Monte Carlo Simulations of Surface Segregation in Cu Ni Alloys
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MONTE CARLO SIMULATIONS OF SURFACE SEGREGATION IN Cu Ni ALLOYS A. PASTUREL *, V.DRCHAL **, J.KUDRNOVSKY , , and P.WEINBERGER * *Laboratoire de Thermodynamique et Physico-Chimie Metallurgiques, ENSEEG BP75, 38402 Saint Martin d' Hares, France "**Institute of Physics, Czechoslovak Academy of Sciences, CS-180 40 Praha 8 Czechoslovakia *** Institut fur Technische Elektrochemie, Technische Universitlit, A-1060 Wien, Austria
ABSTRACT A new method coupling electronic structure calculations with Monte Carlo simulations has been developed to determine surface compositions in Cu-Ni alloys. The calculations are based on an effective Ising model with parameters as defined within the framework of the Generalized Perturbation Method (GPM) and as calculated by means of the tight-binding version of the linear muffin-tin orbital method. The composition profiles are obtained for the fcc(001) surface for three bulk compositions, namely Cu 7 5 Ni 2 5, Cu 50Ni50, and Cu 2 5Ni 75 and compared with available experimental data.
INTRODUCTION Surface of bulk alloys are of great interest from both the theoretical and technological points of view, since many material properties depend on them. Alloying elements and impurities often segregate to the surface or near the surface region; an understanding of surface phenomena requires to answer the following questions: (i) nature of the segregating element, (ii) number of layers affected by the segregation, (iii) concentration profile (oscillating or monotonous ), (iv) influence of the temperature, the bulk concentration and the crystallographic orientation of the surface. One of the most elegant way to solve these problems is the use of the Monte Carlo simulation based on the surface Ising model derived from the knowledge of the underlying electronic structure. Recently, the first-principles version of the Ising model for the surface-related problems was formulated [1] and successfully used to study the formation of ordered surface alloys on clean transition metal surfaces [2]. In this paper, we present an ab-initio study of the surface segregation in CuNi (001) alloys based on the construction of the surface Ising model and its statistical study using the Monte Carlo simulation. From the theoretical point of view, the CuNi system has several attractive features: the elastic contributions are small and so are the relativistic effects for the valence band structure. In addition, magnetic interactions are negligible at least for Cu-rich compositions.
SURFACE ffSING MODEL The detailed knowledge of the underlying electronic structure is a necessary first step in such a study. The method to describe the electronic structure of a semi-infinite disordered alloy with concentration inhomogeneities near the surface was described in detail elsewhere [3,4]. In here we just summarize its basic features. Our approach is based on the local density functional approximation and uses the first-principles tight-binding linear muffin tin orbitals (TB-LMTO) method [5] to calculate the electronic structure. The all
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