Extension of a New Semiempirical Method (BFS) And the Study of Ground State Properties of Binary Alloys
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EXTENSION OF A NEW SEMIEMPIRICAL METHOD (BFS) AND THE STUDY OF GROUND STATE PROPERTIES OF BINARY ALLOYS GUILLERMO BOZZOLO* AND JOHN FERRANTE" Analex Corporation, 3001 Aerospace Parkway, Brook Park, OH, 44142-1003 *National Aeronautics and Space Administration, Lewis Reseaxch Center, Cleveland, OH 44135.
ABSTRACT We extend the method of Bozzolo, Ferrante and Smith (BFS) for the study of alloy energetics to include a description of the local environment in specific ordered structures. The concept of bond-diagrams is introduced and applied to fcc binary compounds. A simple example of the parameterization of the bond-diagrams is done with reference to available first-principles calculations of Ni-Pt ordered alloys. INTRODUCTION
Recently, a new semiempirical method was introduced by Bozzolo, Ferrante and Smith (BFS) for the study of alloy energetics [1-8]. The method is based on the ideas of equivalent crystal theory (ECT) for defect formation energies in elemental solids [9] and uses pure metal properties and only two alloy quantities as input data. In previous work, we applied the method for the study of the heat of formation as well as the concentration dependence of the lattice parameter of several fcc binary alloys [1,2]. More recent applications of BFS deal with the study of the energetics of bcc alloys [3] and the theoretical modelling of the atomic force microscope for bimetallic tip-sample interactions [4]. Also, as a consequence of the ideas underlying the foundation of BFS, we were able to derive a new set of sum rules which allow for direct calculation of alloy bulk properties from information of the pure components in an approximate fashion [5]. In the original formulation of BFS, the contribution to the energy of each individual atom is given by two separate terms: a strain energy, where the neighboring atoms are taken to be of the same atomic species as the reference atom, and a chemical energy contribution where the surrounding atoms retain their chemical identity but are forced to be in equilibrium lattice sites of the reference atom. The strain energy contribution of each atom is obtained by a straightforward application of ECT for pure elements, accounting for the departure from equilibrium of the actual spatial distribution of the atoms in the alloy. Thus, its accuracy depends on the ability of ECT to properly describe the energetics of a single-component defect crystal. The basic concepts in ECT also apply for the calculation of the chemical energy: in this case, the defect is represented by the different chemical species of the neighboring atoms. Being that the strain energy is a pure crystal calculation, any information about a particulax structure is necessarily included in the chemical energy term. This fact introduces a limitation in the power of the original BFS to properly account for the relative location of atoms of different species in a given ordered structure, as ECT only accounts for the number of neighboring atoms of each species, but not their relative locations. Thus, ordered struc
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