PHASE-TRANSITIONS IN NA7-NA9 MICROCLUSTERS

We present a wide range of properties of Na7-Na9 microclusters determined in the framework of our improved molecular-dynamics approach at fixed temperature. The Born-Oppenheimer dynamics was treated by means of an effective many-body potential, used previously for clusters, surface, and bulk properties. We have computed rotational, vibrational, and potential energies and corresponding specific heats, momenta of inertia, shapes parameters, and bond lengths of the Na microclusters over a wide range of temperatures, from 25 to 1500 K. We have determined that sodium microclusters undergo two "phase transitions," one around 100 K from a crystal to a glassy or molten state, and a second one around 800 K, to a fluid state of the cluster. At low temperatures Na microclusters are essentially incompressible, but relatively easy to deform. At high temperatures Na clusters become extremely soft, and evaporation of atoms sets in.