Powder x-ray diffractometry was used to study the effect of intercalating C60 fullerite with helium and argon atoms on the fullerite structure. The samples were saturated at room temperature and normal pressure. The dependences obtained for the lattice parameter and half-width of certain reflections on the intercalation time with helium atoms showed that the voids in the C60 lattice were filled in two stages. Helium filled the octahedral voids relatively rapidly first and then the tetrahedral subsystem, but much more slowly. Both intercalants affected the half-width and intensity of the reflections, the matrix lattice parameters, the phase transition temperature, the volume jump at the transition, and the thermal expansion coefficients of C60 fullerite.

No abstract

The influence of hydrogen sorption in fullerite C60 at pressure of 30 atm and saturation temperature 150–380 °C on its structural and thermodynamic properties was studied using x-ray powder diffraction and photoluminescence methods. The kinetics of hydrogen sorption at different temperatures was studied by monitoring the time dependence of the fullerite lattice parameter. It was found that the sorption mechanism undergoes change upon the temperature increase. Diffusion-controlled filling of the cavities in fullerite lattice with hydrogen molecules at temperatures T ≤ 250 °C is replaced by chemical interaction between hydrogen and fullerene molecules at higher temperatures, resulting in the formation of a new molecular material: hydrofullerite C60Hx. It was established that the transition from physisorption to chemisorption of hydrogen by fullerite (adsorption crossover) occurs in the temperature range 300 °C > T > 250 °C. The hydrogenation of C60 was shown to dramatically increase the volume of the C60 cubic cell, decrease the thermal expansion of the crystals, and suppress the orientational transition and formation of a glass state. The maximum deformation levels of fcc lattice as well as the characteristic times of voids filling and hydrogenation were determined for the cases of diffusion-controlled voids filling and chemisorption, respectively.

Intercalation of C 60 single crystals with helium was studied by powder x-ray diffractometry. It was established that the intercalation is a two-stage process, octahedral cavities are filled first and then tetrahedral ones, the chemical pressure being negative during both stages. For the first time low-temperature (5 K) photoluminescence spectra of helium-intercalated fullerite C 60 were studied. The presence of helium in lattice voids was shown to reduce that part of the luminescent intensity which is due to the emission of covalently bound pairs of C 60 molecules, the so-called «deep traps» with the 0-0 transition energy close to 1.69 eV. The mechanism of the effect of the intercalation with helium on the pair formation in fullerite C 60 is discussed.