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First-principles (FP) energetics of both the constituent elements and the compounds in the Al-Ca binary system are used in the CALPHAD (CALculation of PHase Diagrams) approach of thermodynamic modeling. First-principles calculations are performed using both an all-electron full-potential linearized augmented plane-wave method, as well as an ultrasoft pseudopotential/plane wave method. We perform calculations of T=0 ground state total energies of the pure Al and Ca in fcc, bcc, and hcp structures, and the binary compounds in their observed crystal structures. Al₄Ca, Al₁₄Ca₁₃, and Al₃Ca₈ are modeled in CALPHAD as simple stoichiometric compounds; however, the Laves C15 compound, Al₂Ca, is modeled using two sublattices (Al,Ca)₂(Al,Ca)₁, necessitating first-principles energies of both the stable Al₂Ca compound as well as the three nonstable Al₂Al, AlCa₂, and Ca₂Ca compounds. From …