Hybridization-induced band gaps in transition-metal aluminides

The occurrence of hybridization gaps around the Fermi level in ordered binary and ternary transition-metal aluminides is investigated using first-principles electronic structure calculations. Hybridization between transition-metal atoms and Al and the lack of transition-metal–Al $d-d$ hybridization are both found to be important for the formation of the hollows or pseudogaps in the density of states. Whether these hollows become actual gaps, as predicted for ${\mathrm{RuAl}}_2,$ ${\mathrm{FeAl}}_2,$ and ${\mathrm{Fe}}_2\mathrm{NbAl},$ depends on details of the electronic structure, including the relative positions of the atomic levels, the size of the atoms, the crystal structure, and the number of valence electrons. Although hybridization gaps resulting in true semiconductors are uncommon, at least among the aluminides, hollows in the density of states around the Fermi level are ubiquitous.