Bonding characteristics and site occupancies of alloying elements in different Nb5Si3 phases from first principles

The electronic structures and impurity formation energies of alpha-Nb5Si3, beta-Nb5Si3, and gamma-Nb5Si3 doped with Ti, V, Mo, Cr, W, Zr, and Hf additions in different sublattices have been investigated with the first-principles pseudopotential plane-wave method based on density functional theory. The bonding characteristics of the undoped and doped alpha-Nb5Si3, beta-Nb5Si3, and gamma-Nb5Si3 are analyzed with the valence charge densities and Mulliken overlap populations as well as partial density of states. The results show that the alloying elements with larger atomic size than Nb prefer to substitute the Nb in less closed space and the atoms with smaller atomic size than Nb prefer to substitute the Nb in more closed space in alpha-Nb5Si3, while these alloying elements present the opposite behavior in beta-Nb5Si3 and gamma-Nb5Si3. These behaviors can be explained by bonding characteristics: strong antibonds in beta-Nb5Si3 and gamma-Nb5Si3 are observed, whereas no antibonds in alpha-Nb5Si3 exist. The influences of the additions on phase stability difference between alpha-Nb5Si3 and beta-Nb5Si3 are also discussed.