Kinetics of ⁴He gas sorption by fullerite C₆₀. Quantum effects

Abstract

The kinetics of helium gas sorption by a C₆₀ powder and subsequent desorption of the ⁴He impurity from the saturated powder has been investigated in the temperature interval 2–292 K. Evidence is obtained that supports the existence of two stages in the temperature dependences of sorption and desorption. The stages account for the different times taken by helium to occupy the octahedral and tetrahedral interstices in the C₆₀ lattice. The characteristic times of sorption and desorption coincide. It is found that the temperature dependences of the characteristic times of occupying the octahedral and tetrahedral interstices are nonmonotonic. As the temperature is lowered from 292 to 79.3 K, the characteristic times increase, which indicates a predominance of thermally activated diffusion of helium in C₆₀. On a further decrease to T = 10 K the characteristic times reduce over an order of magnitude. Below 8 K the characteristic times of sorption and desorption are temperature-independent. This suggests a tunnel character of ⁴He diffusion in C₆₀.