Quantum effects in the low-temperature thermal expansion of fullerite C₆₀ doped with a ⁴He impurity

The thermal expansion of fullerite C-60 doped with a He-4 impurity at T similar to 2 K has been investigated by the method of low temperature precision dilatometry in the interval T = 2.2-24 K. The character and the derived values of the thermal expansion coefficients were strongly dependent on the concentration of the He-4 impurity in fullerite. In the interval T = 2.2-5 K the thermal expansion of the He-4(x)-C-60 system is negative, which is attributed to the tunnel movement of the He-4 atoms in the cavity system of the C-60 crystal lattice and at the crystallite surface. The contribution of this process to the thermal expansion decreases as the impurity (He-4) concentration increases and the probability of He-4 tunneling between the crystal lattice cavities and the impurity-free areas of the grain surface diminishes. In the temperature interval T = 4.5-24 K the thermal expansion of the He-4(x)-C-60 system is influenced predominantly by the mutual transformations of different orientation glass modifications of fullerite. Owing to their tunnel character the transformations make a negative contribution to the process of thermal expansion entailing a hysteresis and other anomalies observed in this temperature interval. The intensity of the processes provoked by the tunneling-encouraged phase transformations of the orientational glasses of C-60 increases with the He-4 concentration in fullerite. Published under an exclusive license by AIP Publishing.