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We show specific heat data for Na4C60 and Li4C60 in the range 0.4-350 K for samples characterized by Raman spectroscopy and X-ray diffraction. At high temperatures, the two different polymer structures have very similar specific heats both in absolute values and in general trend. The specific heat data are compared with data for undoped polymeric and pristine C60. At high temperatures, a difference in specific heat between the intercalated and undoped C60 polymers of 100 J K−1 mol−1 is observed, in agreement with the Dulong-Petit law. At low temperatures, the specific heat data for Li4C60 and Na4C60 are modified by the stiffening of vibrational and librational molecular motion induced by the polymer bonds. The covalent twin bonds in Li4C60 affect these motions to a somewhat higher degree than the single intermolecular bonds in Na4C60. Below 1 K, the specific heats ofboth materials become linear in temperature, as expected from the effective dimensionality of the structure. The contribution to the total specific heat from the inserted metal ions can be well described by Einstein functions with TE = 386 K for Li4C60 and TE = 120 K for Na4C60, but for both materials we also observe a Schottky-type contribution corresponding to a first approximation to a two-level system with ΔE = 9.3 meV for Li4C60 and 3.1 meV for Na4C60, probably associated with jumps between closely spaced energy levels inside “octahedral-type” ionic sites. Static magnetic fields up to 9 T had very small effects on the specific heat below 10 K.

Three A4C60 compounds, with A = Li, Na and K, have been studied by impedance spectroscopy between 100 K and 293 K at pressures up to 2 GPa. The results are in very good agreement with earlier DC resistance studies and with data from the literature. For all three materials the measured conductivity can be fitted by a sum of at least two Arrhenius terms. The band gaps derived from the resistance data, 0.3 eV for Na4C60 and 0.5 eV for K4C60, are in excellent agreement with data measured by other methods. For Li4C60, our results disagree with a recent suggestion that the conductivity is dominated by ionic conduction. Although a certain ionic component probably exists we suggest that electronic transport dominates in our samples at and below room temperature because the derived “activation energy” decreases under pressure, the derived “activation volume” is negative, and we observe neither a significant electrode blocking capacitance nor any significant metal transport under DC conditions.

We demonstrate that laser irradiation on the narrow-gap insulator Li4C60 and Na4C60 polymers results in metallic monomeric structures, having the same Raman features as metallic fcc K3C60 and Rb3C60, with orientationally disordered C3-60 anions stable at ambient conditions. The Raman Ag(2) mode indicates that a single phase was obtained. Additionally, a monomeric A4C60-type phase (A = K, Rb) is observed in Na4C60 after irradiation. The electron–phonon coupling constants derived from Raman modes of C3-60 fullerides is found to be higher than that of C3-60 fullerides, agreeing with the trend found in heavy alkali fullerides. We propose possible structures of the metallic phases.

In search of structural phase transformations, the electrical resistances of the alkali metal-intercalated fullerene compounds Li4C60, Na4C60, and Rb4C60 have been measured as functions of temperature and pressure. The compounds have different lattice structures, and in the first two, the fullerene molecules form two-dimensional covalently bonded polymer lattices. No phase changes could be identified over the ranges 100-400 K and 0-2 GPa. All materials are semiconducting under the conditions studied, and we see no strong dependence of the band gaps on pressure., Editors: Alfredo Segura, Valentin G. Baonza, Fernando Rodriguez, Alfonso Muñoz, José Manuel Recio and Bonaventura Guamis.

We have synthesized the series of nominally isoelectronic intercalated fullerene compounds LixNa(4 − x)C60 and investigated these by X-ray diffraction and Raman spectroscopy. All compounds are two-dimensional polymers, with the Li4C60 structure dominating for x > 1 and the Na4C60 structure for x less-than-or-equals, slant 1. We find almost no shift of the Ag Raman modes with x, indicating that the charge transfer is also practically independent of composition. We conclude that the Li4C60 structure is the lowest energy structure for all these compounds, and that the Na-rich compounds choose the Na4C60 structure because of geometrical factors connected with effective ion radii.

We have examined intercalated compounds of C60 containing four light alkali metal atoms per molecule. The single-metal compounds Na4C60 and Li4C60 form two-dimensional polymers with intermolecular links consisting of one and two C–C bonds, respectively. We have here studied the compounds NanLi(4-n)C60, with n between 0 and 4, to find out what parameters define the polymeric structure, in particular the type of intermolecular bonding. The materials have been studied by X-ray diffraction and Raman spectroscopy. Although the results are compatible with a charge transfer model with different charge transfers for Na and Li ions, other models cannot be ruled out because disorder and mixed phases complicate the analysis.

We have studied tetragonal C60 and C60-based polymers doped with Lithium and Sodium. We show that the intercalated phases Li4C60 and Na4C60 both form two-dimensional polymers. X-ray diffraction diagrams for Li4C60 and tetragonal C60 can be accurately indexed assuming tetragonal structures, but for Na4C60 a monoclinic quasi-tetragonal structure is found. We conclude that in Li4C60 the covalent bonds are formed by (2 + 2) cycloadditions, in the same way as in the tetragonal polymer produced by treating pure C60 at high temperature and high pressure, while single C-C bonds connect the fullerene molecules in Na4C60.

We present an analysis of three different two-dimensional polymers, tetragonal C60, Li4C60, and Na4C60. Based on X-ray diffraction and Raman spectroscopy, we conclude that Li4C60 forms a tetragonal structure with intermolecular bonds formed by 2 plus 2 cycloaddition, in the same way as for tetragonal C60. Na4C60, on the other hand, forms a monoclinic structure with single C–C bonds between the molecules. Our Raman spectroscopy results can be interpreted in two ways: either the charge transfer to the C60 molecules is the same in both doped compounds with four electrons/molecule or the electron charge transfer is smaller from the Li ions than from the Na ions., Editors: Daniel Guérard and Francois Béguin

We present 13C NMR results on the intercalated 2D C60-polymers Li4-xNaxC60 (x=0-4 with half integer steps) showing that while Li4C60 forms a tetragonal polymer with double polymer bonds between the molecules, Na4C60 form a monoclinic polymer with single polymer bonds. The chemical shift is much larger for Li-rich samples than for Na-rich samples, indicating perhaps a different type of interaction with the C60 molecules. The crossover between the two structures appears to be at the composition Li2.5Na1.5C60 . We also show NMR data on SWNT filled with C60 molecules, so called peapods, showing that the NMR line of C60 can not be observed when the C60 is inside the tubes. Weinterpret this as resulting from a very long relaxation time, indicating that the dynamics of the C60 molecule is hindered by the nanotube walls., Editors: Hans Kuzmany, Jörg Fink, Michael Mehring and Siegmar Roth

Excitation spectra of polymerized fullerides such as RbC60, Na4C60 and pressurized C60 are studied by inelastic neutron scattering and Raman spectroscopy in view of differences in the interfullerene bonding. Changes in the dynamics are followed by temperature dependent measurements. A detailed analysis is performed by model calculations., Editors: Hans Kuzmany, Jörg Fink, Michael Mehring and Siegmar Roth.