Electronic structure of alkali-metal fluorides, oxides, and nitrides:density-functional calculations including self-interaction corrections

\u3cp\u3eThe recently synthesized compound Na3 N has experimentally been shown to be semiconducting much in contrast to the outcome of standard density-functional theory calculations which find Na3 N to be metallic. To address this obvious contradiction, we have systematically investigated the electronic structure of Na3 N by density-functional calculations employing self-interaction-corrected pseudopotentials which have been shown before to yield results in much better agreement with experiment than standard local-density calculations. To assess the usefulness of such pseudopotentials for a broader class of related materials, we have carried out, in addition, a comparative ab initio study of the electronic structure of the nine alkali-metal fluorides MF, oxides M2 O, and nitrides M3 N with M=Li, Na, and K. We arrive at valence and conduction bands that are in good agreement with the restricted results available in the literature from calculations going beyond the local-density approximation and from experiment. In particular, Na3 N turns out to be clearly semiconducting, as observed in experiment.\u3c/p\u3

Inorganic Niobium and Tantalum Octahedral Cluster Halide Compounds with Three-dimensional Frameworks: A Review on their Crystallographic and Electronic Structures

International audienceThis review summarizes the development of the rich crystal and bonding chemistry of face-capped and edge-bridged inorganic niobium and tantalum octahedral cluster halide compounds, with a particular emphasis on those showing three-dimensional cluster frameworks. Discussion is made on varied structures and bonding which are intimately linked to the valence electron concentration, i.e., the number of electrons that held the octahedral metal cluster architecture. Exploration of the literature indicates that apart from Nb6I11 and derivatives, which show electron-deficient face-capped M6Xi8Xa6 units, compounds containing edge-bridged M6Xi12Xa6 motifs are the most largely encountered. Closed-shell compounds with a valence electron concentration of 16 are predominant, although a few 15-electron open-shell magnetic compounds or even 14-electron closed-shell species have also been reported. Particularly interesting from a structural point of view is the fashion in which these face-capped and edge-bridged clusters “pack” in crystals. The astonishing diversity of structural types, which are observed, is mainly due to the flexibility of the halogen ligands to coordinate in various manners to metal atoms. However, a rigorous structural analysis of these compounds reveals no close relationship between the valence electron concentration and the variability of the intercluster connections and/or the nature of the counterions. Indeed, the main bonding features of these compounds can be understood from the delocalized bonding picture of isolated “molecular-like” M6Xi8Xa6 or M6Xi12Xa6 clusters