Fullerene glasses

We present the phase diagram of (C₆₀)_1-x(C₇₀)._x alloys, obtained from low frequency (1Hz) elastic and thermal expansion measurements. Whereas thermal expansion measurements show only a. fcc to sc phase transition in C₆₀-rich and a hcp to a distorted hcp phase transition in C₇₀-rich samples, low-frequency (1 Hz) elastic compliance measurements show an additional dynamic anomaly for all the samples (except for the pure C₇₀) due to freezing of the molecular motion. This anomaly is centered around T_g≈ 120 K for the pure C₆₀ and rises with rising concentration of C₇₀, reaching T_g ≈ 150 K for mixtures with 95mol% C₇₀. For C₇₀-rich samples with more than 10mol% C₆₀ no orientational phase transition was found. The phase diagram can be described in terms of the orientational glass model.