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

Structure of quench condensed nH₂–N₂ alloys was 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 mild regime, no solid nitrogen reflections from hydrogen-rich deposits were 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 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 mono-phase states based on the nitrogen cubic lattice, which can accommodate up to 70% of deuterium, if grown under mild regime

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 the 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 the x-ray diffraction. Few, often only one reflection can be unambiguously detected for the as-grown alloy samples. In the Kr-H₂ condensates with c < 10%, the x-ray patterns show fine-grain krypton-rich crystallites with rather high actual hydrogen contents as estimated from Vegard`s law. At high nominal hydrogen fractions( %) c ≥ 10 , no the reflections attributable to the krypton lattice were recorded and the incoherent background showed no characteristic swelling around the position of reflection (111) from pure Kr but, instead, the reflections from a hydrogen-rich hcp phase were distinct. As the temperature was steadily raised, first the hydrogen reflections disappeared and then, at a certain temperature, the samples underwent an abrupt transformation, releasing heat and making the krypton component forms larger, x-ray detectable textured crystallites. In the as-grown Ar-H₂ samples, only (111) reflections from the argon-rich phase were recorded. Warmup led to the same consequences, viz., effusion of hydrogen and then recrystallization. In both systems, the recrystallization onset temperature depends substantially on the nominal hydrogen fraction in the gas. The shift of the lattice parameter in the as-grown argon-based phases suggests a strong suppression of the quantum nature of hydrogen in argon lattice environment. The entire set of the experimental findings can be treated as evidence that the quench-condensed hydrogen-containing alloys morphologically resemble helium-impurity solids (gels) whose structure and morphology are currently studied at Cornell University

Properties of solid hydrogen doped by heavy atomic and molecular impurities

Using powder x-ray diffraction we studied the structural characteristics of normal and para hydrogen crystals doped with Ar, Kr, N₂, and O₂ impurities over the range from 5 K to the melting point of the hydrogen matrix. It has been established that in spite of 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 treated as evidence that these impurities form van der Waals complexes with the hydrogen lattice environment

Interlayer Mn–Mn exchange parameter 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 Néel 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

Investigation of the low-temperature mechanical behaviour of elastomers and their carbon nanotube composites using microindentation

The micromechanical properties of epoxy resin elastomers and their carbon nanotube composites were studied using a microhardness tester equipped with low-temperature chamber. X-ray diffraction analysis indicated that all specimens were free of any crystalline components and were amorphous with only short-range order domains. The Vickers microhardness of all samples has been estimated in the temperature range 230–300 K. The measurements demonstrated that at room temperature these materials are elastomers (notably, they are in high-elastic state) and on cooling in the range of 250–270 K the glass transition takes place. Analysis of the temperature dependence of microhardness suggested that the thermomechanical and relaxation properties of the materials studied are consistent with a rheological model of a standard linear solid where the relaxation time (or viscosity) depends exponentially on the temperature in accordance with the Arrhenius equation for the rate of thermally activated process. Empirical estimates for the nonrelaxed and relaxed Young’s moduli and also for the activation energy (U = 0.75 eV) and the period of attempts (τ0 = 10–12 s) of the molecular process which determines the relaxation properties and the glasstransition of the materials have been obtained. The addition of carbon nanotubes into elastomeric epoxy resin had no effect on its micromechanical characteristics as measured by the microhardness tester. It is shown that the conventional microindentation method is an efficient tool of investigating the thermomechanical properties of elastomers nearby and below the glass transition temperature

Process of intercalation of C₆₀ with molecular hydrogen from XRD data

The process of normal hydrogen infusion into a C₆₀ powder at 1 bar and room temperature was monitored using x-ray diffraction. The effect of the intercalation on the lattice proved to be rather weak: the volume expansion upon complete saturation does not exceed 0.13%. The characteristic saturation time was found to be 320 h; the corresponding diffusion coefficient amounts to (2.8 ± 0.8)·10⁻¹⁴ cm²/s. The integrated reflection intensity calculations for completely saturated sample suggest that only octahedral voids are filled under the conditions of experiment. The effect of complete saturation on the rotational subsystem of the C₆₀ fullerite is rather weak: the orientational phase transition shifts by 6 to 7 K to lower temperatures; no essential hysteresis is noticeable. The dopant shows reluctance to leave the sample under a vacuum of 10⁻³ Torr at room temperature

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

The lattice parameter a of fullerite C₆₀ intercalated with N₂ molecules is investigated in the temperature interval 6–295 K by x-ray diffraction. It is found that the interstitial molecular N₂ has a considerable effect on both the temperatures, Tc of the orientational phase transition and Tg of the orientational glass formation. Hysteresis of a(T) has been detected in the Tc and Tg regions, besides, the abrupt change in the volume over the region defining Tc. Complete intercalation of C₆₀ with N₂ molecules results in a 0.2% increase in the lattice parameter, which persists over the whole temperature range. Evidence is also obtained that the interstitial guest molecular N₂ induces a slight deformation of the cubic symmetry of the host C₆₀ lattice

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

Polycrystalline fullerite С₆₀ 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 С₆₀ 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 С₆₀. 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 lowest temperatures studied the thermal expansion of the doped crystal is negative under cool-down

Structure and photoluminescence of helium-intercalated fullerite C₆₀

Intercalation of C₆₀ 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 C₆₀ 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 C₆₀ molecules, the socalled "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 C₆₀ is discussed

The effect of the thermal reduction on the kinetics of low-temperature ⁴He sorption and the structural characteristics of graphene oxide

The kinetics of the sorption and the subsequent desorption of ⁴He by the starting graphite oxide (GtO) and the thermally reduced graphene oxide samples (TRGO, T reduction = 200, 300, 500, 700 and 900 °C) have been investigated in the temperature interval 1.5–20 K. The effect of the annealing temperature on the structural characteristics of the samples was examined by the x-ray diffraction (XRD) technique. On lowering the temperature from 20 to 11–12 K, the time of ⁴He sorption increased for all the samples, which is typically observed under the condition of thermally activated diffusion. Below 5 K the characteristic times of ⁴He sorption by the GtO and TRGO-200 samples were only weakly dependent on temperature, suggesting the dominance of the tunnel mechanism. In the same region (T < 5 K) the characteristic times of the TRGOs reduced at higher temperatures (300, 500, 700 and 900 °C) were growing with lowering temperature, presumably due to the defects generated in the carbon planes on removing the oxygen functional groups (oFGs). The estimates of the activation energy ( Ea) of ⁴He diffusion show that in the TRGO-200 sample the Ea value is 2.9 times lower as compared to the parent GtO, which is accounted for by GtO exfoliation due to evaporation of the water intercalated in the interlayer space of carbon. The nonmonotonic dependences Ea( T) for the GtO samples treated above 200 °C are determined by a competition between two processes—the recovery of the graphite carbon structure, which increases the activation energy, and the generation of defects, which decreases the activation energy by opening additional surface areas and ways for sorption. The dependence of the activation energy on T reduction correlates well with the contents of the crystalline phase in GtO varying with a rise of the annealing temperature