Static and dynamic lattice changes induced by hydrogen adsorption on NiAl(110)

Abstract

Static and dynamic changes induced by adsorption of atomic hydrogen on the NiAl(110) lattice at 130 K have been examined as a function of adsorbate coverage.Adsorbed hydrogen exists in three distinct phases. At low coverages the hydrogen is itinerant because of quantum tunneling between sites and exhibits no observable vibrational modes. Between 0.4 ML and 0.6 ML, substrate mediated interactions produce an ordered superstructure with c(2×2) symmetry, and at higher coverages, hydrogen exists as a disordered lattice gas. This picture of how hydrogen interacts with NiAl(110) is developed from our data and compared to current theoretical predictions.

First principles theory of artificial metal chains on NiAl(110) surface

“…45,46 A large amount of work has also been devoted to study the adsorption of atomic and molecular hydrogen on NiAl͑110͒ at different coverages. [47][48][49][50] In those cases H was not constrained to form 1D chains but was uniformly distributed onto the surfaces. Ad-structures of Ni and Al may be present as defects or steps on the NiAl͑110͒ surface, but generally do not form ordered chains.…”

Structure and dynamics of atomic hydrogen on NiAl(110)

“…A consequence of the atomic arrangement used is that only infinite-wavelength surface vibrations, i.e., ⌫-point phonons, can be studied. This is sufficient for the present purpose because the experimentally measured spectra 8,11 have also been collected at the ⌫ point and have been shown to exhibit only a minor dispersion in the surface unit cell. The height of the unit cell was taken to be 16 Å (28 Å in calculations using the nine-layer slab͒.…”

“…However, it fails completely at a description of the hydrogen induced mode experimentally found at 49.5 meV. In an effort to reconcile the theoretical and experimental findings Hanbicki et al 8,11 drew analogies with the behavior found for hydrogen over low-index Ni surfaces. 13 In this system a strongly anharmonic coupling parallel and perpendicular to the surface is operational.…”

“…[8][9][10][11] A very thorough study of hydrogen adsorption induced changes of static and dynamical properties of the NiAl͑110͒ surface exists. 8,11 The model which emerged from those studies was that hydrogen behaves as an effective potential distributed homogeneously across the surface. Consequently, increased hydrogen coverage results in monotonic changes in all properties.…”

“…10 It is well known that surface atoms can exhibit large anharmonicities in their motion along the surface plane. In such a case the dynamics cannot be studied using harmonic or quasiharmonic treatment, the basis of the hypothesis of Hanbicki et al 8,11 The potential-energy surface of anharmonic systems is much more complicated than in a harmonic case. Static calculations and parametrization of an anharmonic system to obtain quantities of interest, such as vibrational frequencies or anharmonic shifts, may become impractical.…”