APW bands and bonding in transition metal oxides, nitrides and carbides

In order to deepen our understanding of the relationships between APW bands and covalent binding in the carbides, nitrides and oxides of transition elements a model has been constructed where bonds are formed of 2s and 2p anionic states extended towards neighbouring sites by means of six equivalent metallic orbitals. Valence bands are generated in a way similar to that of the bond orbital model for semiconductors. The model explains by the intermediary of electronegativity differences, the correlation between cohesion energies and the valence bandwidths quoted by Slater. Moreover in contrast to the tightbinding approximation, it confers only a secondary role to lattice spacing. For example one verifies that APW bands of the TiO and VO are relatively narrow for the oxide series in spite of a small lattice spacing. This result remains unexplained in a tight-binding scheme. In addition, the model relates s- and p-bands by the conjecture that both are formed from the same equivalent metallic orbitals. It is found that there is an appreciable amount of covalency for s-bonds. This model emphasizes from another point of view the importance, which has previously been recognized by Ramqvist and Phillips, of the electronegativity differences in these compounds. (author)