Theory of distinct crystal structures of polymerized fullerides AC60, A = K, Rb, Cs: the specific role of alkalis

Superexchange and electron correlations in alkali fullerides AC60, A=K, Rb, Cs

Superexchange interactions in alkali fullerides AC(60) are derived for C(60) molecular ions separated by interstitial alkali-metal ions. We use a multiconfiguration approach which comprises the lowest molecular orbital states of the C(60) molecule and the excited s and d states of the alkali-metal atom A. Interactions are described by the valence bond (Heitler-London) method for a complex (C(60)-A-C(60))(-) with two valence electrons. The electronic charge transfer between the alkali-metal atom and a neighboring C(60) molecule is not complete. The occupation probability of excited d and s states of the alkali atom is not negligible. In correspondence with the relative positions of the C(60) molecules and A atoms in the polymer crystal, we consider 180 degrees and 90 degrees (angle) superexchange pathways. For the former case the ground state is found to be a spin singlet separated from a triplet at approximately 20 K. For T<20 K there appear strong spin correlations for the 180 degrees superexchange pathway. The results are related to spin lattice relaxation experiments on CsC(60) in the polymerized and in the quenched cubic phase.

Valence electronic charge density of distorted C60- monomers in polymerized KC60 and RbC60

We investigate the valence electronic charge density of the C(60) (-) monomers in (C(60) (-))(n) polymer chains in K- and RbC(60) by means of a nonorthogonal tight-binding formalism using experimental data on the positions of the carbon atoms. Various configurations of the C(60) cages are considered. Starting from the ideal icosahedral C(60) structure and moving to the realistic, experimentally determined spatial configuration of the C(60) cages in K- and RbC(60), we observe a systematic increase of the electric quadrupole moments on the C(60) (-) monomers. We also confirm the validity of factorizing the charge density of a C(60) (-) monomer into an angular and a radial part.

Metal-insulator and structural phase transition observed by ESR spectroscopy and x-ray diffraction in KC60

We have performed electron spin resonance (ESR) spectroscopy and x-ray diffraction experiments at low temperature on KC60 single crystals. ESR data reveal the occurrence of a metal-insulator phase transition at about 50 K. In the same temperature range, we observe the stabilization of a superstructure which doubles the volume of the unit cell. We suggest that displacements of the K atoms and a modulation of the C60 charge may be involved in the mechanism of this phase transition. These results shed new light on the subtle interplay of structure, dimensionality, and electronic properties in the AC60 fullerides.