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.

Step effects on diffusion near a substrate reconstructive phase transition: H on W(100)

“…The H coverage was calibrated by thermal desorption spectroscopy ͑TDS͒ on the flat surface. Because the W atoms are known to be immobile at temperatures below 200 K, 10 the sample was slowly heated to room temperature and held for about 2 min to equilibrate the surface. It was known from other studies that hydrogen adsorption is dissociative and that hydrogen atoms adsorb on the bridge sites between W atoms at all coverages.…”

“…When the hydrogen coverage exceeds about 0.12 ML, a change of the c(2ϫ2) structure symmetry from p2mg of the clean surface to c2mm is induced. 10,13,14,23 From this change, it was deduced that hydrogen causes W atom displacements to switch from along the ͗110͘ surface directions to along the ͗100͘ surface directions 10,14 to form a dimerlike structure. Again, there are two types of domains that are related by a rotation of 90°about the surface normal.…”

“…However, the degeneracy of the two types of domains can be lifted by properly orienting and spacing surface steps. 10,14,[23][24][25][26] For example, a domain ratio of about 30:1 was reported for the p2mg structure on the clean surface as a result of miscutting the surface along the ͓110͔ direction by 3.25°, which resulted in a terrace width about 28 Å. 23,27 The atomic displacements in the p2mg reconstructed structure are preferentially perpendicular to the step edges for this miscut.…”

“…23,25 For the c2mm structure, miscutting a surface along the ͓100͔ direction, which results in steps along the surface ͓010͔ direction, can break the degeneracy and results in preferential domains with displacements of the W atoms along the step edge. 10,13,14,26 A second purpose of our experiment will be to study the H diffusion anisotropy on single-type domains that are obtained on a stepped W͑100͒ surface.…”

“…In contrast, the nature of the substrate reconstructive phase transition of the W͑100͒ surface is well understood. [7][8][9][10][11][12][13][14][15][16] It provides an ideal model system to investigate the effect of the substrate phase transition on surface diffusion. As reported in a recent Letter, 17 the H diffusion anomaly near the reversible phase transition of W͑100͒ from a (1ϫ1) disordered phase at high temperature to a c(2 ϫ2) phase at low temperature was clearly demonstrated by experiment, in qualitative agreement with the prediction of an earlier theoretical work.…”