Specific heat of rhombohedral C60 polymer in the temperature range of 2-300K

Under high temperature of 700 K and high pressure of 6 GPa, we have prepared a batch of C60 polymer. XRD data confirmed it is rhombohedral phase and solid 13C NMR showed a formation of sp3 bond between two neighbor C60 in (111) plane. We have measured the specific heat of C60 polymer and pristine C60 by PPMS in the range from 2 to 300 K. The experimental result of pristine C60 agreed well with previous report. For C60 polymer, above T=80 K it is found that temperature dependence of the specific heat is similar to that of pristine C60 besides an anomaly from order-disorder phase transition at 260K, but in range from 2 to 80K the specific heat is much less than that of pristine C60. Assuming three- (3D) and two-dimensional (2D) Debye phonon modes to contribute respectively to the specific heat in different temperature zone, the calculated values of specific heat have got a good agreement with the experimental data in the whole temperature range. These results show the 2D planar modes but not 3D modes are a dominator to the specific heat of C60 polymer, and the low-frequency intermolecular modes of C60 lattice are restrained in the case of C60 polymer by sp3 bonds from 2+2 cycloaddition reaction.