PHASE-DIAGRAM AND OVERLAYER STRUCTURES FOR CO ON RH(110)

Low energy electron diffraction (LEED) patterns have been recorded for CO on Rh(110) and analyzed as a function of sample temperature or CO dose using a video-LEED system. The results provide a diagram of boundaries and existence regions for overlayer phases. The overlayer is a disordered lattice gas at absolute CO coverages (theta(CO)) below about 0.08 for all temperatures. A pattern characterized by splitting of c(2 x 2) spots appears most intense for theta(CO) almost-equal-to 0.3-0.6 below 180 K. The ''split'' c(2 x 2) pattern converts to a pattern with diffuse, broad c(2 x 2) spots above 180 K. Overlapping diffraction features from ''(3 x 2)'', ''(4 x 2)'', and ''(5 x 2)'' symmetries are observed for theta(CO) above about 0.62 and below 240 K. The ''(5 x 2)'' predominates by theta(CO) almost-equal-to 0.8. With further increase in theta(CO), (2 x 1) LEED spots appear with the ''(5 x 2)'' features. A pure (2 x 1) pattern appears at monolayer theta(CO). Overlayer structural models with CO molecules in short-bridge sites (off-top models) or tilted from on-top adsorption sites (tilt models) are proposed to account for features in the LEED patterns. Off-top models giving rise to pseudo-hexagonal ordering are favored for the ''split'' c(2 x 2) overlayer. The ''c(2 x 2)'' overlayer contains uniformly sized domains with ideal c(2 x 2) ordering and disordered domain walls. Increasing 3-, 4-, and 5-fold symmetry in off-top adlayers arises from uniform, commensurate compression along [110]. In tilt overlayer models by comparison, the ''(n x 2)'' structures contain CO molecules in rows with local (2 x 1) symmetry separated by 3, 4, or 5 unit cell spacings. A zig-zag tilt model for reconstruction in the (2 x 1) CO phase is proposed.