The experimental study of the thermodynamic and kinetic properties of the $\mathrm{S}\mathrm{b}/\mathrm{S}\mathrm{i}(111)$ interface reveals a surprising behavior: a 2D phase condensates when the Sb coverage increases, indicating strong attractive Sb-Sb interactions, whereas the isotherms present a quasi-Langmuir shape, suggesting that these interactions should be negligible. Ab initio calculations raise this contradiction: while the adsorption site evolves from ternary towards the on-top position with increasing coverage, the character of the Sb-Sb effective interactions changes from repulsive towards attractive, resulting in an almost constant average adsorption energy. A simple (Langmuir) thermodynamic behavior can then be the consequence of a surface phase transition.

• Received 30 July 2004

DOI:https://doi.org/10.1103/PhysRevLett.94.076101