Carbon impurities in MgB2

Abstract

The structural and electronic properties of carbon impurities in ${\mathrm{MgB}}_2$ are investigated by first-principles total-energy calculations. We find that high concentration of C impurities could be incorporated in ${\mathrm{MgB}}_2$ due to their low-formation energies. Substitutional C impurities have lower formation energies than interstitial C. Thus, the majority of C impurities in ${\mathrm{MgB}}_2$ should be substitutional, which results in a lattice parameter decrease in the $\mathrm{a-b}$ plane. C impurities are found to reduce the density of states of ${\mathrm{MgB}}_2$ around the Fermi energy, and therefore, may cause a decrease in critical temperature $T_c.$