  Knowledge of the behavior of hydrogen in metal hydrides is the key for
understanding their electronic properties. So far, no experimental methods
exist to access these properties beyond 100 GPa, where high-Tc
superconductivity emerges. Here, we present an 1H-NMR study of cubic FeH up to
200GPa. We observe a distinct deviation from the ideal metallic behavior
between 64 and 110 GPa that suggests pressure-induced H-H interactions.
Accompanying ab-initio calculations support this interpretation, as they reveal
the formation of an intercalating sublattice of electron density, which
enhances the hydrogen contribution to the electronic density of states at the
Fermi level. This study shows that pressure induced H-H interactions can occur
in metal hydrides at much lower compression and larger H-H distances than
previously thought and stimulates an alternative pathway in the search for
novel high-temperature superconductors.
