Deuterium in 001-oriented Mo0.5V0.5/V: Density profile on the atomic level

We have investigated the deuterium density profile in a Mo0.5V0.5/V superlattice along the direction parallel to the surface normal, using neutron reflectivity. To increase the sensitivity to changes in the deuterium density profile, in particular at the interfaces, we have carefully selected the thickness ratio of the constituents. Furthermore, the alloy composition of the Mo0.5V0.5 layer was chosen so that no depletion zone of hydrogen (deuterium) is expected at the interface in the low concentration limit. The neutron reflectivity results yield, however, a clear evidence of a depletion zone at the interfaces, with an extension of at least one monolayer. The appearance of a depletion layer is inferred to originate from the H-H interaction. Finally, by simultaneously monitoring the expansion and the concentration, we determined the elastic response of the V layers, and we prove the presence of nonlinear elastic response associated to a change of site occupancy