M. A. Strzhemechny

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.

Quench deposited Kr–H2 and Ar–H2 mixtures: in quest of impurity–hydrogen gels

The structure and morphology of low-temperature quench condensed binary alloys of hydrogen with argon and krypton were studied by powder x-ray diffraction. The nominal hydrogen fraction c in both systems was varied from 0 to 50%; the condensation was performed at 5–6 K; both as-prepared and annealed samples were examined by x-ray diffraction. Few reflections (and often only one) can be unambiguously detected for the as-grown alloy samples. In Kr–H2 condensates with c<10% the x-ray patterns show fine-grain krypton-rich crystallites with a rather high actual hydrogen content as estimated from Vegard’s law. For high nominal hydrogen fractions (c⩾10%) no reflections attributable to the krypton lattice were recorded and the incoherent background showed no characteristic swelling around the position of the (111) reflection from pure Kr; instead, the reflections from a hydrogen-rich hcp phase were distinct. As the temperature was steadily raised the hydrogen reflections disappeared first. Then at a certain tempe...

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...

Photoluminescence of ortho-bromobenzophenone

Phosphorescence spectra of crystalline ortho-bromobenzophenone (2-bromobenzophenone, 2BrBP) were measured from 1.6K to room temperature. A cardinal emission mechanism crossover occurs within this temperature range. At low temperature the phosphorescence spectrum is a superposition of two similar sets of equidistant bands spaced by the CO stretch frequency. We ascribe these two sets to the emission of two different conformers, one of which is metastable and does not manifest itself above approximately 70K. The presence of two conformers is explained by the fact that the 2BrBP molecule deforms considerably upon excitation. The emission from the stable conformer survives up to approximately 140K or even higher. At roughly 60K and higher, another type of emission reveals itself in the shape of two very broad, partly overlapping bands, which gains in intensity with increasing temperature. Above 150K only this two-hump feature is observed in phosphorescence spectra. Additional phosphorescence experiments were ...

Properties of solid hydrogen doped by heavy atomic and molecular impurities

The structural characteristics of normal and para hydrogen crystals doped with Ar, Kr, N2, and O2 impurities are studied by powder x-ray diffraction over the range from 5 K to the melting point of the hydrogen matrix. It is established that in spite of the very low solubility of the dopants in solid hydrogen, these impurities appreciably affect the structural characteristics. In particular, only nitrogen impurities do not change the molar volume of the matrix; the other three make the matrix expand. The Ar and Kr impurities also change the c/a ratio of the hcp matrix. The fact that both Ar and O2 have smaller molar volumes than hydrogen may be regarded as evidence that these impurities form van der Waals complexes with the hydrogen lattice environment.

Structure, lattice parameters, and thermal expansion anisotropy of C70 fullerite

X-ray diffraction studies of the two low-temperature phases of especially pure polycrystalline C70 fullerite are carried out in the temperature range 20–310 K. At room temperature a rhombohedral structure is established, and at T<276 K a monoclinic structure. The volume jump at the low-temperature phase transition is determined to be ∼8.5 cm3/mole, or 1.7%. The temperature dependence of the lattice parameters and of the linear and volume thermal expansion coefficients is investigated in the two phases. For both the intermediate and the low-temperature modifications a substantial anisotropy of the thermal deformation of the crystals along individual crystallographic directions is observed. The results are analyzed with the use of previous structural studies. It is shown that in the absence of rotations at low temperatures the lattice is unstable to monoclinic distortions, the estimated sign and order of magnitude of which agree with the observations.

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.