Thanks Jess.
The abstract of the associated article says:
"Accompanying...simulations are in excellent agreement with experiment and reveal the structure to consist of an hexagonal close packed gold lattice with atomic hydrogen disordered in the interstices."
The capacity to occupy interstices is a property shared by the seven SeSPONCH nonmetals.
The compounds involved include interstitial hydrides (e.g. VH0.05; NbH0.11; TaH0.22), and refractory carbides, nitrides (TiN; ZrN), oxides (ThO2; m.p. 3300 °C), phosphides (PuP) and sulfides (Ce2S3; ~ 2000 °C). Here, selenium is a borderline interstitial-compound-former.
René