Adsorption-site mixing at a continuous order-disorder phase transition

The temperature-dependent probability of occupation of the two threefold-coordinated adsorption sites was determined for the system S/Ru(0001) at a coverage of 0.33, analyzing energy-dependent intensity curves [I(E) curves] in low-energy electron diffraction of the ordered {}3 ×{} {}3 R30^∘{} phase and of the disordered system. We show that this probability agrees fully with a lattice gas model containing a finite difference in adsorption energy between these sites.