An x-ray study of palladium hydrides up to 100 GPa: Synthesis and isotopic effects

The stable forms of palladium hydrides up to 100GPa were investigated using the direct reaction of palladium with hydrogen (deuterium) in a laser-heated diamond anvil cell. The structure and volume of PdH(D)x were measured using synchrotron x-ray diffraction. The Pd atoms remain on a fcc lattice. The stoichiometry of the hydride is inferred from the volume expansion due to the hydrogen solubility in the Pd lattice. No evidence for hydrogen to palladium ratio greater than 1 is observed for both isotopes. An inverse isotope effect on the formation enthalpy of the stoichiometric Pd hydride is disclosed by measuring the equilibrium formation pressure of PdH and of PdD, 1.9GPa and 2.7GPa, respectively. An isotopic shift between the compression curves of PdH and of PdD is also measured, in good agreement with ab initio calculations quantifying the contribution of the hydrogen zero-point vibrational energy.