Isotherms of Pd–H clusters with different sizes differ from bulk isotherms: they show a largely enhanced solubility in the low-concentration
regime. The isotherms resemble those of bulk above the critical point, they show no ﬂat plateau region, but a slope. However, the existence of
a hysteresis gives evidence for a phase transition even in small Pd–H clusters. Structural studies of 6.0 nm clusters show a transition between
two cubic phases. 3.8 nm Pd–H clusters always show an icosahedral structure in the low and high concentration regime. For an intermediate
size of 5.0 nm Pd–H clusters, the lattice structure changes during H-absorption, from cubic to, most probably, icosahedral. The slope in the
cluster isotherm’s two-phase region is here attributed to stress emerging between the surfactant shell and the cluster during hydrogen-loading.
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