Vibrational behavior of Na4C60 in the monomer and two-dimensional polymer states

We have studied the phonon excitation spectrum of Na4C60 fullerenes in the temperature range 300 K less than or equal to T less than or equal to 600 K ,using neutron and Raman spectroscopy. Strong intensities in the gap region 10 meV < (h) over bar omega < 30 meV show direct evidence for the existenc e of intermolecular excitations arising from covalent intercage bonds below T approximate to 550 K. Both charge transfer and cage distortion are refle cted in the spectra. The analysis of our spectra in terms of model calculat ions confirms the proposed structure of a two-dimensional polymer with "sin gle" covalent interfullerene bonds [G. Oszlanyi et al., Phys. Rev. Lett. 78 , 4438 (1997)]. The single bonds show similar restoring forces as in RbC60 [H. Schober et al, Phys. Rev. B 56, 5937 (1997)] or RbC70 dimers . Temperat ure-dependent studies reveal the details of the polymerization process. Mon omer and polymer phases are separated by a region of mixed states as in RbC 60 The polymerization is fast compared to the other network forming fullere ne systems malting quenching of the monomer phase at normal cooling rates u npracticable. [S0163-1829(99)10825-7].