SIZE-INDEPENDENT FCC-TO-ICOSAHEDRAL STRUCTURAL TRANSITION IN UNSUPPORTED SILVER CLUSTERS - AN ELECTRON-DIFFRACTION STUDY OF CLUSTERS PRODUCED BY INERT-GAS AGGREGATION

The structure of free silver clusters, produced in an inert-gas-aggreg ation source and flowing in a molecular beam has been studied by elect ron diffraction. Large clusters (up to 11 nm in diameter) of both icos ahedral and fee structure are observed. Cluster structure is investiga ted as a function of evaporation temperature and molecular weight of t he inert gas in the source. An increase in evaporation temperature lea ds to increasing formation of fee particles in the beam, without a sig nificant change in particle size. The existence of large icosahedra is ascribed to the formation of metastable structures, grown under condi tions dominated by kinetic factors. Furthermore, these large icosahedr a show pronounced relaxation, indicating that they are subject to sign ificant internal strain. This strain appears to be accommodated by def ormation of the lattice, rather than by the inclusion of defects. The diffraction findings are supported by transmission-electron-microscopy measurements made on samples taken from the beam.