on the samples show only trace amounts of impurity phases. The C fraction that substitutes the B atoms into the MgB2 lattice, as determined from the now well established correlation between the 'a' lattice parameter and x in Mg(B1-xCx)2 is found to be very close to the targeted values. The temperature dependence of the normalized resistance is similar to that seen in C doped MgB2 single crystals along the ab plane. Rn/R(300 K), where Rn is the resistance just above TC, increases with x