Pressure-temperature phase diagram of the rotor-stator compound C60-cubane

When the rotor-stator compound C60⋅C8H8 is treated at temperatures above about 450 K under high-pressure conditions, fullerene-based copolymers are formed. Earlier results have shown the existence of two types of polymers, a “low-pressure” pseudo-cubic copolymer similar to that formed at atmospheric pressure, and a “high-pressure” phase tentatively identified as a pseudo-orthorhombic copolymer structurally based on the known low-temperature phase. In this communication we show that the reaction boundary between the polymers occurs at pressures near 1 GPa. From this result it follows that the slope of the cubic-orthorhombic phase line in the pristine material is 305 K GPa−1, significantly larger than the slope observed for the corresponding rotational transition in pure C60.