Carbon solubility and superconductivity in MgB2

Successful replacement of B by C in the series MgB_2?xC_x for values of x upto 0.3 is reported. Resistivity and ac susceptibility measurements have been carried out in the samples. Solubility of carbon, inferred from the observed change in the lattice parameter with carbon content indicates that carbon substitutes upto x=0.30 into the MgB₂ lattice. The superconducting transition temperature, T_c measured both by zero resistivity and the onset of the diamagnetic signal shows a systematic decrease with increase in carbon content upto x=0.30, beyond which the volume fraction decreases drastically. The temperature dependence of resistivity in the normal state fits to the Bloch–Gruneisen formula for all the carbon compositions studied. The Debye temperature, θ_D, extracted from the fit, is seen to decrease with carbon content from 900 to 525 K, whereas the electron–phonon interaction parameter, λ, obtained from the McMillan equation using the measured T_c and θ_D, is seen to increase monotonically from 0.8 in MgB₂ to 0.9 in the x=0.50 sample. The ratio of the resistivities between 300 and 40 K versus T_c is seen to follow the Testardi correlation for the C substituted samples. The decrease in T_c is argued to mainly arise due to large decrease in θ_D with C concentration and a decrease in the hole density of states at N(E_F).