60 30 09 v 1 2 4 M ar 1 99 6 On metallic clusters squeezed in atomic cages

The stability of metallic clusters of sodium (N a) in the octahedral cages of N a-doped fullerites N a 6 C 60 and N a 11 C 60 is discussed within a Thomas-Fermi model. It is shown that the tetrahedral N a 4-cluster in N a 6 C 60 has an electric charge ∼ +2.7 (in electron charge units), while the body-centered cubic N a 9-cluster in N a 11 C 60 is almost electrically neutral. The nature of the metallic clusters is currently receiving a great deal of interest.[1] Metallic clusters trapped in zeolites cages provide the opportunity of studying them by various magnetic resonance techniques,[2] − [4] and, in this respect, the nature of the chemical bonding, the distinction between metallic and covalent bondings and the degree of ionicity of the clusters are of utmost importance. Recently, the existence of Na-clusters in the octahedral cages of Na-doped C 60-fullerite has been suggested from X-ray analysis.[5] , [6] Similar clusters of calcium (Ca) in Ca-doped C 60 has previously been pointed out.[7] Tetrahedral-shaped Na 4-clusters seem to build up in the octahedral cages of the (face-centered cubic) f cc-Na 6 C 60 ,[5] with the Na atoms disposed alternately on the corners of a cube of side 2 ˚ A. Similarly, body-centered cubes of 9 sodium atoms (Na 9-cluster) seem to occur in the octahedral cages of the f cc-Na 11 C 60 ,[6] with the cube side 3.2 ˚ A. This is 1