Magic alkali-fullerene compound clusters of extreme thermal stability

Abstract

The thermal stability of free pure C60-, as well as C60-alkali, and -alkaline-earth metal compound clusters is investigated. We find that small (C60)m-clusters (m⩽6) decay at comparatively low temperatures below 400K, 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 (C60)mBa2m−1 and (C60)mK2m, which reach values as high as 1780K. 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 C60 and the metal atoms, on the one hand, and entropic contributions to the Gibbs free energy, on the other hand, determine the unusual stability.

Experimental and Theoretical Investigations of the Thermodynamic Stability of Ba−C60 and K−C60 Compound Clusters

“…Previously, we have demonstrated that the thermal stability of C 60 clusters can be significantly enhanced by adding alkali or alkaline-earth metal atoms, thus forming superstable metalO(C 60 ) m compound clusters. 1 Whereas the decay temperatures for (C 60 ) 2 , (C 60 ) 3 , and (C 60 ) 4 were found to be 189, 270, and 340 K, respectively, much higher annealing temperatures could be reached via addition of K or Ba. Figure 1 illustrates the effect of the temperature of the heating cell on the mass spectra of Ba n (C 60 ) m clusters (in all cases the cooling cell was maintained at 150 K).…”

“…However, recent studies have pointed out that simple geometric and electronic shell-filling arguments may not be generally applicable to determine which clusters are magic. 1,2 In a recent communication on metalfullerene clusters we have demonstrated that the experimental setup drastically affects the observed cluster distribution. 1 In the experiments of Martin et al, 7,8 the clusters were produced in a low-pressure, inert gas condensation cell cooled by liquid nitrogen.…”

“…[1][2][3][4][5][6] For example, chemically inert clusters may be ideal building blocks for tailored nanomaterials. Thus, it is important to uncover general principles which may be used to explain enhanced cluster stability and to understand how the experimental conditions affect which superstable (magic) clusters are produced.…”

“…A new experimental setup containing a novel heating/cooling stage mounted directly after the cluster source has revealed a different set of magic peaks for BaϪC 60 and KϪC 60 compound clusters. 1 Here, the clusters were heated to a temperature varying between 150 to 1800 K via thermalization with a He bath for a time span on the order of ca. 1 ms.…”

Adsorption of sodium and cesium on aggregates of C60

“…One or two additional A + ions are needed to provide for the net charge of the "magic" mono-and dications. However, a different pattern was observed in a later study of C 60 -potassium complexes by the MPI group [7,8]. By thermalizing the neutral precursors in helium at 900 K, subsequent cooling and soft single-photon ionization the researchers inferred that neutrals with the composition (C 60 K 2 ) m (m = 2, 3, 4) are extremely stable.…”