Mechanical properties of C60 single crystals

The Vickers microhardness HV of C60 single crystals grown from the vapor phase in vacuum has been measured in the temperature range 77-300 K. The temperature dependence of HV manifests two distinct features: a step-wise change in the microhardness of about 30% in the temperature region of the face-centered cubic-simple cubic (fcc → sc) transition (Tc ≈ 260 K) and a kink in the HV(T) dependence at T0 ≈ 180 K. The influence of structural defects, environmental factors (light, atmosphere), and helium intercalation on the microhardness was investigated. Besides, the crystals were subjected to compressive deformation along 〈1 1 0〉 at room temperature. The yield stress was about 2.85 MPa and the activation volume determined by the stress relaxation technique was about 60b3. It is assumed that the dislocation glide is controlled by the local high-density defects or by the lattice potential relief. The fracture of crystals was often caused by the splitting along the {1 1 1} planes, which are the cleavage planes in fullerite C60.