Metallic superlattices: quasi two-dimensional playground for hydrogen

Abstract

Hydrogen in superlattices, whose constituents have different hydrogen affinities, comprises a new class of hydrogen absorbing materials. The period of the absorption potential can be made as small as two monolayers, which implies highly anisotropic host mediated hydrogen–hydrogen (H–H) interactions. As the correlation between the hydrogen atoms can be influenced in an almost arbitrary way, it is possible to create an ensemble whose nature is neither three- nor two-dimensional, but something in between. By changing the strain state of the constituents, the interaction between the hydrogen atoms in vanadium can be changed from being attractive (for biaxial tensile strain) to repulsive (for biaxial compressive strain). This has been observed experimentally for, e.g., 2 nm vanadium layers in Mo–V and Fe–V superlattices. The biaxial strain state is shown to have a gigantic effect on the hydrogen induced lattice expansion. By changing the strain state from tensile to compressive, in ≈2 nm V layers, the hydrogen induced lattice expansion is altered by a factor of ten. This feature is expected to affect the diffusion properties of hydrogen in these structures dramatically. The principal orientation of the crystal axis of the host material, with respect to the boundary planes, is found to have large impact on the host mediated H–H interactions. The theoretical understanding of hydrogen in quasi-two-dimensional potentials is still rudimentary, and more theoretical and experimental work is required for a detailed understanding of these problems. In the present work, we will treat the implications of the extension and amplitude of the hydrogen absorption potential, and we will demonstrate the existence and impact of weak and strong modulation of the host potential on the hydrogen uptake. The influence of the strain state on the hydrogen absorption and the host mediated H–H interactions will also be treated, and we will show that the phase diagram can, in some cases, be altered in a controllable way.