Complex Hydrogen Substructure in Semimetallic RuH.

When compressed in a matrix of solid hydrogen, many metals form compounds with increasingly high hydrogen contents. At high density, hydrogenic sublattices can emerge, which may act as low-dimensional analogues of atomic hydrogen. We show that at high pressures and temperatures, ruthenium forms polyhydride species that exhibit intriguing hydrogen substructures with counterintuitive electronic properties. RuH is synthesized from RuH in H at 50 GPa and at temperatures in excess of 1000 K, adopting a cubic structure with short H-H distances. When synthesis pressures are increased above 85 GPa, we observe RuH which crystallizes in a remarkable structure containing corner-sharing H octahedra. Calculations indicate this phase is semimetallic at 100 GPa.