Monte Carlo and Molecular Dynamics Validation of an N-Body Potential for Cu 3 Au
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MONTE CARLO AND MOLECULAR DYNAMICS VALIDATION OF AN N-BODY POTENTIAL FOR Cu 3Au CARLOS REY-LOSADA*, MARC HAYOUN** AND VASSILIS PONTIKIS** * Dep. de Ffsica de la Materia Condensada, Facultad de Ffsica, Universitad de Santiago, E - 15706 Santiago de Compostela, Espafia 00 Section d'Etude des Solides Irradi6s, CEA-CEREM, CNRS URA No 1380, Ecole Polytechnique, 91128 Palaiseau Cedex, France. ABSTRACT We present the results of Monte Carlo and Molecular Dynamics simulations of some thermodynamical properties of Cu 3 Au. The calculations rely on an empirical n-body potential that reproduces satisfactorily the critical temperature, Tc, the temperature dependence of the lattice constant and atomic vibrational amplitudes as well as Cowley's short-range order parameters above Tc. Our results show that relaxation effects decrease considerably the formation energy of antisite defects and therefore should explicitly be considered for a realistic description of the transition. INTRODUCTION The compound Cu3Au has often served as the prototype of alloy systems showing orderdisorder phenomena and has therefore been widely studied both, experimentally and theoretically. Moreover, computer simulations using the Monte Carlo technique (MC) on a fixed lattice showed that the order-disorder transition as well as short and long-range correlations can thereby be predicted thus offering matter of comparison with the experiments. The numerous advantages offered by computer simulations [1] have stimulated the development of interatomic potentials for alloys on an empirical [2] or semi-empirical [34] basis. Such potentials prove useful where an 'ab-initio' approach is impractical that is in applications requiring the use of large systems e.g. in numerical studies of interface properties. It is worth to notice that n-body semi-empirical potentials appear to describe quite satisfactorily the properties of noble metal alloys and in particular those of the Au-Cu system [4]. More specifically, simulations using such potentials suggest that relaxations effects are important in Cu3Au, due to the large difference in atomic sizes between Au and Cu atoms, in agreement with X-ray diffuse scattering experiments [5, 61. The present work is devoted to the development of a composition dependent n-body potential adapted to Cu3Au that also satisfactorily predicts physical properties of the pure elements. Its validation is based on a systematic comparison between experimental and computed physical properties including the critical temperature, the temperature dependence of the lattice constant and atomic mean square displacements (MSD) and Cowley's shortrange order parameters above Tc. Excellent agreement is obtained between experimental and computed thermodynamical data. Moreover, special attention is given to the relaxations associated with antisite defects. To this purpose we compare relaxed and unrelaxed values of the formation energy of a single chemical defect in an otherwise perfect system. The significant difference found between these values rises doubts
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