Abstract
For many chemisorption systems several different substrate binding sites (e.g., on-top 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 favour occupation of the sites with the lowest vibrational frequencies. In this letter it is argued 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(1 1 1).
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