Résumé
Periodic density functional theory calculations are used to discuss the existence of metal tetraborides MB 4 with divalent metals. Tetraborides which contain metal atoms inserted in a three-dimensional boron network made of B 6 octahedra and B 2 dumbbells exhibit a pseudo energy gap for a count of 60 valence electrons per M 4(B 6) 2(B 2) 2 formula unit. Such a count satisfies the stability electron requirement for B 6 2− (20 electrons) octahedra and B 2 2− (8 electrons) units and allows the filling of two supplementary low-lying bands deriving from the valence metallic d atomic orbitals. This favored electron count is not reached for CaB 4 which is then formally deficient by one electron per metal atom. This indicates that CaB 4 is unlikely to exist without n-doping.