A NEW PHASE-TRANSITION IN THE T-P DIAGRAM OF C-60 FULLERITE

The earlier high-pressure study of C60 concentrated on the orientation al transition and room-temperature transformations observed in diamond anvils. This work presents a part of the T-P phase diagram for solid C60 at pressures up to 20 kbar in the temperature range extended to 70 0 K. A new phase region is outlined herein. The new phase is retained to ambient conditions and preliminarily characterized. The new phase i s more dense than the initial C60 by at least 8.5% and less compressib le by approximately 1.5 times. Its crystal structure is very different from the well known face-centered cubic and simple cubic phases, and its recovery to a cubic phase is associated with an endothermal heat e ffect of approximately 23 J g-1. The nature and the thermodynamic stab ility of the new phase are discussed in connection with polymerization of C60 molecules, which seems likely.