Complex Hydrogen Substructure in Semimetallic RuH 4

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. Ru H 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.