I. V. Legchenkova

Hysteretic phenomena in Xe-doped C60 from X-ray diffraction

Polycrystalline fullerite C60 intercalated with Xe atoms at 575 K and a pressure of 200 MPa was studied by powder x-ray diffraction. The integrated intensities of a few brighter reflections have been utilized to evaluate the occupancy of the octahedral interstitial sites in C60 crystals, which turned out to be (34±4)%, and in good agreement with another independent estimate. It is found that reflections of the (h00) type become observable in Xe-doped C60. The presence of xenon in the octahedral sites affects both the orientational phase transition as well as the glassification process, decreasing both characteristic temperatures as well as smearing the phase transition over a greater temperature range. Considerable hysteretic phenomena have been observed close to the phase transition and the glassification temperature. The signs of the two hysteresis loops are opposite. There is reliable evidence that at the lowest temperatures studied the thermal expansion of the doped crystal is negative under cool-down.

Structure and photoluminescence of helium-intercalated fullerite C60

The intercalation of C60 single crystals with helium is studied by powder x-ray diffractometry. It is 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. The low-temperature (5 K) photoluminescence spectra of helium-intercalated fullerite C60 are studied for the first time. The presence of helium in lattice voids is shown to reduce that part of the luminescent intensity which is due to the emission of covalently bound pairs of C60 molecules, the so-called “deep traps” with the 0–0 transition energy close to 1.69 eV. The mechanism of the effect of intercalation with helium on the pair formation in fullerite C60 is discussed.

X-ray studies of the effects of intercalation of C60 fullerite crystals by Ne atoms

The process of intercalation of polycrystalline C60 fullerite by Ne atoms at room temperature and a gas pressure of 1atm and the temperature dependence of the lattice parameter in the temperature interval 30–293K are investigated by an x-ray method. It is established that the occupation of the octahedral interstitial cavities of the fcc lattice of C60 by neon is accompanied by a noticeable increase in the lattice parameter and volume. These changes as a function of saturation time are described satisfactorily by an exponential law. The intercalation time constant τ is determined to be 179±24h, which is almost two orders of magnitude longer than the time τ obtained at increased pressure. The maximal saturation of fullerite by neon reaches nearly (48±3)%. The presence of a neon impurity in the C60 lattice has practically no influence on the intensity of the x-ray scattering, but as a consequence of the nonuniformity of the impurity distribution in the first stage of intercalation it appreciably alters the w...

Process of intercalation of C60 with molecular hydrogen according to x-ray diffraction data

The process of normal hydrogen infusion into a C60 powder at 1bar and room temperature is monitored using x-ray diffraction. The effect of the intercalation on the lattice proves to be rather weak: the volume expansion upon complete saturation does not exceed 0.13%. The characteristic saturation time is found to be 320h; the corresponding diffusion coefficient amounts to (2.8±0.8)×10−14cm2∕s. The integrated reflection intensity calculations for a completely saturated sample suggest that only octahedral voids are filled under the conditions of the experiment. Complete saturation has only a weak effect on the rotational subsystem of the C60 fullerite: the orientational phase transition shifts by 6to7K to lower temperatures; no substantial hysteresis is noticed. The dopant shows reluctance to leave the sample under a vacuum of 10−3Torr at room temperature.

Dynamics of He atoms adsorbed on a carbon nanotube

The spectrum of a helium atom adsorbed on an armchair single-walled carbon nanotube is calculated. It is shown that a helium adatom tunnels over the surface of the nanotube within a band of width 10 K (for 4He) to 14 K (for 3He). The undersaturated system of helium adatoms on nanotubes can be used for studying quantum transport. The dynamics of this system can also be an important contributor to the large negative expansion coefficients of nanotube bundles with adsorbed helium.

Saturation of fullerite C60 with hydrogen: Adsorption crossover studies

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 cu...

Effect of nitrogen sorption mechanisms on the properties of fullerite C60 over a wide range of temperatures

X-ray diffractometry is used to study the effect of the adsorption of nitrogen at a pressure of 30 atm and temperatures of 200–550 °C on the structural and thermodynamic properties of fullerite C60. The sorption kinetics of nitrogen at different temperatures are studied, and the lattice parameter is plotted as a function of the time for fullerite to saturate with nitrogen. The sorption mechanism is found to change with increasing saturation temperature. The diffusive filling of lattice octahedral voids by nitrogen atoms at temperatures below 450 °C is supplanted at higher temperatures by a chemical interaction of nitrogen with fullerite molecules leading to the formation of a new molecular compound, fullerite nitride C60Nx. The transition from physisorption to chemisorption of nitrogen by fullerite (the adsorption crossover) takes place at saturation temperatures of 450 > T > 400 °C. When C60 molecules are nitrogenated, the volume of the cubic cell increases dramatically, while the intensity of the x-ray ...

Effect of impurity oxygen molecules on the structural and thermodynamic properties of fullerite C60

Intercalation of fullerite C60 with oxygen molecules is performed by two methods—under ordinary conditions of saturation at room temperature and gas pressure 1atm as well as at elevated temperature (373K) and pressure (10atm). The time dependences of the lattice parameter of fullerite during intercalation are determined. This made it possible to evaluate the activation energy and the coefficient of diffusion of oxygen molecules in fullerite C60. It is shown that increasing the temperature and pressure of the gas increases the diffusion coefficient almost five-fold: from D≈(1.6±1.9)⋅10−14cm2∕s at atmospheric pressure and room temperature to D≈(7.1±1.9)⋅10−14cm2∕s for pressure P=10atm and temperature T=373K. The oxygen concentration in octahedral voids and the temperatures of phase transitions in solutions are determined on the basis of an analysis of the results obtained together with published data on the effect of atomic and molecular impurities on the properties of C60. The effect of oxygen impurity on ...

Novel mechanism of the negative thermal expansion of doped fullerite C60

A model of an atomic impurity in an octahedral void of fullerite C60 is suggested. The problem is solved in the spherical oscillator approximation, which is appropriate for the larger rare gas atoms. It is shown that such impurities can contribute to the negative thermal expansion at low temperatures and produce a Schottky-like maximum at higher temperatures.

Influence of CO molecular impurity on the structural and thermodynamic properties of fullerite C60, in a broad range of sorption temperatures

An x-ray diffraction study of how sorption of CO gas at a pressure of 30 atm in the temperature range of 150–600 °C influences the structural characteristics of polycrystalline and single crystal fullerite C60. The sorption kinetics are studied by constructing a dependence of the lattice parameter on the time it takes for fullerite to be saturated by CO molecules. At temperatures Tsorb > 300 °C there is an observed dissociation of carbon monoxide, accompanied by the precipitation of carbon powder and the chemical interaction of atomic oxygen with C60 and CO molecules, and possibly with the carbon condensate. These processes have a strong influence on the structural characteristics of fullerite, thus creating, in part, a nonmonotonic dependence of the parameter and lattice matrix volume on the impurity saturation temperature. The concentrations of solid solutions C60(CO)x poly- and single crystal samples are determined in the physisorption range for two modes (150 and 250 °C). It is found that the CO impur...