The order-disorder phase transition of the Rh(lll)-(root 3 x root 3)R30 degrees-CO phase was investigated by He scattering. The critical temperature turned out to be 330+/-5 K, while the critical exponents are consistent with a phase transition belonging to the 3-state Potts universality class. The temperature dependence below the critical temperature of both CO phases was studied using elastic He scattering and LEED. These measurements demonstrate that a simple Debye-Waller evaluation of the diffraction data is not appropriate to provide information on the vibrational properties of the CO molecules. From LEED measurements at low temperatures (70 K) and various CO partial pressures there is evidence of the formation of a disordered second CO layer in equilibrium with the gas phase. The isosteric heat of CO adsorption in the second layer is about 6 kJ mol(-1).
