C. Goze

Insulator-metal transition in Rb4C60 under pressure from 13C-NMR

Abstract We present an NMR study of Rb 4 C 60 performed under pressure up to 12 kbar. The temperature dependence of the 13 C spin-lattice relaxation rate is activated at ambient pressure and becomes Korringa-like under pressure. The behaviour of the relaxation is interpreted in terms of one channel due to excited intramolecular triplet states above the Jahn-Teller ground state plus another one related to electron-hole excitations through an indirect band gap. In spite of a finite value of the density of states at the Fermi level indicated by the NMR data under pressure, no superconductivity could be detected at 15 kbar above 0.4 K.

Quantum size effects induced by confinement of C60 in MCM41

Abstract Siliceous mesoporous MCM-41 type solids of pore diameters 17 and 26 A have been used for occlusion of fullerene C60 molecules by sublimation of C60 at 873 K under vacuum. The resulting nanocomposite exhibits quantum size effects in its electronic and magnetic properties. The C60 adsorbed by the MCM-41 shows an intense ESR spectrum between 4 and 300K. The electron spin resonance linewidth is greater for the sample with pore diameter 17 than for that with diameter 26 A, suggesting that the linewidth is inversely dependent on the particle size. Furthermore, the linewidth shows no temperature dependence in the range 30 to 300K, indicating a quenching of the bulk C60 relaxation mechanisms. The intensity of the signal obeys Curies law over this range. These effects are consistent with the modification of the electronic band structure of the fullerene as a result of spatial confinement.

On the structural distortion in Rb3C60 and K3C60 revealed by 87Rb- and39K-NMR

Abstract The splitting of the tetrahedral Rb - and K - N M R lines in Rb 3 C 60 and K 3 C 60 is analyzed in terms of the mechanism of alkali-cation vacancies. The calculations indicate the presence of vacancies only in the tetrahedral positions, and the inter-atomic potentials between the alkali cations and the fullerene molecules account satisfactorily for the temperature dependence of the position of the distorted line.

NMR studies of alkali fullerides: Rb1C60 and Cs1C60

Abstract We report on 13 C and 87 Rb NMR measurements on Rb 1 C 60 rocksalt, polymerized and quenched phases, as well as on polymerized Cs 1 C 60 at room temperature. We present static and high-resolution MAS NMR signatures of these phases in various conditions, and we discuss the different line positions. The study of the temperature dependence of 87 Rb NMR on Rb 1 C 60 has evidenced the high stability in air of all the Rb 1 C 60 phases.

Insulator-metal transition in Rb4C60 under pressure : Jahn-Teller theory versus NMR experiments

Abstract NMR experiments on Rb 4 C 60 performed under pressure are presented. The temperature dependence of the 13 C NMR relaxation rate T 1 −1 shows two activated behaviors coexisting at ambient pressure; the lowest activation energy disappears under pressure in favor of the emerging Korringa law. The theory is suggested to describe the Rb 4 C 60 as a Jahn-Teller crystal with corresponding consequences for electronic band spectra and for self-trapped states. Experiments are interpreted in terms of two relaxation channels: one due to intramolecular triplet exitons; the other being related to electron-hole excitations through an indirect band gap.

High resolution 13C NMR investigation of A6C60 (A=K, Rb, Cs) and Ba3C60

Abstract We report the result of 13 C nuclear magnetic resonance (NMR) measurements on A 6 C 60 ( A =K, Rb, Cs) and Ba 3 C 60 . By using high-resolution magic angle spinning, we were able to identify an isotropic line around 156 ppm for all investigated compounds. NMR spectra of the saturated alkali compounds are quite similar. The corresponding isotropic lines show three narrow components consistent with orientationally ordered C 60 molecules leading to three non-equivalent carbon sites in these compounds as reported by x-ray studies. No line splitting was observed for the Ba 3 C 60 isotropic line.

Off-centre sites in alkali fullerides

Abstract It is shown that, under certain conditions, the octahedrally-coordinated alkali cation A in the A 3 C 60 fullerides may acquire off-centre positions, placed along the high-symmetry directions.

Raman studies of C60, phototransformed C60 and AC60 phases

Abstract We report a room-temperature Raman investigation of C 60 , stable-in-air phototransformed C 60 and AC 60 phases (A=Rb,Cs) as excited at 776 nm. The existence of a stable-in-air phototransformed C 60 state is confirmed. The low-frequency Raman spectrum of AC 60 exhibits a well-defined peak at about 33 cm −1 . The assignment of this mode is discussed.

High-resolution 13C NMR in alkali intercalated C60

Abstract We present the results of 13 C NMR measurements on AxC60 compounds (A = K, Rb and x = 3, 4, 6). By using high-resolution MAS NMR spectroscopy we were able to identify and characterize the different, thermodynamically stable phases of AxC60. Based on an analysis of the different NMR spectra we discuss the effects of the intercalation on the electronic properties and molecular dynamics in these compounds.