Covalent bonding and band-gap formation in ternary transition-metal di-aluminides: Al₄MnCo and related compounds

In this paper we extend our previous study of the electronic structure of and bonding mechanism in transition-metal (TM) di-alumininides to ternary systems. We have studied the character of the bonding in Al4MnCo and related TM di-aluminides in the C11_b (MoSi₂) and C54 (TiSi₂) crystal structures. A peculiar feature of the electronic structure of these TM di-aluminides is the existence of a semiconducting gap at the Fermi level. In our previous work we predicted a gap in Al₂TM compounds where the TM atoms have eight valence electrons. Here we demonstrate that the semiconducting gap does not disappear if the TM sites are occupied by two different TMs, provided that the electron-peratom ratio is conserved. Such a replacement substantially increases the class of possibly semiconducting TM di-aluminides. Substitution for 3d TMs of 4d or 5d TMs 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 dominantly covalent character. This is confirmed not only by the enhanced charge density halfway between atoms, but also by the clear bonding-antibonding splitting of the electronic states. If the gaps between split states that correspond to all bonding configurations in the crystal have a common overlap at the Fermi level, the intermetallic compound becomes a semiconductor. However, the results of the total-energy calculations suggest that the existence of a band gap does not necessarily imply a stable structure. Strong covalent bonds can exist also in Al-TM structures where no band gap is observed.