  Understanding diffusion of large solutes such as hydrogen and lithium in
solids is of paramount importance for energy storage in metal hydrides and
advanced batteries. Due to its high gravimetric and volumetric densities,
magnesium is a material of great potential for solid-state hydrogen storage.
However, the slow hydrogen diffusion kinetics and the deleterious blocking
effect in magnesium have hampered its practical applications. Here, we
demonstrate fast lateral hydrogen diffusion in quasifree magnesium films
without the blocking effect. Massive concomitant lattice expansion leads to the
formation of remarkable self-organized finger patterns extending over tens of
micrometers. Detailed visualization of diffusion fronts reveals that the
fingers in these patterns follow locally the direction of hydrogen diffusion.
Thus, the streamlines of the diffusion process are self-recorded by means of
the finger pattern. By inclusion of fast hydrogen diffusion objects or local
gaps, the resulting streamlines exhibit a clear analogy to optical rays in
geometric optics. The possibility to spatially manipulate hydrogen diffusion
opens an avenue to build advanced hydrogen storage systems, cloaking and active
plasmonic devices, as well as prototype systems for computational models.
