Sorption studies of helium and neon by crystals of C60 and C70

Abstract

We present sorption measurements for3He and4He in the temperature range of 1.5 K to 4.1 K, and for20Ne in the temperature range of 22 K to 27 K by crystals of C60, C70 and crystals of the mixture of these two molecules, 80% C60, 20% C70 We analyze these data by taking into account the non-ideality of the gas in equilibrium with the adsorbate. We calculate chemical potentials and isosteric heats. We find that there is no obvious evidence of intercalation of helium in these crystals at low temperatures. At higher temperatures there are some anomalies in the helium isotherms, and indication of excess sorption. The isosteric heat shows a minimum in this region which can be interpreted as penetration of the helium into a region of repulsive potential. We also find that levels of sorption, at the same chemical potential difference from saturation, are higher for4He than for3He. They are also higher for4He on C70 than for the other crystals. For neon our work is concentrated around the triple point. We find that the isotherms indicate the formation of liquid or solid films. Below the triple point, and above a few atomic layers, the neon film does not grow uniformly.