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 c rystal structures of a given AB compound are used to define a long-ran ge Ising Hamiltonian which correctly represents atomic relaxations. Th is allows us to accurately calculate structural energies of relaxed su bstitutional A1-xBx systems containing thousands of transition-metal a toms, 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 transi tion temperatures T(c), and (iii) the T > T(c) short-range-order param eters. The method is illustrated for a transition-metal alloy (Cu1-xPd x) and a semiconductor alloy (Ga1-xInxP). It extends the applicability of the local-density method to finite temperatures and to huge substi tutional supercells. We find for Cu0.75Pd0.25 a characteristic fourfol d splitting of the diffuse scattering intensity due to short-range ord er as observed experimentally.