The effect of nano-confinement on hydrogen uptake in metallic superlattices

Sammanfattning: The absorption of hydrogen is exothermic in vanadium whereas it is endothermic in iron and chromium. Investigations of the hydrogen uptake within Fe/V(001) and Cr/V(001) superlattices allow therefore a detailed exploration of finite size effects and the influence of boundaries on hydrogen absorption. Fe/V(001) and Cr/V(001) superlattices can be grown as single crystal structures with a small mosaic spread, as determined by X-ray reflectometry and diffraction. Furthermore when the thickness ratio of the constituents is kept constant the crystal quality can be retained in the range from a few up to 40 monolayer repeat distances (Λ). Neutron reflectometry was used to simultaneously determine the volume expansion and concentration of hydrogen in the vanadium layers. Large differences are found in the expansion of Fe/V(001) and Cr/V(001) superlattices, in good agreement with density functional theory (DFT) calculations. The findings are consistent with tetrahedral and octahedral site occupancy in Cr/V(001) and Fe/V(001) superlattices, respectively. Full fitting of the reflectivity pattern is required to obtainan accurate measure of expansion if the number of repeats is small. Under these conditions, the shift of the first order superlattice peak can be an inaccurate measure of the volume changes. By using a specially designed neutron scattering chamber, allowing simultaneous neutron and optical transmission measurements, it is found that the optical transmission scales linearly with hydrogen concentration. By comparing the experimental results to ab-initio DFT calculations, it is shown that optical transmission scales with electron density changes in the samples, explaining the linearity with concentration. This change is dominated by the hydrogen induced expansion of the lattices and depends therefore strongly on the site occupancy of the hydrogen. Finally, X-ray diffraction was used to address the local strain fields and the α to β phase transition, typically observed in bulk vanadium. Below 448 K the results are consistent with an α to β phase co-existence, separated along the surface normal of the samples.