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

Low-energy-electron-diffraction intensity measurements and multiple scattering analysis are used to determine the multilayer surface relaxation of clean and hydrogen-dosed Nb(100) as a function of temperature. Accurate characterization of residual surface impurity concentration (oxygen) based on Auger electron spectroscopy is used to obtain a meaningful extrapolation of the first-layer relaxation to the clean surface value: d₁₂=1.481±.05 Å corresponding to Δ₁₂=−10±3%, a 10% relaxation relative to the bulk value d₀=1.645 Å. This experimental result for d₁₂ can be used to judge the accuracy of recent ab initio calculations for Nb(100). Temperature-dependent changes in surface relaxation resulting from hydrogen dosing of Nb(100) 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 (Δ∼4.5%) that occurs when Nb is hydrated to form βNbH. The observed relaxation of the hydrogen-dosed near-surface interplanar separation to the clean surface value for T>400 K is consistent with the subsurface “hydrogen valve” model that has been used to account for unusual hydrogen uptake kinetics associated with Nb(100).