Face-related segregation reversal at Pt50Ni50 surfaces studied with the embedded atom method

The segregation to the three low-index surfaces of a Pt50Ni50 single crysta l is modelled by Monte Carlo simulations combined with the embedded atom me thod (EAM). Using the best fit EAM parameters from the literature for the s ix transition metals of the Ni and Cu groups does not yield satisfactory re sults. In this work the EAM parameters are recalculated and optimised exclu sively for the Pt-Ni alloy system under study. Only then does EAM reliably reproduce the driving forces for segregation. The experimental results [Y. Gauthier et al., Phys. Rev. B 31 (1985) 6216; Y. Gauthier et al., Phys. Rev. B 35 (1987) 7867; S.M. Foiles, in: P.A. Dobs on, A. Miller (Eds.), Surface Segregation Phenomena, CRC Press, Boca Raton, FL, 1990, p. 79] reveal a face-related segregation reversal for the Pt,,Ni ,, single crystal. It appears from the simulations that this is caused by a relatively small difference in surface energy in close competition with th e elastic strain release. At the open (110) surface the difference in surfa ce energy dominates causing Ni segregation. At the (100) and (111) surfaces the difference in surface energy is overpowered by the elastic strain lead ing to Pt segregation. The simulations are in good agreement with the exper imental results and reproduce quantitatively the Ni segregation to the (110 ) surface and the Pt segregation to the (100) and (111) surfaces. Only at t he (110) surface significant relaxations are predicted in good agreement wi th experimental evidence. Atomic vibrations can be included by allowing a large number of very small displacements or with a more classical treatment of vibrational entropy. Bo th approaches yield the same results and show that the inclusion of atomic vibrations is important only for the (110) surface and tend to attenuate th e Ni segregation profile. (C) 1999 Published by Elsevier Science B.V. All r ights reserved.