Electronic Structure, Multiatom Interactions, Stability of Ordered Structures and Short Range Order in Binary Transition
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ELECTRONIC STRUCTURE, MULTIATOM INTERACTIONS, STABILITY OF ORDERED STRUCTURES AND SHORT RANGE ORDER IN BINARY TRANSITION METAL ALLOYS 2
A. BIEBER 1,2 AND F. GAUTIER 1 C.R.M.-CNRS, 6 rue Boussingault 67083 Strasbourg Cedex France (LA 306) 4 rue Blaise Pascal 67070 Strasbourg Cedex France
2 L.M.S.E.S.
ABSTRACT 'lultiatom interactions required in the statistical models used for the study of both the stability of ordered structures in the ground state and the chemical short range order, have been computed from the electronic structure for transition metal alloys. It is shown that : i) the most important interactions coming from "d" bands are those corresponding to self-retraced paths, the contributions of compact clusters being negligible ; ii) in general the results obtained using only second order pair interactions are valid provided that the concentration dependence of the pair interactions is taken into account. However in a few peculiar cases these results may be qualitatively modified.
INTRODUCTION The computation of the ordering phase diagrams in binary alloys AcBI1c has been studied by means of generalized 3D Ising models in the framework of different approximate methods [1-3]. All these models involve phenomenological cluster (multiatom) interactions. These latter can be qualitatively important since they can modify the nature of the ground state, the phase diagrams and the nature of the phase transitions [4]. In binary alloys they are currently invoked to explain the asymmetry of the phase diagrams about the concentration c = 0.5 [5]. There is no means to decide whether, and how much, the relevant cluster interactions (CI) are concentration dependent [I] ; moreover there is no definite criteria for decreeting a priori which clusters should be retained in the configurational free energy [1]. It is therefore of central importance to determine the CI from microscopic models which are based on the electronic structure and which can be related to an anoroDriate statistical model. We have therefore investigated the role of multiatom interactions, in determining both the stability of ordered structures(OS) in the ground state and the short range order (SRO) [6],by me; this ans of the generalized perturbation method discussed previously L,8 latter enables to compute the CI from the electronic structure[9]. We have found that, although the interactions of some clusters are not negligible as compared to the pair interactions , in general the overall effect of the higher order interactions leads only to small quantitative modifications both for the stability and for the SRO [6] ; only in a few cases,for c=0.5. we have found a strong qualitative change. Our aim in this paper is to briefly summarize the general results and to discuss in some detail the results obtained for c = 0.5. For simplicity we restrict ourselves to non magnetic systems. MULTIATOM INTERACTIONS AND CONFIGURATIONAL ENERGY Cluster Interactions For binary systems, in which only chemical interactions exist, every atomic configuration is completely cha
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