Electron Ptychography Images Hydrogen Atom Superlattices and 3D Inhomogeneities in Palladium Hydride Nanoparticles

When hydrogen atoms occupy interstitial sites in metal lattices, they form metal hydrides (MH_x), whose structural and electronic properties can differ significantly from those of the host metals. Determining where the hydrogen is located within the MH_x is crucial for predicting and understanding the resultant unique physical and electronic properties of the hydride. Yet, directly imaging hydrogen within a host material remains a major challenge due to its weak signal in conventional X-ray and electron imaging techniques. Here, we employ electron ptychography, a scanning transmission electron microscopy (STEM) technique, to image the three-dimensional (3D) distribution of H atoms in palladium hydride (PdH_x) nanocubes, one of the most studied and industrially relevant MH_x materials. We observe an unexpected one-dimensional superlattice ordering of hydrogen within the PdH_x nanocubes and 3D hydrogen clustering in localized regions within the PdH_x nanocubes, revealing spatial heterogeneity in metal hydride nanoparticles previously inaccessible by other methods.