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Abstract

The thermal stability of free pure C-60-, as well as C-60-alkali, and
-alkaline-earth metal compound clusters is investigated. We find that
small (C-60)(m)-clusters (m <= 6) decay at comparatively low
temperatures below 400 K, as a consequence of weak intermolecular van
der Waals interaction. Adding barium or potassium to the clusters
dramatically increases the decay temperatures for "magic"
configurations of (C-60)(m)Ba2m-1 and (C-60)(m)K-2m, which reach values
as high as 1780 K. Contrary to common belief, the superstable compound
clusters are not characterized by filled geometrical or electronic
shells. Density functional calculations show that the delicate
interplay of ionic (K, Ba) and covalent (Ba) interaction between C-60
and the metal atoms, on the one hand, and entropic contributions to the
Gibbs free energy, on the other hand, determine the unusual stability.
(c) 2006 American Institute of Physics.