Carbon impurities in MgB2

The structural and electronic properties of carbon impurities in MgB2 are investigated by first-principles total-energy calculations. We find that high concentration of C impurities could be incorporated in MgB2 due to their low-formation energies. Substitutional C impurities have lower formation energies than interstitial C. Thus, the majority of C impurities in MgB2 should be substitutional, which results in a lattice parameter decrease in the a-b plane. C impurities are found to reduce the density of states of MgB2 around the Fermi energy, and therefore, may cause a decrease in critical temperature T-c. (C) 2002 American Institute of Physics.