TEMPERATURE BEHAVIOR OF THE (ROOT-3X-ROOT-3)R30-DEGREES-1CO AND THE (2X2)-3CO OVERLAYERS ON RH(111) - A COMBINED HAS AND LEED INVESTIGATION

The order-disorder phase transition of the Rh(lll)-(root 3 x root 3)R3 0 degrees-CO phase was investigated by He scattering. The critical tem perature turned out to be 330+/-5 K, while the critical exponents are consistent with a phase transition belonging to the 3-state Potts univ ersality class. The temperature dependence below the critical temperat ure 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 equilibri um with the gas phase. The isosteric heat of CO adsorption in the seco nd layer is about 6 kJ mol(-1). (C) 1997 Elsevier Science B.V.