Intralayer carbon substitution in the MgB₂ superconductor

We report that the ternary MgB_2-xC_x compounds adopt an isostructural AlB₂-type hexagonal structure in a relatively small range of nominal carbon concentration, x<0.1. The lattice parameter α decreases almost linearly with increasing carbon content x, while the c parameter remains unchanged, indicating that carbon is exclusively substituted in the boron honeycomb layer without affecting the interlayer interactions. The superconducting transition temperature T_c decreases quasilinearly as a function of the carbon concentration, with a slope steeper than that for the isoelectronic Mg_1-xAl_xB₂.