Hybrid surface structures formed on Cu(001) and Ag(001) by alkali-metal adsorption

We have determined the surface structure sequence 2 × 1 → 3 × 3 → 4 × 4 formed on Cu(001) with increasing Li coverage at 300 K by using low-energy electron diffraction analysis. In the 2 × 1 structure all Li atoms occupy substitutional sites, whereas the 3 × 3 and 4 × 4 structure similar to each other contain both substitutional Li atoms and Li adatoms. It is concluded that the surface accommodates additional Li atoms at the cost of the average adsorption energy in the sequence above. Another sequence 2 × 1 → 3 × 3 is found for Na adsorption on Ag(001) at room temperature. The 3 × 3 surface structure is considered to be the same one formed in Cu(001)-Li. Different sequence 2 × 1 → 3 × 1 is observed for K adsorption on Ag(001). Structure models for the 3 × 1 is proposed. We conclude that the 3 × 3 and 3 × 1 structures following the same 2 × 1 structure are formed basically due to the same cause and that different structures are realized owing to the size effect.