Cluster structure and elastic properties of superhard and ultrahard fullerites

Velocities of the longitudinal and shear sound waves are measured in ultrahard fullerites created by static high-pressure-high-temperature treatment under P=13 GPa and T=1670–1870 K. The highest value of 26.0 km/s for the longitudinal waves is measured, that is about 40% more than in diamond. Bulk modulus of different ultrahard fullerites covers the range of about 600–1700 GPa. The highest hardness is about 30×103 kg/mm3. We ascribe these unique properties to formation of 20–30 atoms clusters by the walls of adjacent molecules under process of 3D-cross-linking. Most distinctly these clusters declare themselves in the cubic structure with the lattice parameter about 6 A and 32 atoms per unit cell.