Electronic structure and chemical-bonding mechanism of ${\mathrm{Cu}}_3$N, ${\mathrm{Cui}}_3$NPd, and related Cu(I) compounds

The electronic structure and the chemical-bonding mechanism of ${\mathrm{Cu}}_3$N, ${\mathrm{Cu}}_3$NPd and related Cu(I) compounds, such as ${\mathrm{Cu}}_2$O, are studied on the basis of band-structure calculations, using both the linearized augmented plane wave and linear combination of atomic orbitals (LCAO) methods. In accordance with experimental observations, ${\mathrm{Cu}}_3$N is found to be a semiconductor, while ${\mathrm{Cu}}_3$NPd should exhibit a semimetallic conductivity. The chemical bonding is investigated using various methods, among them are the valence charge partitioning scheme of Bader and a basis set reduction technique built on the LCAO method. A partly ionic, partly covalent bonding is found. The admixture of the Cu (4s, 4p) states to the Cu 3d–N 2p bands resulted to be essential for the covalent bonding effect, since pure 3d-2p bands, with bonding and antibonding states fully occupied, do not lead to a covalent energy gain. This specific hybridization appears to be the origin of the twofold dumbbell like Cu(I) coordination observed in ${\mathrm{Cu}}_3$N and other Cu(I) compounds. In ${\mathrm{Cu}}_3$NPd, a covalent to metallic bonding between the ${\mathrm{Cu}}_3$N host crystal and the interstitial Pd atoms is found, which is mainly caused by Pd 5s and 5p states hybridizing to Cu 3d states. © 1996 The American Physical Society.