Icosahedral, decahedral, fcc, and defecthyphen;fcc structural models for ArNclusters,Ngsim;500: How plausible are theyquest;

摘要

Different structural models for solid argon clusters with more than 500 atoms, that have been proposed previously on the basis of energy considerations, but whose origin and growth histories are not well understood, are compared with other models that have been devised mainly to overcome these difficulties, at the expense, however, of a smaller binding energy. Diffraction functions have been calculated for a variety of structures and sizes, and are compared with the observed electronhyphen;diffraction patterns obtained by Fargesetal. lsqb;Adv. Chem. Phys.70, 45 (1988)rsqb;. Since the nonsplitting of the first peak in these patterns up toNsim;3000 suggests the absence of themdash; energetically most favorablemdash; decahedra, and other features reflect an increasing fraction of clusters withmdash; at that size unfavorablemdash; bulklike fcc structure, it seems appropriate to relax the energy criterion and to consider other properties as well, notably the rate of growth. Defecthyphen;fcc clusters, obtained from perfect fcc crystallites by introducing at least two crossing stacking faults, derive this property from a modified surface structure, involving immobile, nonvanishing, stacking fault resisting steps. Calculated diffraction functions compare favorably with observed patterns forNsim;600 andNsim;3000. A growth mechanism is proposed which can explain the disappearance of multishell icosahedra and the appearance of (defecthyphen;) fcc clusters atNsim;500, with no structural transition involved.