BaxC60 fullerides: ? Electronic peculiarities of the C60 molecule and their consequences for the solid state

Abstract

The electronic structure of BaxC60 fullerides was studied theoretically under special consideration of π electronic effects in the C60 molecule. Band structure data were derived by an intermediate neglect of differential overlap (INDO) crystal orbital (CO) approach. Different electronic configuration were evaluated in the Ba-doped C60 fullerides. BaxC60 solids with x=0, 3, 4, 6 are insulators. For a Ba5C60 model extrapolated from the crystal structure of Ba6C60, a finite band gap is also predicted. For a Ca5C60-like structure of Ba5C60, a quasi-degeneracy between a metallic configuration and an insulating Mott-like state was found. With an increasing Ba-to-C60 charge transfer (CT), sizable changes in the π system of C60 occur. In the neural molecule and for not too high an electron count, the π electrons form more or less electronically isolated hexagon–hexagon (6–6) “double” bonds with only minor hexagon–pentagon (6–5) “double-bond” admixtures. In the vicinity of C6012−, the 6–6 bonds have lost most of their double-bond character while it is enhanced for the 6–5 bonds. In highly charged anions, the π electron system of the soccer ball approaches a configuration with 12 decoupled 6π electron pentagons. For electron numbers between C60 and C6012−, the net π bonding is not weakened. The INDO CO results of the BaxC60 solids are supplemented by INDO MO and ab initio (3-21 G* split-valence basis) calculations of molecular C60 and some highly charged anions. Ab initio geometry optimizations show that the bond alternation of C60 with short 6–6 and long 6–5 bonds is inverted in C12−60. The high acceptor capability of C60 is explained microscopically on the basis of quantum statistical arguments. In the π electron configurations of C60 and C6012−, the influence of the Pauli antisymmetry principle (PAP) is minimized. The quantum statistics of (π) electron ensembles with a deactivated PAP is of the so-called hard-core bosonic (hcb) type. In these ensembles, the on-site interaction is fermionic while the intersite interaction is bosonic. Energetic consequences of the quantum statistical peculiarities of π systems are explained with the aid of simple model systems; we selected annulenes and polyenes. Computational tools in this step are Green's function quantum Monte Carlo (GF QMC) and full configuration interaction (CI) calculations for the π electrons of the model systems. These many-body techniques were combined with a Pariser–Parr–Pople (PPP) Hamiltonian. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 333–373, 1997