  Hydrogen as a fuel can be stored safely with high volumetric density in
metals. It can, however, also be detrimental to metals causing embrittlement.
Understanding fundamental behavior of hydrogen at atomic scale is key to
improve the properties of metal-metal hydride systems. However, currently,
there is no robust technique capable of visualizing hydrogen atoms. Here, we
demonstrate that hydrogen atoms can be imaged unprecedentedly with integrated
differential phase contrast, a recently developed technique performed in a
scanning transmission electron microscope. Images of the titanium-titanium
monohydride interface reveal remarkable stability of the hydride phase,
originating from the interplay between compressive stress and interfacial
coherence. We also uncovered, thirty years after three models were proposed,
which one describes the position of the hydrogen atoms with respect to the
interface. Our work enables novel research on hydrides and is extendable to all
materials containing light and heavy elements, including oxides, nitrides,
carbides and borides.
