Monte Carlo calculations of adsorbate structures and the role of the vibrational entropy in phase transitions at surfaces

Abstract

I have used the Monte Carlo method to study the ordered structures of CO on Pt(111). The calculations are based on a recently constructed potential energy surface for CO on Pt(111) and on CO-CO interaction potential deduced from the variation of the CO binding energy with coverage. The results are consistent with most known experimental facts and give additional information about adsorbate structures not directly available from experimental data. For many chemisorption systems several different substrate binding sites (e.g., atop and bridge sites) can be occupied simultaneously. The resonance frequencies of the low-frequency adsorbate vibrational modes (frustrated translations and rotations) may differ strongly between the different symmetry sites, and this introduces an important vibrational entropy term in the free energy for the adsorbate system, which at high temperature tends to favor occupation of the sites with the lowest vibrational frequencies. I show that this is a strong driving force for phase transitions in many adsorbate systems and in particular for the order-disorder transition of CO on Pt(111).