Mesoporous tantalum oxide–sodium fulleride composites were synthesized by solution impregnation and characterized by elemental analysis, X-ray diffraction (XRD), Raman spectroscopy, nitrogen adsorption–desorption, X-ray electron photoelectron spectroscopy (XPS), superconducting quantum interference device (SQUID) magnetometry, variable-temperature electron transport measurements, and solid-state 13C and 23Na NMR spectroscopy. The room temperature conductivity pattern as a function of sodium reduction level displayed a minimum at n = 3.0 and a maximum at n = 4.5, where n is the formal charge on the fulleride. The variable-temperature conductivity measurements demonstrated that the n = 0.5 and n = 4.5 materials were semiconductors. Solid-state 23Na NMR spectroscopy of the n = 0.5 composite exhibited three Na environments in the composites: two associated with the tantalum oxide walls and a third associated with the fulleride. The n = 3.0 and n = 4.5 materials showed a large build-up of Na ions in the wall with no visible Na resonances associated with the fulleride, suggesting a structure in which the fulleride units exist as naked anions in one-dimensional chains surrounded first by a layer of Na ions and then by a layer of mesoporous tantalum oxide. Solid-state 13C NMR experiments showed more than one fulleride species in both the n = 0.5 and the n = 4.5 composites, as well as pure C60 in the n = 0.5 material, but almost exclusively C603– in the n = 3.0 material. The retention of carbon throughout reduction suggests that polymerization may have occurred, however this could not be verified by 13C NMR spectroscopy, because the region where sp3 fullerene resonances normally appear was obscured by solvent peaks.

Structural and Spectroscopic Studies on Mesoporous Tantalum Oxide–Sodium Fulleride Composites with Conducting Fulleride Columns in the Pores

Solid-state 23Na and 13C NMR characterization of Na3C60

Shielding diamagnetism and X-ray powder diffraction were used to characterize alkali-metal fullerides of sodium, potassium, rubidium, and cesium. The superconducting phase in both the potassium and rubidium systems has the composition A3C60 and a face centered cubic structure with alkali metal filling all octahedral and tetrahedral sites of close packed C60, layers. High pressure magnetic susceptibility measurements on K3C60 showed a negative pressure dependence on the superconducting transition temperature of −0.78 K/kbar. No evidence for superconductivity was observed in either the sodium or cesium systems, even though Na3C60 appears isostructural with K3C60 and Rb3C60.

Structure and properties of superconducting and nonsuperconducting alkali-metal fullerides axc60 (A = Na, K, Rb, or Cs)

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Structure and properties of superconducting and nonsuperconducting alkali-metal fullerides axc60 (A = Na, K, Rb, or Cs)