First-principles calculation of phase stability and cohesive properties of Ni-Sn intermetallics

The cohesive properties of Ni-Sn intermetallics (stable, metastable, and virtual), hitherto unexplored by density-functional theory (DFT) methods, are reported. Specifically, the total energies and cohesive properties of Ni, Sn, and 27 Ni-Sn intermetallics are calculated from first-principles, using ultrasoft pseudopotentials (USPP) and both local-density approximation (LDA) and generalized-gradient approximation (GGA) for the exchange-correlation functional. Among the intermetallics considered, the ground-state structures are consistent with experimental observations; however, not all of them are registered in the equilibrium-phase diagram. An important result of this systematic study, using both USPP-LDA and USPP-GGA, is that oC 20-NiSn₄ is predicted to be the ground-state structure. Only recently, this phase has been observed as a product of the interfacial reaction in Ni/Sn diffusion couples. In addition, we find that the thermodynamic stability of a tetragonal phase, tP 10-NiSn4, is very similar to that of oC 20-NiSn₄. The elastic stability of both tP 10-NiSn₄ and oC 20-NiSn₄ is confirmed by calculating their single-crystal elastic constants. The calorimetric data for the enthalpy of formation of stable intermetallics show an agreement that is better for those calculated with USPP-LDA than those calculated with USPP-GGA. In general, the experimental lattice parameters of stable and metastable phases are found to lie between those calculated using USPP-LDA and those calculated using USPP-GGA; however, in several cases, the values calculated using USPP-GGA agree to within 1 pct of the experimental data. The Ni₃Sn₂(ht) ⇌ Ni₃ Sn₂(lt) transformation is discussed in terms of supergroup-subgroup relations. The bonding between the Ni and the Sn is discussed based on the analyses of the density of states (DOS) and the charge densities.