BONDING IN THE MOLYBDENUM SILICIDES

Full-potential linear muffin-tin orbital calculations are reported for the bulk properties, density of states, and electron density in A2 Mo, A15 Mo3Si, D8(m) Mo5Si3, C11(b) MoSi2, and A4 Si. Both the magnitude and the directional anisotropy of the valence charge density associated with bonds in the three silicides suggest a hierarchy in bond strength, ordered from strongest to weakest: Mo-Mo, Mo-Si, and Si-Si. A combined Mulliken and tight-binding analysis shows this behavior to coincide with variations in occupation of the respective types of bonding states, which is ultimately traced to Mo(Bd) site energies that are nearly degenerate with the Fermi level, in contrast to Si(3p) site energies lying about 5 eV higher. These characteristics point to the dominance of Mo-Si bonding in C11(b) MoSi2, consistent with the near pd hybridization gap in this material, which appears to explain the unique position of the disilicide in systematics of the molybdenum silicide bulk properties. Finally, having identified the critical elements of the electronic structure in the molybdenum silicides, this work provides a physically motivated basis upon which to improve models used in the study of large-scale Mo-Si interface problems.