Carbon miscibility in the boron layers of the MgB₂ superconductor

A detailed structural analysis of the MgB_2-xC_x series at low doping levels (0 ≤ x ≤ 0.1) is presented. High-resolution synchrotron x-ray powder diffraction experiments show an extremely small miscibility of carbon within the boron sheets despite the structural analogy between MgB₂ and graphite. Multiphase behavior is detected for x ≥ 0.04. The coexisting phases, isostructural with the parent MgB₂ compound, differ mainly in their in-plane lattice constants, as evidenced by the splitting of the (hk0) Bragg reflections. All phases contain carbon in substitution for boron but in different concentrations.