Electronic structure and properties ofd- andf-shell-metal compounds

Bonding in rock-salt–structure compounds is described. For NaCl, the bonding is in terms of electrons occupying chlorine p bands, lowered in energy by interaction with the sodium s states, and an overlap repulsion arises from nonorthogonality of states on neighboring ions. For transition-metal compounds d-like states are added, with their coupling with the nonmetallic p states. This coupling is taken to be of the form ${\eta }_{\mathrm{pdm}}$ ${ħ}^2$($r_p$ $r_d^3$ ${)}^{1/2}$/(${\mathrm{md}}^4$), with $r_p$ and $r_d$ tabulated for each element. An additional overlap repulsion, proportional to ${ħ}^2$ $r_p$ $r_d^3$/(${\mathrm{md}}^6$) and to the number of electrons occupying the corresponding bands, also arises from this interaction. The simplest systems, such as KF, CaO, ScN, and TiC, contain eight valence electrons per atom pair and all but TiC are insulating. The extra energy from the covalent pd coupling is calculated; it decreases the lattice distance and increases the cohesion and bulk modulus. With a total valence of 9–12, the excess electrons occupy nonbonding bands making the compound metallic, but not significantly modifying the bonding properties.