The calculation of adsorption parameters using embedded atom methods

Abstract

A comparative investigation to assess embedded atom methods for the calculation of the suitability of various adsorption properties of systems consisting of homo- and heteronuclear bee metals have been undertaken. Homonuclear systems consisting of a tungsten adatom and an infinite tungsten monolayer adsorbed on a tungsten {110} substrate, were considered. For heteronuclear systems, the case of a tantalum adatom adsorbed on a tungsten {110} surface was analysed. The embedded atom methods introduced by Gollisch, by Johnson and Oh, and by Finnis and Sinclair were applied to the homonuclear system. The latter two models were also applied to the heteronuclear system. For both systems the potential of Finnis and Sinclair have been extended for application to {110} tungsten surfaces, introducing an additional parameter fitted to the empirical value of the adatom-surface migration energy. Appropriate definitions for the desorption energy and the adatom-surface migration energy in terms of N-body potentials, were formulated. In addition, the vibration frequencies and coefficients for a truncated Fourier representation of the adatom-surface interaction potential were also calculated numerically. The potential models were assessed on their ability to reproduce the experimental values of the adatom-surface migration energy and the desorption energy, as well as the presence of a double well in the potential surface at an adsorption site on tungsten {110} substrates. The surface modified model of Finnis and Sinclair appeared to be the most suitable potential for application to the homonuclear system, whereas, for the heteronuclear system, no specific model could be identified as being the most appropriate. For the homonuclear system, all N-body potentials yielded more realistic values of the adatom-surface migration energy, the desorption energy and the adatom-substrate equilibrium height than the conventional Morse and Lennard-Jones pair potentials. The magnitude of optimized Fourier coefficients for truncated Fourier series representations of the adatom-substrate interaction potential were shown to decay rapidly with harmonic order. This justifies the implementation of Fourier series truncated at low harmonic order for the theoretical description of isolated adsorbates on crystal surfaces. The relative values of the normalized Fourier coefficients appeared to be largely determined by the symmetry of the substrate. The success of embedded atom methods in describing surface phenomena was attributed mainly to their ability to incorporate environmental effects on bonding; while requiring only slightly more computational effort than pair potentials.