On the ionicity of carbon in Be2C. A theoretical investigation of its electronic structure

Abstract

A self-consistent APW band structure calculation for insulating Be2C is reported. From the APW results local partial densities of states have been obtained showing a considerable beryllium admixture in the carbon 2p band. An LCAO tight binding fit and a subsequent Mulliken analysis of the eigenvectors reveals that the carbon carries a charge of approximately 3.5 e. The electrons remaining on the beryllium atom are predominantly of p character giving rise to a covalent pp interaction which is responsible for pronounced features in the valence bands not present in a completely ionic description. A further analysis of the ionicity is made on the basis of a fit of the structure factors derived from the APW valence electron density using a superposition of atomic or ionic form factors. To obtain an optimum fit the electronic charge on the carbon has to be decreased to approximately 3.0. This reduction of charge, compared with the LCAO fit, is traced back to the fact that the atomic form factors have been calculated from a nonorthogonal basis, whereas an orthogonal one underlies the LCAO fit.