Bonding characteristics and site occupancies of alloying elements in different Nb5 Si3 phases from first principles

Abstract

The electronic structures and impurity formation energies of α- Nb5 Si3, β- Nb5 Si3, and γ- Nb5 Si3 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 α- Nb5 Si3, β- Nb5 Si3, and γ- Nb5 Si3 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 α- Nb5 Si3, while these alloying elements present the opposite behavior in β- Nb5 Si3 and γ- Nb5 Si3. These behaviors can be explained by bonding characteristics: strong antibonds in β- Nb5 Si3 and γ- Nb5 Si3 are observed, whereas no antibonds in α- Nb5 Si3 exist. The influences of the additions on phase stability difference between α- Nb5 Si3 and β- Nb5 Si3 are also discussed.