  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.
