Extended Data Fig. 9: Detailed atomic structure and local H concentration map of a PdH_x nanoparticle obtained in the GLC, showing large local fluctuations.

a, Layer-by-layer representation of the traced atomic coordinates and fitted hcp positions for two major domains of the PdH_x nanoparticle shown in Fig. 4. The atoms belonging to two major domains are shown in orange and green. Each domain was separately fitted with a hcp structure, and black dots represent atoms fitted to the hcp positions. Gray dots represent the atomic positions not assigned to any domain. The layers were sliced from the top of the nanoparticle along the hexagonal c axis and plotted in vertical order starting from the top left panel. b, Layer-by-layer representation of the local H concentration map calculated from MC simulations using Pd coordinates constructed from the AET results, where the layers are ordered corresponding to a. The local fluctuation of H correlated with the large fluctuation in lattice constants (0.1–0.2 Å) observed in the statistical ex situ analysis of hcp particles (Fig. 3 and Extended Data Figs. 5a and 6f–k) as well as in situ analysis of particle growth, showing fluctuations in the d-spacing during particle growth (Fig. 1c–d). These local fluctuations might stem from the nonuniform H distribution in the liquid owing to the stochastic collisions of incident electrons and water molecules. A high H molar concentration appears near the Pd atoms with a large local lattice constant, where the H concentration reaches up to 150% of the Pd concentration. Approximately 20% of the total H atoms are found to occupy the surface region. The lower spatial H concentration marked at the surface region is due to the difference in accessible free volume per Pd atom, where surface Pd atoms have a larger Voronoi volume than the inner Pd atoms owing to less overlap with other Pd atoms. Scale bar, 1 nm.