Phase Transitions in Chemisorbed Hydrogen Layers

A brief survey is given about hydrogen‐transition metal systems which exhibit adsorbed hydrogen phases with long‐range order that can undergo order‐order and order‐disorder transitions depending upon the surface coverage θHand the temperature, respectively. A distinction has to be made between genuine lattice gas phases in which the H atoms simply occupy lattice sites with identical local symmetry without affecting the positions of the underlying metal atoms and those chemisorbed H phases which involve a distortion of the surface geometry of the substrate leading to the well‐known surface reconstruction. The systems H/Ni(111), H/Pd(111), H/Pd(100) and H/Fe(110) belong to the first category whereas H/Ni(110) and H/Pd(110) represent examples of the latter category, i.e., that involves surface reconstructed phases. As far as they have been determined the phase diagrams of the ordered H phases will be described and interpreted in terms of the mutual H‐H interactions.