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.

A Monte Carlo simulation study of H2 layers on NaCl(001)

“…Our calculation of the molecule-molecule and molecule-surface interactions potentials has been described in detail elsewhere [6]. A brief summary of our potential model will be presented below.…”

A Monte Carlo simulation study of Nitrogen on LiF(001)

“…One of the interesting systems that widely investigated is the N 2 /NaCl [2] system. Previous Monte Carlo simulations of the adsorbed molecules systems were able to reproduce most of the experimental results [1][2][3], in addition the simulations of the N 2 /NaCl [2] show that this system undergo an order-disorder phase transition with nonuniversal critical exponents and fall in the same universality class as the XY model with cubic anisotropy. The values of the critical exponents depend on the strengths of an anisotropic external potential provided by the substrate (NaCl).…”

“…In the last twenty years, experimental techniques have been improved and made a wealth of information on adsorbate structures at low temperatures, Epitaxial growth and 2D phase transitions, exhibited by molecules adsorbed on solid surfaces [1][2][3][4][5][6][7][8][9][10][11][12][13], available. This was attracting considerable attention for both experimentalists and theoreticians.…”

Monte Carlo Simulations of Adsorbed Molecules on Ionic Surfaces

“…To study the physical properties of a system with a large number of atoms or molecules interacting with each other, MC methods can be readily applied whereby possible configurations of the system can be sampled according to their Boltzmann probability distribution via the use of random numbers [1] [3] [11] [12]. Metropolis Monte Carlo simulations have been performed to study the structures and phase transitions of adsorbed molecules on solid surfaces such as HBr/LiF(001) [13], CO 2 /NaCl [14], CO/NaCl [15] , CO/LiF [16], CO/MgO [17], N 2 /NaCl [18], N 2 /LiF [19], H 2 /NaCl [20], D 2 /MgO [21], H 2 /LiF [22,23]. They have also been used to study critical phenomena near their transition temperatures for many models such as the Ising, XY, and Heisenberg models [12].…”

“…Monte Carlo simulations of CO [17] 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 as shown in Fig. 8: a perpendicular site and a tilted site (polar angle of 31°).…”