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

Abstract

A novel experimental set-up was used to study superstable (magic) Ba−C60 and K−C60 compound clusters. The most stable systems observed cannot be rationalized by simple electronic or by geometrical shell filling arguments. Annealing the clusters past the temperature necessary for the fragmentation of the initial metastable clusters formed at the source reveals information about their thermodynamic stability. Higher temperatures yield larger species, suggesting that similar experiments may be used to rationally produce nanoscale clusters with highly desirable properties. Density functional calculations reveal ionic (K, Ba) and covalent (Ba) bonding between C60 and the metal atoms. The entropic contribution to the Gibbs free energy is shown to be essential in determining absolute and relative cluster stabilities. In particular, we demonstrate that at higher temperatures the entropy favors the formation of larger clusters. A simple criterion which may be used to determine the absolute and relative stabilities of general multicomponent clusters is proposed.

Adsorption of sodium and cesium on aggregates of C60

“…In other words, the adatoms decorate a hemisphere of C 60 [11,12,37]. For C 60 Na 6 and C 60 K 6 , on the other hand, it is found that adatoms preferentially bind to hexagonal sites and stay away from each other as far as possible [8,36,37,41] 5 .…”

“…Li + prefers triangular interstitial sites whereas Na + and K + prefer tetrahedral sites. Zurek et al [8] find that the adatoms in (C 60 ) 2 K 4 reside in the groove between the two C 60 . Apparently the enhanced adsorption energy at groove sites [45] outweighs the tendency for ions to spread out uniformly due to Coulomb repulsion.…”

“…Apparently the enhanced adsorption energy at groove sites [45] outweighs the tendency for ions to spread out uniformly due to Coulomb repulsion. However, two K atoms added to (C 60 ) 2 K 4 will reside at opposite ends of the C 60 dimer, on the molecular axis [8].…”

“…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.…”

“…Chandrakumar and Ghosh [9] report that the charge on a single sodium atom adsorbed on C 60 derived by the Mulliken method is 0.89. Weis et al [10] find that the Mulliken charge is >0.90 per potassium atom for C 60 K n , n 3; the same holds for (C 60 ) 2 K n for n 6 [8]. Considering adsorption of up to 12 Li, Na or K atoms on C 60 , Hamamoto et al [11] find that the Mulliken charge is smallest for lithium.…”