Surface structures and phase diagram for the H/W(001) chemisorption system

Abstract

The chemisorption of hydrogen on tungsten (001) induces displacive rearrangements (’’reconstruction’’) of the substrate surface, depending on the temperature T and the adsorbate coverage ϑ. By means of LEED and other techniques, the surface phase diagram in the T–ϑ plane (ϑ<0.7, T<500 K) has been studied. The predominant structure is (√2×√2) which is stabilized by hydrogen in the region ϑ<0.3, but is transformed to an incommensurate structure at higher coverages. Adsorption below ?200 K produces no ordered phase. The LEED patterns from a terraced surface exhibit reduced symmetry and reveal the direction of the lattice distortion; the atomic displacements are found to be along 〈10〉 for the H-induced structures, in contrast to the 〈11〉 displacements on clean W(001). The substrate rearrangement is believed to be a major cause of the observed coverage dependence of the desorption energy and other adsorbate properties.