Alkali fullerides

Takenobu T, Muro T, Iwasa Y, Mitani T (2000) Antiferromagnetism and phase diagram in ammoniated alkali fulleride salts. Phys Rev Lett 85 381-384... 

Holczer, K., Klein, O., Huang, S.-M., Kaner, R. B., Fu, K.-J., Whetten, R. L. Diederich, F. 1991 Alkali-fulleride superconductors synthesis, composition, and diamagnetic shielding. Science, Wash. 252, 1154-1157. 

Intense efforts in the last decade have exhaustively mapped the electronic and superconducting properties of intercalated alkali fullerides and the occurrence of the metal-antiferromagnetic insulator transition as a function of inter -fullerene separation, orientational order/disorder, valence state, orbital degeneracy, low-symmetry distortions and metal-C60 interactions [6-12]. 

The superconducting properties displayed by the series of ammoniated alkali fullerides, (NH3)xNaA2C60 (0.5[Pg.132]

The smallness of the C atom displacements expected theoretically as a result of the Jahn-Teller distortion (JTD) makes it obvious that the direct detection of JTD will be very challenging. As the coupling between molecules in a metallic phase will add new degrees of freedom and considerably complicate the problem, the study of a simple model system where the C"d could be considered as nearly isolated is highly desirable. Such phases have been obtained in C60 salts, principally with Q0 and C, where large counter anions lead to a distance between C60 centers of typically 12.6 A compared to 10 A in alkali fullerides (for a review on C60 salts, see ref. [9]). 

In this review we shall focus on some of these new forms of solid carbon. The emphasis is on the physical properties of electrically conducting fullerides, fulleride polymers and nanotubes, but the neutral fullerene polymers, dimers and onion-like structures are also included for completeness. This paper is by no means a review of all important work in the domain. We fully realize that in choosing the material we had to be subjective and we selected material best known to us. A few other short reviews have been published recently on fullerene polymers on the optical properties of polymeric fullerenes [15] and on the physical properties of conducting fullerenes [16,17]. There are extensive recent reviews on the pressure and heat induced polymers [18]. We did not include in the paper the physical and chemical properties of alkali fullerides with variously charged monomer ions. These are the subject of other reviews and are described in detail in a recent monograph [19]. In particular, there are comprehensive reviews [20,21] on experiments and theories aimed at the understanding of the mechanism of superconductivity. 

The AC o polymers are resistant to oxidation in air and remain intact in solvents like toluene [11], This is in contrast to all other known alkali fullerides, including the fee high temperature AC o phases, which rapidly degrade in air. The air stability facilitates experimentation and the stability and the insolubility in solvents provides a mean for separating the polymer phase from other phases. Doping pure with precisely weighed amounts of alkalis at high temperatures is the usual synthesis of AQg compounds with A = K, Rb, Cs. The exact stoichiometry is hard to achieve and most early data are on mixtures of different phases. 

It is worthwhile to point out that the alkali fullerides are extremely air-sensitive. All sample preparations with alkali metals must be carried out in a glove box with oxygen levels on the order of one part per million. However, prepared samples can survive some exposure to air.[Ho91a ]... 

We have been concentrating exclusively on the superconducting AaCeo phases, but there are other alkali fullerides. KeCeoj RbeCeO) a-nd CseCeo all have a body-centered cubic structure, with alkali ions in distorted tetrahedral interstices.[Zh91a] In that structure, the fullerenes are all fixed in the same orientation, in contrast to the disorder and motion observed in KsCeo-lStSl, Ty91 ] The leCeo phases are insulators, as would be expected from the fact that six donated electrons will completely fill the bands created by the fullerene and t g molecular orbitals. 

Ho91a K. Holczer et ai, Alkali-Fulleride Superconductors Synthesis, Composition, and Dia- Mu92... 

TV92 R. Tycko et al, Electronic Properties of Normal and Superconducting Alkali Fullerides Probed by C Nuclear Magnetic Resonance, Phya. Rev. Lett. 68, 1912-1915 (1992). 

The results of nuclear magnetic resonance (NMR) measurements on alkali fullerides K cC o reported. The NMR spectra demonstrate that material with 0 < X < 3 is in fact a two-phase system at equilibrium, with x = 0 and x = 3. NMR lineshapes indicate that C o Ions rotate rapidly in the KsC q phase at 300 K, while 50 ions in the insulating KaC o phase are static on the time scale of the lineshape measurement. The temperature dependence of the spin-lattice relaxation rate in the normal state of is found to be characteristic of a metal, indicating the... 

Electronic Properties of Normal and Superconducting Alkali Fullerides Probed by Nuclear Magnetic Resonance... 

Alkali-Fulleride Superconductors Synthesis, Composition and Diamagnetic Shielding... 

Unfortunately, the simplest fulleride salts, the monovalent ACeo (A = K,Rb,Cs) alkali fullerides, exist in a polymerized phase at room temperature (see Sect. 4.2) and depolymerize only above 400 K where the rotation of the balls averages out any distortion. Infrared spectra of monoanions at this high temperature show unperturbed icosahedral symmetry [47,48]. 

We have seen above that close-lying thermally accessible orbitals can give rise to many unusual phenomena in fulleride solids. We now briefly discuss two further consequences of the presence of such states possible chemical reactions (as, e.g. polymerization) and the coexistence of several phases in a solid at the same temperature (segregation). Both are a source of new information but unfortunately they also prohibit a full systematic investigation of the monomeric alkali fulleride salts series. 

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