M. Belli

Unusual polymerization in the Li4C60 fulleride

Li4C60, one of the best representatives of lithium intercalated fullerides, features a novel type of 2D polymerization. Extensive investigations, including laboratory x-ray and synchrotron radiation diffraction, 13C NMR, MAS and Raman spectroscopy, show a monoclinic I2/m structure, characterized by chains of [2+2]-cycloaddicted fullerenes, sideways connected by single C-C bonds. This leads to the formation of polymeric layers, whose insulating nature, deduced from the NMR and Raman spectra, denotes the complete localization of the electrons involved in the covalent bonds.

The structural and electronic evolution of Li4C60 through the polymer–monomer transformation

In this paper, we combine synchrotron powder x-ray diffraction, 7Li nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) experiments to study the structural evolution of Li4C60 and how its electronic ground state depends on the crystal symmetry. The compound in the two-dimensional polymer phase with mixed interfullerene bonding motifs is a band gap insulator. EPR, however, reveals the presence of intrinsic centers originating from broken C60–C60 bonds and local Li off-stoichiometry that create states in the band gap and account for the complex temperature dependence of the spin susceptibility as well as the residual temperature dependence of the 7Li NMR shift. At low temperatures, the Li+ ions are statically disordered on the 7Li NMR timescale. The observed 7Li NMR line narrowing at T>200 K is ascribed to the Li+ diffusion dynamics and above room temperature the polymer phase is already a good ionic conductor. Heating the sample to temperatures above ~470 K results in gradual depolymerization to the metallic monomer fcc high temperature structure. The transformation is first order and polymer as well as monomer phases coexist over a broad temperature interval (130 K).

Recovering metallicity in A 4 C 60 : The case of monomeric Li 4 C 60

The restoration of metallicity in the high-temperature, cubic phase of Li4C60 represents a remarkable feature for a member of the A4C60 family (A = alkali metal), invariably found to be insulators. Structural and resonance technique investigations on Li4C60 at T > 600 K, show that its fcc structure is associated with a complete (4e) charge transfer to C60 and a sparsely populated Fermi level. These findings not only emphasize the crucial role played by lattice symmetry in fulleride transport properties, but also re-dimension the role of Jahn-Teller effects in band structure determination. Moreover, they suggest the present system as a potential precursor to a new class of superconducting fullerides.

Recovering metallicity in A4C60 : The case of monomeric Li4C60

The restoration of metallicity in the high-temperature, cubic phase of Li4C60 represents a remarkable feature for a member of the A4C60 family (A = alkali metal), invariably found to be insulators. Structural and resonance technique investigations on Li4C60 at T > 600 K, show that its fcc structure is associated with a complete (4e) charge transfer to C60 and a sparsely populated Fermi level. These findings not only emphasize the crucial role played by lattice symmetry in fulleride transport properties, but also re-dimension the role of Jahn-Teller effects in band structure determination. Moreover, they suggest the present system as a potential precursor to a new class of superconducting fullerides.

Unconventional isotope effects in superconducting fullerides

Although widely accepted, the phonon-mediated, BCS-like theory of superconducting A3C60 fullerides (A=K, Rb, Cs) cannot reproduce correctly all their parameters, even in its strong-coupling, Migdal-Eliashberg limit. The fundamental difficulty, ascribed to intrinsically close phonon and electron energy scales (respectively at 0.2 and 0.25 eV), has been overcome by dynamical mean-field theories (DMFT), which, unlike ME, consider electron-phonon and electron-electron interactions on an equal footing. The unconventional phenomena predicted in the new framework include, among others, isotope effects on spin susceptibility, totally absent from standard theories. We have tested these predictions, finding a significant dependence on the isotopic mass in both Tc and, more importantly, in the normal-state Pauli susceptibility χP. The comparative measurement of χP in two different K3C60 samples (85%13C-enriched vs. natural abundance), both by SQUID magnetometry as well as by 13C NMR Knight shift, definitely confirms the presence of an isotope effect on susceptibility, although a quantitative agreement with theory is still missing.

New Polymeric Phase in Low‐Doped Lithium Intercalated Fullerides

Abstract The low‐doped Li x C60 compounds (x≤6) were investigated using laboratory X‐ray and synchrotron radiation diffraction, 13C NMR and Raman spectroscopy. Li4C60 shows an unusual 2D polymerisation in which the C60 units are connected both by [2+2] cycloaddition and by single carbon–carbon bonds, a unique feature among the known polymerised fullerene compounds. This picture is fully supported also by static NMR and Raman measurements. The charge transfer to C60 in the polymeric phase was evaluated from the shift of the Ag(2) mode. The depolymerisation process was investigated as well; despite the presence of two different bonds, the polymer‐to‐monomer transition induced by thermal treatments is a single‐step phenomenon.

Recovering metallicity inA4C60: The case of monomericLi4C60

The restoration of metallicity in the high-temperature, cubic phase of Li4C60 represents a remarkable feature for a member of the A4C60 family (A = alkali metal), invariably found to be insulators. Structural and resonance technique investigations on Li4C60 at T > 600 K, show that its fcc structure is associated with a complete (4e) charge transfer to C60 and a sparsely populated Fermi level. These findings not only emphasize the crucial role played by lattice symmetry in fulleride transport properties, but also re-dimension the role of Jahn-Teller effects in band structure determination. Moreover, they suggest the present system as a potential precursor to a new class of superconducting fullerides.