Muon spin relaxation reveals the hydrogen storage mechanism in light alkali metal fullerides

We report a muon spin relaxation investigation of Li6C60 and Na10C60 fullerides, which have been recently demonstrated to be efficient and reversible H2 absorbers above 570 K. We prove that, differently from other fullerides, a sizeable fraction of implanted muons form C60 muonium adduct radicals, with hyperfine coupling depending on the C60 hydrogen coverage. Surprisingly, the fraction of radicals was found to increase up to 65% when lowering T to 5 K in Na10C60Hy. This indicates that hydrogen interaction in these systems is enhanced even at cryogenic temperatures, while the high T needed for hydrogen absorption is only required to overcome the H2 dissociation barrier mediated by alkali metals.