Covalent bonding and bandgap formation in transition-metal aluminides: Di-aluminides of group VIII transition metals

In this paper we study the electronic structure, electron density distribution and bonding mechanism in transition-metal (TM) di-aluminides Al₂TM formed by metals of group VIII (TM Fe, Ru, Os) and crystal structures of TM di-silicides C11_b (MoSi₂), C40 (CrSi₂) and C54 (TiSi₂). A peculiar feature of the electronic structure of these TM di-aluminides is the existence of a semiconducting gap at the Fermi level. A substitution of a 3d TM by 4d or 5d metal enhances the width of the gap. From the analysis of the charge-density distribution and the crystal-orbital overlap population we conclude that the bonding between atoms has strong covalent character. This is confirmed not only from the enhanced charge density halfway between atoms, but also by a clear bonding-antibonding splitting of the electronic states. Groups of bonding and antibonding states corresponding to a particular bonding configuration of atoms are separated by a gap. As such a gap is observed in all bonding configurations among atoms in the unit cell it results in a gap in the total density of states. The bandgap exists at a certain electron per atom ratio e/A≈4.67 and also occurs in TM di-aluminides of groups VII and IX. For group VIII TM di-aluminides the Fermi level falls just in the gap.