Hardening of fullerite C60 during temperature-induced polymerization and amorphization under pressure

We report a detailed study of Vicker's hardness and ultrasonic elastic moduli for carbon phases prepared by heating fullerite C-60 at pressures of 3.5, 5, and 8 GPa. It is shown that the transformation of two-dimensional C-60 polymers into graphite-like amorphous carbon is accompanied by an increase in hardness of 100-200 times, as well as an increase in bulk and shear moduli by 4-5 and 2-3 times, respectively, with no density jump taking place. It is proposed that the high hardness (up to 40 GPa) of the disordered phases synthesized is caused by the three-dimensional ordering of sp(2)-based network. It was found that, in the 3.5-8 GPa interval, the mechanical properties of the phases obtained depend basically on the temperature rather than on the pressure of synthesis. (C) 2000 American Institute of Physics. [S0003-6951(00)01706-X].