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In situ X-ray diffraction (XRD) and gravimetric hydrogen uptake measurements of d∼2–3nm spherical PdH_x particles have been studied in the temperature and pressure range of 323<T<428K and 0<P<10bar. The Pd particles were protected from sintering with a hydrogen-permeable carbon coating. While only containing ∼300–1000 atoms, the Pd particles were found to exhibit the same fcc structure and lattice constant as the bulk. Our isothermal studies show that, with increasing x, these highly crystalline PdH_x nanoparticles also exhibit a complete transformation from the dilute α solid solution phase to the more concentrated β hydride phase. However, we observed that the character of the α–β phase transition in these nanoparticles is very different from that in the bulk. Indeed, the hydrogen uptake isotherm exhibits a noticeable positive slope in the α+β co-existence region. Furthermore, we also observed a …