Low temperature thermal expansion of pure and inert gas-doped Fullerite C60

The low temperature (2-24 K) thermal expansion of pure (single crystal and polycrystalline) C60 and polycrystalline C60 intercalated with He, Ne, Ar, and Kr has been investigated using high-resolution capacitance dilatometer. The investigation of the time dependence of the sample length variations on heating shows that the thermal expansion is determined by the sum of positive and negative contributions, which have different relaxation times. The negative thermal expansion usually prevails at helium temperatures. The positive expansion is connected with the phonon thermalization of the system. The negative expansion is caused by reorientation of the C60 molecules. It is assumed that the reorientation is of quantum character. The inert gas impurities affect very strongly the reorientation of the C60 molecules especially at liquid helium temperatures. A temperature hysteresis of the thermal expansion coefficient of Kr- and He- C60 solutions has been revealed. The hysteresis is attributed to orientational polyamorphous transformation in these systems.Comment: 18 pages, 12 figure

The effect of the thermal reduction on the kinetics of low-temperature 4He sorption and the structural characteristics of graphene oxide

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Low Temperature Physics 43, 383 (2017) and may be found at https://doi.org/10.1063/1.4979362The kinetics of the sorption and the subsequent desorption of 4He by the starting graphite oxide (GtO) and the thermally reduced graphene oxide samples (TRGO, Treduction = 200, 300, 500, 700 and 900 °C) have been investigated in the temperature interval 1.5–20 K. The effect of the annealing temperature on the structural characteristics of the samples was examined by the x-ray diffraction (XRD) technique. On lowering the temperature from 20 to 11–12 K, the time of 4He sorption increased for all the samples, which is typically observed under the condition of thermally activated diffusion. Below 5 K the characteristic times of 4He sorption by the GtO and TRGO-200 samples were only weakly dependent on temperature, suggesting the dominance of the tunnel mechanism. In the same region (T < 5 K) the characteristic times of the TRGOs reduced at higher temperatures (300, 500, 700 and 900 °C) were growing with lowering temperature, presumably due to the defects generated in the carbon planes on removing the oxygen functional groups (oFGs). The estimates of the activation energy (Ea) of 4He diffusion show that in the TRGO-200 sample the Ea value is 2.9 times lower as compared to the parent GtO, which is accounted for by GtO exfoliation due to evaporation of the water intercalated in the interlayer space of carbon. The nonmonotonic dependences Ea(T) for the GtO samples treated above 200 °C are determined by a competition between two processes—the recovery of the graphite carbon structure, which increases the activation energy, and the generation of defects, which decreases the activation energy by opening additional surface areas and ways for sorption. The dependence of the activation energy on Treduction correlates well with the contents of the crystalline phase in GtO varying with a rise of the annealing temperature.Financial support from Spanish Ministry MINECO and the European Regional Development Fund (Project No. ENE2013-48816-C5-5-R), the Regional Government of Aragon and the European Social Fund DGA-ESF (Project No. T66) and Targeted Comprehensive Fundamental Research Program of NASU (Project No. 6/16-H) is gratefully acknowledged.Peer reviewe

PHOTOLUMINESCENCE AND STRUCTURE OF C60C_{60} INTERCALATED WITH HELIUM

Author Institution: Verkin Institute for Low Temperature Physics and Engineering; Department of Mechanical Engineering, Northwestern UniversityPowder x-ray diffractometry was employed to study infusion of He into $C_{60}$ fullerite. It has been shown that the intercalation at a pressure of 1 Bar is a two-stage process, the first stage being the saturation of the octahedral voids, virtually complete after 55 hr. Photoluminescence spectra were taken at 5 K from $C_{60}$ with completely saturated octahedral voids. Helium in the lattice voids is shown to reduce that part of the luminescent emission which is due to 0-0 transitions around 1.69 eV from the so-called deep traps, or according to existing notions, the covalently bound pairs of $C_{60}$ molecules. The effect of He intercalation on polymeric dimer formation is ascribed to the changes in the pentagon to hexagon configuration ratio caused by the intercalation-related increase of the lattice parameter and the formation of bound states of He atoms in the $C_{60}$ lattice voids