Cluster structure of ultrahard fullerite revealed by Raman spectroscopy

We have discovered Raman peculiarities of sp3-bonded carbon nanoclusters forming ultrahard fullerite synthesized at pressure up to 85 GPa. The nanoclusters differ in the values of bulk modulus B0 ranging from 580 to 730 GPa. We found that the Raman frequency, corresponding to the C-C stretching mode of the nanoclusters, varies linearly from 1485 to 1640 cm−1, depending on the excitation wavelength used, which ranges from 785 nm to 257 nm. In particular, the nanoclusters with higher B0 have a higher Raman frequency. We also observed a resonant Raman spectrum when excited at a wavelength of 1064 nm. This showed a plateau from 1400 to 1720 cm−1, covering all Raman band positions of the nanoclusters from 1485 to 1640 cm−1. In addition, two Raman bands at 1285 and 1600 cm−1 appear at the resonance. The presence of several types of carbon sp3-bonded nanoclusters is confirmed by high-resolution transmission electron microscopy. Using moment tensor potentials, a class of machine learning interatomic potentials, we calculated the phonon density of states of the nanoclusters formed during the 3D polymerization of C60. The presence of high frequency modes in the resonant Raman spectra suggests the existence of bonds with force constants higher than those in a single crystal diamond. In addition, we observed a stiffening effect when analyzing the dependence of the mechanical stiffness on the cluster size, with smaller clusters exhibiting a more rigid structure.