XANES study of hydrogen incorporation in a Pd-capped Nb thin film

Abstract

X-ray absorption near-edge structure (XANES) measurements were used to probe the H-charging-induced electronic structure changes of a 2400 \AA{} Nb film capped with Pd. These results are discussed in terms of ab initio linear augmented plane-wave (LAPW) band-structure calculations for this material. The $\mathrm{P}\mathrm{d}\ensuremath{-}{L}_{3}$-edge XANES clearly manifested the spectral ($\mathrm{P}\mathrm{d}\ensuremath{-}d$ state related) changes expected for Pd-hydride formation, a white line feature degradation, and the appearance of a Pd-H antibonding feature at 6 eV above the threshold. The $\mathrm{N}\mathrm{b}\ensuremath{-}{L}_{2,3}$ edge changes with H charging show a distinct enhancement of the white line strength; a feature 6 eV above the edges, associated with Nb-H antibonding states in analogy with the Pd results; the suppression of a threshold-onset feature of Nb metal; and a shift of the centrum of the white line feature towards the threshold. Comparison of the Nb sphere projection of the ${d}_{3/2}$ component of the LAPW density of states (DOS) to the $\mathrm{N}\mathrm{b}\ensuremath{-}{L}_{2}$-edge spectra yields good basic agreement with the observed spectral changes. In particular, the substantial theoretical reduction in the DOS at, and just above, the Fermi energy ${(E}_{f})$ is directly related to the near threshold $\mathrm{N}\mathrm{b}\ensuremath{-}{L}_{2,3}$ spectral changes. The more modest white line enhancement in the theoretical DOS is noted and discussed. $\mathrm{N}\mathrm{b}\ensuremath{-}K$-edge XANES are also discussed in terms of the Nb-site $p$-state projected LAPW DOS. This last comparison indicates a $p$-state reduction near ${E}_{f}$ upon H charging of the Nb.