Mechanism of three-dimensional polymerization of fullerite C-60 at high pressures

0A series of polycrystalline phases corresponding to different stages of three-dimensional polymerization and destruction of C-60 molecules has been synthesized by heating fullerite C-60 under a pressure P=12.5 GPa. The structure of the phases can be identified as fee, and the lattice period decreases with increasing heating temperature. A model of three-dimensional polymerization in which the lattice parameter is a continuous function of the fraction of covalently bonded molecules is proposed. The model makes it possible to estimate the number of atoms in the sp(3) state. The hardness of the polymerized fee phases is studied on the basis of percolation of rigidity. It is shown experimentally that the period a approximate to 13.8 Angstrom is the threshold for the formation of a three-dimensionally rigid C-60 polymer. It is found that the thermal stability of the strongly and weakly polymerized phases is qualitatively different. (C) 1996 American Institute of Physics.