Pokrovsky-Talapov commensurate-incommensurate transition in the CO/Pd(100) system

X-ray-diffraction measurements have been carried out for a monolayer of CO on the Pd(100) surface. Because of equilibrium with the gas phase, at constant pressure a series of structures is formed with varying temperature (varying coverage). One structure at 350 K selected for crystallographic analysis, is found to be a commensurate lattice with p2gg symmetry of CO molecules bound in substrate-bridge sites. The vibration amplitudes of the molecules are substantially larger in-plane than out-of-plane. Upon cooling, a phase transition is crossed beyond which the structure becomes incommensurate. Symmetry considerations and the measured exponent of 0.5+/-0.05 establish this transition to be in the Pokrovsky-Talapov universality class.