FULLERENE C-60 UNDER THE INFLUENCE OF HIGH-PRESSURE TOGETHER WITH HIGH-SHEAR STRESSES - HOW TA SCRATCH DIAMOND

Crystalline fullerene C-60 has been studied at room temperature in an optical diamond anvil cell, which allows the development of high press ures up to 40 GPa together with high shear stresses, by rotating one a nvil with respect to the other. Three transformations were observed at (2.3+/-0.3), (6.0+/-0.5) and (18+/-2) GPa. The first transformation i s reversible and leads to a state which we suggest to be a crystalline phase III, which is missing at low temperature and atmospheric pressu re. The other two transformations are irreversible, and Raman and elec tron microscopy measurements performed at atmospheric pressure show th at the obtained states 3 and 4 are amorphous. State 3 reverts to usual fee C-60 under the influence of the electron beam, whereas state 4 do es not. The main result is that the rotating anvil surface exhibits ci rcular traces which are scratches and plastic deformations due to stat e 4, which is therefore harder than diamond under our experimental con ditions.