Unusual thermal stability of a site-ordered MC60 rocksalt structure (M=K, Rb, or Cs)

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.

The LUMO-derived band of the [Formula: see text] phases

The LUMO-derived band of the [Formula: see text] compound was investigated by means of x-ray and UV photoemission and photoelectric yield spectroscopies. A double-peak structure is found, with peak maxima at 1.1 eV and 0.35 eV. The relative magnitude of the two peaks is strongly temperature dependent: a large transfer of spectral weight from the lower- to the higher-binding-energy peak takes place upon cooling to 110 K. The peaks are tentatively attributed to the dimer and the polymeric phase respectively. However the behaviour as a function of temperature does not agree with the metastability of the dimer phase at room temperature. The position of the Fermi level is compatible with a correlated system on the border of the metal - insulating transition.

Molecular dynamics, phase diagrams and electronic properties of fullerenes and alkali fullerides: insights from solid-state nuclear magnetic resonance spectroscopy

Solid-state nuclear magnetic resonance (NMR) measurements have made important contributions to the current understanding of the structural, dynamical, and electronic properties of fullerenes (C60 and C70) and the alkali fullerides AxC60 (A = alkali metal). These measurements and their interpretation are reviewed. One- and two-dimensional 13C NMR lineshapes and spin-lattice relaxation rates provide evidence for rapid molecular rotations and orientational order-disorder transitions in the fullerenes and alkali fullerides. The kinetics of molecular reorientations are determined from the NMR data. 13C and alkali metal NMR spectra indicate that the alkali fullerides are stoichiometric compounds. Each stable, stoichiometric phase has distinctive NMR signatures. 13C and alkali metal NMR spectra and relaxation measurements provide valuable and unique information about the electronic properties of the metallic, superconducting, and non-metallic phases of the alkali fullerides.

Single-Crystalline (KC 60 ) n : A Conducting Linear Alkali Fulleride Polymer

Single crystals of a linear cycloadduct conducting polymer, (KC(60))n, have been grown that are a few tenths of a millimeter in length. Partial oxidation under toluene transformed these crystals into bundles of fibers. The degree of polymerization exceeded 100,000.