Structural Changes in Ag,Fe,Mn, and Ge Microclusters

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AbstractThe structures of Ag,Fe,Mn, and Ge microclusters were determined using EXAFS. The measurements were performed over a wide range of clusters sizes.The clusters were prepared using the gas aggregation technique and isolated in solid argon at 4.2 K. A strong contraction of the interatomic distances was observed for Ag dimers and multimers. Silver clusters larger than 17 A mean diameter show a small contraction of the nn distance and a structure consistent with an fcc lattice. By contrast smaller clusters show the presence of a small expansion and a strong reduction or absence of nnn in the EXAFS signal. This points towards a different crystallographic structure for Ag microclusters. In iron clusters we observed a gradual reduction of the nn distance as the cluster size decreases. The interatomic distance for iron dimers was determined to be 1.94 A, in good agreement with earlier measurements. The iron microclusters show a bcc structure down to a mean diameter of 9 A. Iron clusters with 9 A mean diameter show a structure consistent with an fcc or hcp lattice.Mn microclusters show a simpler crystallographic structure than α-Mn. The measurements on Ge clusters show the presence of only nearest neighbors. There was clear evidence of temporal annealing as determined by variations in the near edge structure of the K-absorption edge. Absorption edge measurements were also performed on free Ge clusters traveling perpendicular to the direction of the synchrotron radiation beam.