First-Principles Study of Phase Stability in PD-RH Alloys
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FIRST-PRINCIPLES STUDY OF PHASE STABILITY IN PD-RH ALLOYS 4 3 2 2 1 D.D. Johnson , P.E.A. Turchi , Marcel Sluiter , D.M. Nicholson , F.J. Pinski , G.M. Stocks
'Sandia National Laboratories, Livermore, CA 94550; 2 Lawrence Livermore National Laboratory, Livermore, CA 94551; 3 Oak Ridge National Laboratory, Oak Ridge, TN 37831; 4 University of Cincinnati, Cincinnati, OH 45221.
We present a study of the mixing energies and the effective cluster interactions which form the configurational part of the internal energy of Pd-Rh substitutional alloys. We discuss the tendency towards phase-separation and more generally phase stability. The effects of a substitutional ternary addition on the tendencies toward order or phase-separation are also reported. The Korringa-Kohn-Rostoker Coherent-Potential Approximation (KKR-CPA) is used to investigate the electronic structure effects and energetics of the random alloy. Moreover, we use the Generalized Perturbation Method (GPM), using the KKR-CPA random alloy as a reference medium, to investigate the effective interactions which determine phase stability. We briefly comment on other factors which may give important contributions to the total free-energy of the alloy. We also contrast the GPM with the Connolly-Williams approach for calculating phase diagrams from first-principles. Finally, we explore the inadequacies of the frozen-potential and Harris approximations to the energetics of alloying. Since the 1920's, when Mott and Jones began investigating the implications that quantum mechanics has in determining the properties of metals, origins of alloy phase-stability have been ascribed to various phenomena related to the electronic structure. In the last two decades, with the advent of efficient and relatively accurate electronic-structure methods based on the local density approximation, many properties of materials have been explained in terms of the underlying electronic structure due to alloying or symmetry changes. In the last decade, it has been a goal to connect these electronic-structure (ground-state) energies (assuming little temperature dependence of these energies) with various statistical mechanics techniques (such as, Monte Carlo or Cluster Variational Method (CVM)), to obtain a good description of the freeenergy. Numerous methods have been put forth which, in principle, follow this approach, such as the concentration fluctuation theory of Gybrffy and Stocks [1], the method of Connolly and Williams [2], and the GPM of Duscatelle and Gautier [3]. In this report, we will use the GPM, which is based on the random state described by the KKR-CPA, in order to calculate from firstprinciples the energetics associated with alloy phase stability, such as energies of mixing and effective cluster interactions. The CVM will then be used to calculate the free energies and hence phase stability for PdRh alloys. Before discussing PdRh alloys, their stability, and effects of ternary additions, several remarks are in order to introduce our approach and to contrast some aspects with t
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