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

The earlier high-pressure study of C60 concentrated on the orientational 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 700 K. A new phase region is outlined herein. The new phase is retained to ambient conditions and preliminarily characterized. The new phase is more dense than the initial C60 by at least 8.5% and less compressible 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 effect of approximately 23 J g-1. The nature and the thermodynamic stability of the new phase are discussed in connection with polymerization of C60 molecules, which seems likely.