First-principles interatomic potentials for transition-metal aluminides: Theory and trends across the 3d series

In this paper the first-principles generalized pseudopotential theory (GPT) of transition-metal interatomic potentials [J. A. Moriarty, Phys. Rev. B 38, 3199 (1988)] is extended to AB binary compounds and alloys. For general transition-metal (TM) systems, the GPT total-energy functional involves a volume term, central-force pair potentials, and angular-force multi-ion potentials, which are both volume (Ω) and concentration (x) dependent and include all sp, sp-d, and d-d interactions within local density-functional quantum mechanics. The formalism is developed here in detail for intermetallic systems where A is a simple metal and B is a transition metal and applied to the prominent special case of the transition-metal aluminides TM_xAl_1-x, where sp-d hybridization is especially important. Emphasis is given to the aluminum-rich 3d binary systems for x<0.30, which appear to be well described at the pair-potential level without angular forces and for which the present GPT potentials can be used directly in atomistic simulations. Volume terms and pair potentials for all of the 3d aluminides have been calculated and their behavior with atomic number, Ω, and x is elaborated through illustrative applications to the cohesive and structural trends across the 3d series. More extensive applications to the Co-Al and Ni-Al phase diagrams will be given elsewhere.