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Abstract

Two samples of Au₅₅(PPh₃)₁₂Cl₆ organometallic clusters prepared by different methods were investigated by
X-ray powder diffraction (XRD). The measured diffraction curves were fitted by a set of Debye functions for Au_n with the “magic” numbers N = 13,55,147,... for cuboctahedra and icosahedra; N = 54,181 for decahedra; and N = 13, 57, 154 for hexagonal close-packed clusters. It was clearly shown that most Au₅₅(PPh₃)₁₂Cl₆ clusters do not have an fcc structure, but a good fit was achieved using the icosahedral model. The nearest-neighbor distance is 0.278 nm, i.e. ~3% smaller than that of the bulk gold material (0.288 nm); however, the center of the first coordination shell is uncontracted within the limits of error (0.5%). In the sample that was not filtered by an anotop filter, an additional maximum in the small-angle range was observed. It is attributed to a disordered fcc-type supercluster of (Au₅₅)₅₅ with a cluster-cluster distance of 2.50 nm. The diffraction curve could only be explained by a nonuniform cluster size, verified by TEM micrographs. About 40 wt % of the material belongs to clusters with nuclearities N larger and smaller than 55. Occasionally, TEM reveals large (≈40 nm), two-dimensional, densely stacked arrays of clusters. The lattice fringes of 2.14 nm are in very good agreement with the value 2.50 X (3/4)^1/2 expected from XRD.