Cohesive properties and electronic structure of Heusler L21-phase compounds Ni2XAl (X=Ti, V, Zr, Nb, Hf, and Ta)

The ternary (Heusler) L21-phase compounds Ni2XAl (group-IVA or -VA element X=Ti, V, Zr, Nb, Hf, and Ta) have been found to greatly increase the high-temperature creep strength of NiAl. We report here the cohesive properties and electronic structures of these potential high-temperature structural materials in both the Heusler L21 and the B2 phases determined by means of the all-electron total-energy self-consistent linear muffin-tin orbital method based on the local-density approximation. Our results show that the calculated equilibrium lattice constants of the L21-structure compounds are in very good agreement with experiment. For Ni2TiAl and Ni2VAl, the lattice constants are found to match that of NiAl (mismatch is 1.7% and 1.0%, respectively). But for the other four compounds, the mismatch is found to be larger (3.65.6%). The difference of the lattice constants in the B2 and the L21 structures, however, is very small (less than 1%). The formation energy is found to be consistently in favor of the L21 phase. The Ni d and X d hybridization contributes the most to the cohesion of these compounds whose stability is also made certain by the well-separated filled bonding and empty antibonding states in the density of states. The density of states of the B2 phase is quite similar to that of L21, but distinctions exist due to their structural differences. The rigid-band approximation is found to work well for these compounds.