Extended Data Fig. 2: H occupancy in finite-sized PdH_x nanoparticles with different crystal packing.

a–f, Partitioning of H atoms in PdH nanoparticles with different particle sizes and crystal packing with these H atoms occupying various Pd interstitial sites in nanoparticles with high H content (x = 1) (a–c) and low H content (x = 0.2) (d–f), respectively. The H sites are categorized by the number of coordinating Pd atoms, where surface (a, d), tetrahedral (b, e), or highly occupied (trigonal bipyramidal/octahedral) (c, f) represent 0–3, 4, or 5–6 Pd atoms within the coordinating distance from the H atom, respectively. g, h, Representative coordination structure of PdH showing H atoms in the octahedral (white) and tetrahedral (green) interstitial sites formed by fcc (g) and hcp (h) crystal packing. Pd atoms are shown as large spheres, and the colors represent different Pd layers. The 1-4-layer overlap (ABCA’) in the fcc Pd structure provides extra stabilization for octahedral H atoms, while the 1-3-layer overlap (ABA’) in the hcp Pd structure provides extra stabilization to tetrahedral (T) and trigonal bipyramidal (TB) H atoms, resulting in distinct H atom occupancy, especially in small nanoparticles. At a diameter of 1.2 nm, almost half of the H atoms reside on the surface, regardless of the crystal packing.