In-situmeasurements of the lattice parameter of supported palladium clusters

Palladium clusters have been epitaxially grown under ultra-high vacuum (UHV) on MgO(100) surfaces cleaved in situ. The lattice parameter was measured in situ by surface electron energy-loss fine-structure spectroscopy (SEELFS) above the palladium N23 edge. The size distribution of the clusters was obtained from ex-situ transmission electron microscopy. An expansion of the palladium lattice, increasing on decrease in the mean cluster size, has been observed. From the UHV in-situ measurements the lattice dilatation is attributed to pseudomorphism. With the SEELFS technique the Debye-Waller factor was also determined. An increase in the mean square displacement of the atoms was observed when the cluster size decreased; the latter was explained by the increase in the proportion of surface atoms and by the effect of the lattice dilatation.