N. N. Galtsov

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.

Intercalation of fullerite C60 with N2 molecules. An investigation by X-ray powder diffraction.

The lattice parameter a of fullerite C-60 intercalated with N-2 molecules is investigated in the temperature interval 6-295 K by x-ray diffraction. It is found that the interstitial molecular N-2 h ...

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...

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.

The effect of molecular impurities CO and CH4 on the structural characteristics of the C60 fullerene around the orientational phase transition

X-ray studies of structural characteristics of С60(С)0.9 and С60(CH4)0.5 solid solutions were carried out in around the orientation phase transition in the C60 fullerene. It was established that the filling of octahedral cavities of the fullerene lattice by molecules of CO and CH4 results in a significant increase of the lattice parameter and the thermal expansion coefficients of the ordered phase and in a decrease in the volume change and phase transition temperature TC. In this case there also occurs a noticeable “smearing” of the phase transformation. Based on these results and on the data in the literature contradictions in the conclusions about the structural peculiarities of the formation of a glass state in the CO–C60 solutions were analyzed and discussed. Linear dependences of orientational transition temperature TC and glass transition temperature Tg on CO concentration in the octahedral cavities were obtained. Concentrations of CO in the previous study of specific heat of solid mixtures were cal...

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 ...

X-ray studies of phase transitions in solid oxygen

X-ray studies of the phase composition, lattice parameters, and molar volumes are carried out at the phase transitions in solid oxygen. In the region of the low-temperature transition a two-phase state is observed in a region having a width of 2 K on cooling of the samples and less than 0.8 K on heating. The width of the temperature hysteresis and the jump in volume at the transition are determined. The mean value of the volume jump is ∼0.5±0.2%. The structural characteristics found are convincing evidence that the low-temperature magnetic transformation in solid oxygen, like the high-temperature orientational transition, is a first-order transition. It is shown that at relatively high rates of passage through the β–γ transition, owing to the significant jump in volume and the high stress level, it is possible to supercool the γ phase down to helium temperature and to superheat the intermediate β phase almost to the melting temperature.

Structure of the solid phases of SiH4

X-ray studies of polycrystalline samples of monosilane SiH4 are done in the existence regions of the low-temperature and high-temperature phases with the use of photometric and diffractometric methods. It is found that the unit cells of both phases have monoclinic symmetry. The phase transition from the low-to the high-temperature phase is accompanied by an appreciable volume jump ΔV∕V=1.9% and a change of the number of molecules in the cell from 4 to 32. The unit cell in the low-temperature phase (at T=6K) has parameters a=8.198±0.002A, b=4.601±0.001A, c=7.364±0.002A, β=90.32±0.08°, volume V=41.85cm3∕mole, and density 0.78±0.05g∕cm3. The high-temperature phase is substantially orientationally disordered, containing Z=32molecules per cell and has the parameters a=13.660±0.003A, b=12.767±0.003A, c=13.333±0.003A, β=93.66±0.06°, volume V=43.67cm3∕mole, and density 0.735g∕cm3 at T=65K. The structural and a number of physical characteristics of silane are compared with the corresponding properties of other cry...

Structure of quench condensed nH2–N2 binary alloys: isotope effect

The structure of quench condensed nH2–N2 alloys is investigated by powder x-ray diffraction over a wide range of temperatures, compositions, and rates of deposition. The structure of the deposits is shown to depend on the condensation regime. Under a mild regime, no solid nitrogen reflections from hydrogen-rich deposits are observed. When the temperature is increased to the hydrogen triple point, nitrogen lines appear at substantially larger angles compared to pure N2, which suggests presence of dissolved hydrogen. The quantum nature of this hydrogen is suppressed in the nitrogen matrix. The samples grown in the mild regime exhibit the highest apparent solubility (up to 25%) of hydrogen in solid N2. When H2 is substituted by D2, the isotope effect consists in a wider range of single-phase states based on the nitrogen cubic lattice, which can accommodate up to 70% deuterium, if grown under the mild regime.

Structural and high-frequency (0–110 MHz) resistive characteristics of MgB2 in the temperature range 5–300 K

The structure and resistivity of polycrystalline MgB2 and its impedance at frequencies of 9–110 MHz are determined in the temperature range 5–300 K. It is shown that the lattice type and symmetry of the superconducting phase of MgB2 remain unchanged over this temperature range. At the superconducting transition temperature Tc=39.5 K a structural instability is observed which is accompanied by an overshoot of the measured lattice parameters. It is concluded that the strain of the crystals upon a change in temperature is of a substantially anisotropic character. Measurements of the temperature and frequency dependence of the surface resistance Rs(T,f ) in the superconducting state reveal a transition from the Pippard nonlocal limit at T≪Tc to the London local limit near Tc. At T/Tc<0.76 the value of Rs(T) is well described by an exponential dependence exp(−Δ(T)/kT) in accordance with the BCS theory.