Temperature-dependent multilayer relaxation of clean and hydrogen-dosed Nb(100)

Abstract

Low-energy-electron-diffraction intensity measurements and multiple scattering analysis are used to determine the multilayer surface relaxation of clean and hydrogen-dosed Nb͑100͒ as a function of temperature. Accurate characterization of residual surface impurity concentration ͑oxygen͒ based on Auger electron spectroscopy is used to obtain a meaningful extrapolation of the first-layer relaxation to the clean surface value: d 12 = 1.481± .05 Å corresponding to ⌬ 12 = −10± 3%, a 10% relaxation relative to the bulk value d 0 = 1.645 Å. This experimental result for d 12 can be used to judge the accuracy of recent ab initio calculations for Nb͑100͒. Temperature-dependent changes in surface relaxation resulting from hydrogen dosing of Nb͑100͒ manifest an expansion of the near-surface lattice resulting from subsurface hydrogen atoms. The hydrogen-induced expansion of near-surface interplanar separation is determined to be 3 ± 1% at T =125 K, 4±1% at T = 300 K, and −1±1% at T = 400 K. The measured hydrogen-induced surface lattice expansion is consistent with the bulk lattice constant change ͑⌬ ϳ 4.5% ͒ that occurs when Nb is hydrated to form ␤NbH. The observed relaxation of the hydrogen-dosed near-surface interplanar separation to the clean surface value for T Ͼ 400 K is consistent with the subsurface "hydrogen valve" model that has been used to account for unusual hydrogen uptake kinetics associated with Nb͑100͒.

First-Principles Study on the Initial Oxidization of a Nb(100) Surface

“…Two atomic layers at the bottom are fixed to mimic bulk, while the remaining upper layers are optimized. It is found that the structure relaxation mainly locates at the upper seven atomic layers for the clean Nb(100) surface, in agreement with previous results. , The four-coordinated hollow site (H 4 ), the distorted four-coordinated hollow site (H 4d ), and the bridge site (B) are taken into account for the on-surface adsorption of O atoms (Figure ). A 5 × 5 × 1 Monkhorst-Pack grid is used for the Brillouin-zone sampling .…”