A model for adsorption-induced surface reconstruction. Ground-state and phase diagrams

In this paper we study the adsorption-induced surface reconstruction phenomenon by using the so-called four-position (4-P) model discussed by C. Zuppa et al. [Phys. Rev. B 51 (1995) 2618] for the H-2/W(001) system. In the framework of the lattice-gas scheme, the model contains four equilibrium positions for the W surface atom and the adsorption sites are located between them. Adsorption-induced reconstruction phenomenon occurs when the surface metal atoms structure goes through a second-order phase transition from a c(2 x 2) order to disorder. We calculate the ground-state map and, as a result, three possible fundamental structures emerge for both metal and adsorbed atoms. Through Monte Carlo simulations and finite-size scaling analysis, we calculate the critical temperature and consequently the phase diagram for the surface metal structure as well as for the adsorbed atoms. As a novel result we observe, in the Framework of this model, that reconstruction transition is only possible if the adsorbed layer presents a (2 x 1) ordered phase. Other adsorbate structures, such as a c(2 x 2) phase, do not affect the critical behavior of the surface metal atoms arrangement, however, it is shown that the phase diagram for the adsorbed phase is strongly modified by the surface metal atoms structure. (C) 1998 Elsevier Science B.V.