Ab initio study of C-substituted MgB2

Using selected supercells of C-substituted magnesium diboride Mg(B1-xCx)(2), the electronic properties, lattice dynamics are calculated from first principle density functional methods. It is found, near the substituted C, an important relaxation of the nearby borons with a strong shortening of the C-B bonds. Upon doping, the Fermi level approaches progressively the top of the a bands, with a significant interaction of the sigma and pi states at crossing energies. The contribution of the sigma bands to the density of state at E-F, is slightly changing at low doping (x <= 0.125), while the B-B stretching mode strongly coupled to the sigma holes, get progressively harder with an increasingly reduced anharmonicity. The frequency of this mode determined by the frozen phonon method is found to be close to experimental Raman data by Sakuntala et al. [19]. (c) 2005 Elsevier Ltd. All rights reserved.