Determination of (√5 × √5)R26.7° structures formed on Cu(0 0 1) by coadsorption of Bi and K(Cs): On-top site adsorption of K(Cs)

By using low-energy electron diffraction (LEED), we observed a (√ 5× √5)R26.7° pattern in the coadsorption of Bi and K (or Cs) on Cu(001) at room temperature. Both (√5×√5)R26.7°-Bi,K and -Bi,Cs structures were determined by a tensor LEED analysis. The two structures are ordered mixed phases and similar to each other: Bi atoms are located at surface fourfold hollow sites and K (or Cs) atoms occupy on-top sites of the substrate surface Cu atoms. Significant rumpling of 0.13 Å (-Bi,K) or 0.19 Å (-Bi,Cs) is found in the top Cu layer. It is suggested that the on-top site occupation of K or Cs atoms is stabilized by additional bonds with surrounding Bi atoms. The Hamilton-ratio tests suggest that K atoms displace laterally from the exact on-top position, but that Cs atoms are upright on the on-top sites.