Dihydride formation in the palladium-rhodium alloys under high hydrogen pressure

Palladium, rhodium and their alloys were studied in a diamond anvil cell in a high pressure hydrogen atmosphere up to 20 GPa by X-ray diffraction at room temperature. Formation of a monohydride with H/(Rh + Pd) = 1 ratio was observed for each alloy at hydrogen pressures less than 3 GPa. The corresponding volume expansion of the metal lattice was 2.2-2.8 Å3 per metal atom. A formation of the dihydride with H/(Rh + Pd) = 2 was observed at a hydrogen pressure around 10 GPa for the alloys with rhodium content Rh/(Rh + Pd) ≥ 50 at.%. The dihydride formation is accompanied by a volume expansion of metal lattice by 3.2-3.9 Å3 per metal atom. For Pd-rich alloys with Rh/(Rh + Pd) ≤ 25 at.% and pure Pd the dihydride formation was not observed up to maximum reached hydrogen pressure. All observed phases had a fcc structure of metal lattice.