The ordered overlayer structures formed by Cs adsorbed on a Ru (0001) surface were analyzed by use of low-energy electron diffraction (LEED). The phase diagram reflects the dominance of dipole-dipole repulsions between the adparticles and comprises quasiliquid configurations characterized by diffraction rings up to a coverage =0. 17, followed by a (22) structure with maximum intensity of the diffraction spots at =0. 23. Beyond =0. 25, a series of structures with rotated unit cells is identified which are followed by a (3 3) R30^ structure around =0. 33 (of the first monolayer). In the (22) phase the Cs atoms are located in on-top sites with a Ru-Cs bond length of 3. 250. 08, corresponding to a hard-sphere radius of 1. 9 for the Cs atom. In the (3 3) R30^ structure, on the other hand, the adatoms occupy threefold hollow hcp sites with Ru-Cs bond lengths of 3. 520. 02, corresponding to a Cs hard-sphere radius of about 2. 2. The increase in bond length and effective radius of the adparticle is paralleled by the transition of the character of bonding from more ``ionic'' at =0. 25 (large dipole moment) to more ``metallic'' at =0. 33 (dipole moment reduced by about 30%). The associated change of the type of adsorption site (from on-top to hollow) is qualitatively rationalized by a model according to which inherently less favorable sites may become preferred due to improved effective screening of the dipole-dipole repulsion by the location of substrate atoms in the region between neighboring adatoms.
Over et al. (Wed,) studied this question.
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