Ordered phases and phase transitions of cesium on the Cu(111) surface

Low‐energy electron diffraction (LEED) experiments have been carried out to study the ordered phases and phase transitions of cesium on the Cu(111) surface in the temperature region between 80 and 450 K. The cesium on the 80‐K surface forms an orientationally ordered incommensurate structure (IS) at coverage θ>0.12. The overlayer lattice parameter of the IS phase decreases continuously with coverage until a Cs–Cs spacing 7% smaller than in bulk cesium is reached. By increasing the temperature, the IS phase undergoes several transitions as a function of coverage: (i) a better ordered phase at 135 K, (ii) a disordered phase through an intermediate phase (‘‘hexatic’’ phase) at θ<0.21 or without the intermediate phase at θ>0.21, and (iii) a (2×2) phase at θ>0.25 and T≊260 K and a (6×6) phase at T≊355 K. The cesium on the surface also forms an antisymmetrical metallic domain phase and a coexisting phase of ( (4)/(3) (3)^1/2× (4)/(3) (3)^1/2)‐R30° and (2×2).