Ground State Searches in Fcc Intermetallics
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GROUND STATE SEARCHES IN FCC INTERMETALLICS 4 3 1 C. WOLVERTON( ), G. CEDER(2), D. DE FONTAINE( ), and H. DREYSSIt( )
(I) Dept. of Physics, Univ. of California, Berkeley, CA 94720 and Materials Science Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720 (2) Dept. of Materials Science, Masachusettes Institute of Technology, Cambridge, MA 02139 (3) Dept. of Materials Science and Mineral Engineering, Univ. of California, Berkeley, CA 94720 and Materials Science Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720 (4) Laboratoire de Physique du Solide, Universit6 de Nancy, Vandoeuvre les Nancy, France ABSTRACT A cluster expansion is used to predict the fcc groutnd states, i.e., the stable phases at zero Kelvin as a function of composition, for alloy systems. TFile internetallic structures are not assumed, but derived rigorously by minimizing the configurational energy subject to linear constraints. This ground state search includes pair and multiplet interactions which spatially extend to fourth nearest neighbor. A large number of these concentration-independent interactions are computed by the method of direct configurational averaging using a linearizedmuffin-tin orbital Hamiltonian cast into tight binding form (TB-LMTO). The interactions, derived without the use of any adjustable or experimentally obtained parameters, are compared to those calculated via the generalized perturbation method extention of the coherent potential approximation within the context of a KKR Hamiltonian (KKR-CPA-GPM). Agreement with the KKR-CPA-GPM results is quite excellent, as is the comparison of the ground state results with the fcc-based portions of the experimentally-determined phase diagrams under consideration. INTRODUCTION The study of alloy phase stability is of tutmost practical and technological importance. Recently, it has become possible, through several techniques, to perform first principles electronic structure calculations of ordered and disordered alloys. When a particular alloy system is studied theoretically, the first order of business is to solve the ground state problem. In other words, one first finds the minimum energy structures at all compositions and T=OK, and then perforns non-zero temperature calculations using only these phases. A commonly used "method" for finding the ground states involves selecting several structures suspected of being the lowest energy states, calculating the energies of these structures, and then simply assuming that the true ground states are the ones in the set with the lowest calculated energies. This type of argument is, of course, not sound as it assumes what is to be proved. Thus, using this simple approach, it is quite likely that the true ground states of an alloy system will be missed. Given an alloy system, finding the minimum energy structures with respect to all possible topological vatiations would, at best, be a Herculean task. Fortunately, however, the ground states of many alloys are superstructures of the fcc, bcc, or hcp lattices. The problem of determining
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