Yu. E. Stetsenko

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.

Hydrogen absorption and desorption kinetics in fullerite C60 single crystals. Low-temperature micromechanical and structural characteristics of the interstitial solid solution C60(H2)x

The microhardness HV and lattice parameter a of C60 single crystals are measured at room temperature as functions of the hydrogen saturation time t for several values of the saturation temperature (250, 300, and 350°C) at a fixed hydrogen pressure p=30atm. According to the measurements of HV and a, the kinetics of hydrogen absorption is described by a simple exponential law with a single, temperature-dependent characteristic time. In highly saturated samples the microhardness is 4 times greater than for the initial C60 crystal, while the lattice parameter is 0.2% larger. The temperature dependence of the microhardness HV and lattice parameter a of C60(H2)x crystals is investigated in the temperature interval 77–300K. The introduction of hydrogen lowers the temperature of the fcc–sc phase transition, and the transition becomes strongly broadened in temperature. The dependence of the microhardness of the saturated sample on the hold time in air at room temperature is described by the sum of two exponentials...

Orientational glassification in fullerite C60 saturated with H2: Photoluminescence studies

Using one-photon excitation we studied photoluminescence of C60 saturated with molecular hydrogen over a temperature range from 10 to 230 K. Saturation of samples was done at 30 atm and at temperatures low enough (T < 250 °C) to exclude chemical sorption. The samples were saturated during periods of varied duration τ to reach different occupancy levels. To check the reliability of our luminescence results and their interpretation, our spectra for pure C60 were compared with data known in the art, demonstrating good compatibility. The luminescence spectra were attributed according to the approach of Akimoto and Kan’no by separating the total spectra into two components of different origin. The A-type spectra, associated with exciton transport to deep traps, become prevalent over the B-type emission above 70 K. The integrated intensity I as a function of the temperature T of the luminescence measurements I(T) remained at a constant level up to the orientational vitrification point of about 100 K when the sa...

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 ...

Interlayer Mn–Mn exchange parameter in MnPS3 from x-ray diffraction data

The interlayer distance in MnPS3 was measured by x-ray diffraction as a function of the temperature in the vicinity of the Neel point of 78 K. A well detectable magnetic striction of about 0.0185% has been documented. Using the known values of the external-pressure driven compression and reasonable estimates of the range parameter of the separation dependence of the relevant Mn–Mn exchange parameter J′, we have estimated J′ to be about 1.0 K. From our analysis of the interlayer magnetic coupling, an inference is drawn that the Mn–Mn interlayer exchange can be strongly directional.

Specific features of the glass transition in C60 fullerite saturated with carbon monoxide molecules: Photoluminescence studies

Low temperature (20–230 K) spectral-luminescence studies were conducted on C60 fullerite saturated with carbon monoxide in a physisorption regime. Substantial changes in the photoluminescence characteristics of C60–CO solutions with different impurity concentrations were found already for short intercalation times. Strong dependence of the CO solubility on the saturation temperature was revealed by analyzing the contribution of “deep X-traps” to the luminescence. Furthermore, it was found that filling of the octahedral voids by CO molecules occurs with a lower gradient of the impurity distribution into the bulk C60 crystals as compared with N2. The temperature dependences of the integral emission intensity for the samples with different concentrations of carbon monoxide were studied. For the first time, using the spectral-luminescence method, it was revealed that CO molecules, in contrast to H2 and N2, exhibit a significant effect on the formation of the orientational glass and the rotational dynamics of ...