Pressured-Induced Structural Metastability in Crystalline C60

We show that at hydrostatic pressures as low as 2 kbar the normal low temperature simple cubic phase of crystalline ${\mathrm{C}}_{60}$ is metastable with respect to a new structure which manifests itself through dramatic changes in the intermolecular and intramolecular Raman spectrum. The new phase appears when the sample temperature is raised above the orientational glass transition temperature. Once formed, it remains metastable with respect to the normal simple cubic phase down to atmospheric pressure if the temperature is held below 90 K. The microscopic origin for this new phase is discussed.