Cesium adsorption on graphite (0001) surface: The phase diagram

Low-energy electron diffraction (LEED) has been used to study the two-dimensional structure of cesium on the graphite (0001) surface. Six different superstructures have been observed under cesium adsorption: (√7 × √7 )-R19°, 2×$2^{\mathrm{*}}$, 2×2, (√3 × √3 )-R30°, incommensurate hexagonal and disordered. All but the latter have hexagonal symmetry. The distance between the near-neighbor cesium adatoms in the hexagonally ordered structures decreases from ∼19 Å for the lowest-exposure incommensurate hexagonal structure measured, to 4.26 Å for the √3 × √3 -R30° structure. The √3 × √3 -R30° structure is the most stable structure existing over a coverage range of 0.1 to 0.2 monolayers and a temperature range of 80 to 350 K. A bcc metallic Cs phase has also been observed at high Cs coverages. A limited phase diagram has been generated for cesium adsorption on the (0001) surface of graphite from the LEED data.