1ST-PRINCIPLES SIMULATED-ANNEALING STUDY OF PHASE-TRANSITIONS AND SHORT-RANGE ORDER IN TRANSITION-METAL AND SEMICONDUCTOR ALLOYS

Total-energy local-density calculations on approximately 20 periodic crystal structures of a given AB compound are used to define a long-range Ising Hamiltonian which correctly represents atomic relaxations. This allows us to accurately calculate structural energies of relaxed substitutional A1-xBx systems containing thousands of transition-metal atoms, simply by adding up spin products in the Ising Hamiltonian. The computational cost is thus size independent. We then apply Monte Carlo and simulated-annealing techniques to this Ising Hamiltonian, finding (i) the T = 0 ground-state structures, (ii) the order-disorder transition temperatures T(c), and (iii) the T > T(c) short-range-order parameters. The method is illustrated for a transition-metal alloy (Cu1-xPdx) and a semiconductor alloy (Ga1-xInxP). It extends the applicability of the local-density method to finite temperatures and to huge substitutional supercells. We find for Cu0.75Pd0.25 a characteristic fourfold splitting of the diffuse scattering intensity due to short-range order as observed experimentally.