Structures and stability of CO layers on the MgO(001) surface

Abstract

Monte Carlo simulations of CO/MgO(001) show that below 41 K the CO molecules form a c(4×2) structure with six molecules per unit cell distributed into two kinds of adsorption sites: a perpendicular site and a tilted site (polar angle of 31°). Both sites are localized near Mg2+ ions. The occupancy of perpendicular sites to tilted sites occurs in the ratio of 1:2. At 41 K the c(4×2) phase undergoes a phase transition into a less dense, disordered phase accompanied by the expulsion of some molecules to form a partial second layer. The density of the remaining disordered layer is the same as for a p(3×2) phase and portions of the disordered layer show regions of short range ordering with either the c(4×2) or p(3×2) structures. The p(3×2) phase contains four molecules per unit cell and also consists of perpendicular and tilted sites, but in the ratio of 1:1. This structure was found to be stable up to 50 K after which the expulsion of some molecules and disordering of the layer occurred. A model to test the relative stability of these two phases by examining the difference in Gibbs free energy is constructed and shows that below 41 K the c(4×2) phase is the most stable but above 41 K the p(3×2) phase is the most stable. However, at low pressures the model suggests that the p(3×2) phase will not be observed and the layer will instead transform from the c(4×2) phase to a disordered phase at 41 K. This result reconciles the findings of low-energy electron diffraction (LEED) experiments [p(3×2) phase observed] with those of helium atom scattering (HAS) and polarization infrared spectroscopy (PIRS) experiments (disordered phase observed). It is proposed that the c(4×2)→p(3×2) transition is part of an infinite sequence of transitions involving (n×2)-type structures which, under suitable conditions of temperature and pressure, constitutes an example of the devil’s staircase phenomenon. Such a phenomenon has been suggested by previous LEED experiments.