A310

Morten Bo MADSEN and Steen MORUP

Laboratorv of Applied Physics 11, Technical Unit,ersitv of Denmark, DK 2800 l~vnghv. Denmark

Received 10 July 1984; accepted for publication 29 August 1984

A sample of synthetic goethite with particle dimensions of the order of 100-1000 nm has been studied by use of X-ray diffraction, MOssbauer spectroscopy, and electron microscopy. It is found that each of the particles consists of a bundle of needle-shaped microcrystals with much smaller dimensions. The MOssbauer spectra show that the magnetic properties cannot be explained in terms of a normal superparamagnetic behaviour. However, the model for "superferromagnetism", in which the magnetic coupling among the crvstallites is taken into account, gives an excellent fit to the results.

256 Surface Scicnce 156 (1085) 256 264 North-Holland, Amsterdam

ON THE CRYSTAL STRUCTURE OF SMALL GOLD CRYSTALS AND LARGE GOLD CLUSTERS L.R. WALLENBERG and J.-O. BOVIN

Inorganic Chemist£v 2, Chemical Center. P.O. Box 124, S-221 00 Lund, Sweden

and G. SCHM1D

Inorganic Chemistry, Uni~ersi O, of Essen, Uni~ersiti~ts.~tras.~'e 5 7, 1),4300 E~sen /, t-i,d. R~ T. ~f Germany Received 16 July 1984; accepted for publication 29 August 1984

A large gold cluster with the formula Au55[(C6H5 )~P]~2CI~,, was imaged with a high-resolution transmission electron microscope (HRTEM) and a proposed model was confirmed. Growth of gold crystals of approximately 4.0 nm size could be followed, row by row of atoms, with the use of a low light level TV camera and on-line image processing.

Surface Science 156 (1985) 265-274 265 North-Holland, Amsterdam

IN-SITU TEM EVIDENCE OF LATTICE EXPANSION OF VERY SMALL SUPPORTED PALLADIUM PARTICLES

K. H E I N E M A N N * and H. POPPA

Stanford~NASA Joint Institute for Surface and Microstructure Research. Moffett I")eld. ('altfornia 94035, USA Received 15 August 1984

A direct, in-situ TEM technique was employed to demonstrate that under otherwise identical conditions the lattice parameter of palladium particles in the 1 2 nm size range (supported in random orientation on ex-situ prepared muscovite mica) is expanded by (2.9+0.2)% when compared to particles of 5 nm mean size in which case the lattice parameter is believed to have reached the bulk value. The expansion is not due to pseudomorphism and is considered to only secondarily be a size effect. It is instead believed to be primarily due to a transformation of the 1.5 nm as-deposited Pd particles with predominantly composite crystal structures into larger particles with true fcc structure and with inherently smaller lattice parameter.