POKROVSKY-TALAPOV COMMENSURATE-INCOMMENSURATE TRANSITION IN THE CO PD(100) SYSTEM/

X-ray-diffraction measurements have been carried out for a monolayer o f CO on the Pd(100) surface. Because of equilibrium with the gas phase , at constant pressure a series of structures is formed with varying t emperature (varying coverage). One structure at 350 K selected for cry stallographic analysis, is found to be a commensurate lattice with p2g g symmetry of CO molecules bound in substrate-bridge sites. The vibrat ion amplitudes of the molecules are substantially larger in-plane than out-of-plane. Upon cooling, a phase transition is crossed beyond whic h the structure becomes incommensurate. Symmetry considerations and th e measured exponent of 0.5+/-0.05 establish this transition to be in t he Pokrovsky-Talapov universality class.