The osmium-hydrogen system was studied at pressures up to 186 GPa by in situ synchrotron x-ray diffraction in a diamond anvil cell. Hydrogen solubility in the hexagonal close-packed (hcp) osmium metal at room temperature was found to be negligibly small in the studied pressure range. After laser heating of the osmium sample at the maximal H2 pressure, it transformed to hydride with a face-centered cubic (fcc) metal lattice. The equation of state V(P) for this hydride was then measured at room temperature and decreasing pressure. The hydrogen-induced volume expansion of the Os lattice proved to be weakly dependent on the pressure, and its estimated value of 1.35Å3/Os atom at 100 GPa suggested the formation of monohydride OsH with hydrogen atoms occupying all octahedral interstices in its fcc metal lattice. The OsH sample began to gradually lose hydrogen at pressures below about 55 GPa and completely decomposed to hcp-Os and molecular H2 at about 25 GPa.

Mikhail A. Kuzovnikov, Vasily S. Minkov, Stella Chariton, Vitali B. Prakapenka, Mikhail I. Eremets, Synthesis of osmium hydride under high hydrogen pressure, (2020) Phys. Rev. B, Vol. 102, No. 21, 214109, abstract

Periodic table of hydrides. The cell color for each element indicates the composition of its highest hydride, which is stable at some pressure and temperature. Thick black rectangles outline elements, which highest hydride is stable at high pressure only. The compositions of nonstoichiometric hydrides are rounded off. Hydrides containing molecular H2 units are not shown.