The earlier high-pressure stndy of Cw concenmted on the orientational transition and mom-temperature transformations observed in diamond anvils. This work presents a parl of the T-P phase diagram for solid Cw at pressures up to 20 kbar in the temperahlre 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 Cm by al least 8.5% and less compressible by -1.5 times. Its crystal svucmre is very different from the well known face-centred cubic and simple cubic phases, and its recovely to a cubic phase is associated with an endothermal heat effect of -23 I g-', The nature and the lhemcdynamic stability of the new phase are discussed in connection with polymerization of & molecules, which seem likely.

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The temperature dependence of the attenuation and sound wave velocity of compacted pure C,, fullerite is measured in the temperature interval from 100 to 300 K at a frequency of about 5 MHz. Two internal friction peaks are observed at 250 and 215 K. The conclusion is made that the internal friction peak at 250 K is connected with the structural phase transition and the peak a t 215 K is due to the relaxation by reorientation of C 6 , molecules in the stress field of ultrasonic waves. The magnitudes of the elastic moduli of C,, fullerite are determined by sound velocity measurements. The results are compared with theoretical estimations of the elastic moduli and experimental data of other authors.

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