PHASE-DIAGRAM, STRUCTURE, AND DISORDER IN C-60 BELOW 300 K AND 1 GPA

Earlier structural studies have shown that the pentagon-to-hexagon orientation ratio in the orientationally ordered simple cubic phase of C-60 decreases under pressure. From anomalies observed in the compressibility and thermal conductivity of C-60 under pressure we have deduced a pressure-temperature phase diagram for this substance in the range below 300 K and 1 GPa (10 kbar). We conclude that C-60 forms a nevi,completely ''hexagon'' ordered structural phase above about 0.6 GPa at 150 K (1.2 GPa at 300 K), end that the glass transition shifts upwards in T under pressure by 54 K GPa(-1). However, above 0.1 GPa, pentagon-to-hexagon orientation relaxation seems to occur on heating at an almost pressure independent temperature near 100 K.