CHANGES OF MORPHOLOGY AND CAPPING OF MODEL TRANSITION-METAL CLUSTERS

We investigate changes of morphology between icosahedra, cuboctahedra, and decahedra in transition metal clusters containing 13, 55, and 147 atoms modeled by two different classes of empirical potential. Highly cooperative processes exist which lead to a fundamental change in str ucture via a single transition state. As the size increases, these pat hways break down into a series of steps, and intermediate stationary p oints appear; this happens at smaller size for clusters described by t he Sutton-Chen potentials. Comparison of our calculated barriers and r ates with previous results; from electron microscopy and calculations of diffusion and interlayer transport mechanisms generally gives quali tative agreement. However, neither potential predicts that AU(55) or A U(147) cuboctahedra will be observable experimentally. A combination o f substrate interaction and capping probably explains this discrepancy . Finally, we predict that the icosahedral-morphology is likely to be 'frozen in' at relatively low particle size and that layer by layer gr owth of Mackay icosahedra is possible via appropriate surface rearrang ements.