Influence of deuterium-induced volume changes on optical transmission in Fe/V (001) and Cr/V (001) superlattices

Abstract

The deuterium-induced changes of the optical transmission in Fe/V (001) and Cr/V (001) superlattices are found experimentally to be dominated by the volume changes of the vanadium layers and thus indirectly linked to concentration. The deuterium-induced expansion is 67% larger in Cr/V 2/14 monolayers (ML) as compared to Fe/V 2/14 ML. This large difference can be explained by a difference in the site of deuterium from tetrahedral in Fe/V to octahedral in Cr/V. First-principles calculations based on this assumption give quantitative agreement with both the measured optical transmission and the deuterium-induced expansion coefficient. Placing hydrogen in the middle of the vanadium layers results in total energies at 0 K that favor tetrahedral occupancy at low concentrations, although the energy difference is of the order of the thermal energy available in the experiments. Hence small changes in strain, defect concentration, and/or vibrational spectrum of the superlattices may tip the balance to octahedral occupancy at low concentrations. Given this link to concentration and the linear scaling, optical transmission can, therefore, be used in a straightforward way to obtain pressure-composition isotherms also in thin metal films that do not undergo metal-insulator transitions upon hydrogenation.