Structural studies of impurity-helium solids

Low-temperature high-energy (50 keV) electron diffraction study of size-dependent structures and growth mechanisms of neon samples in multiporous «amorphous» carbon films is presented. Electron diffractograms were analyzed on the basis of the assumption that there exists the cluster size distribution in deposits formed in substrate and multi-shell structures such as icosahedra, decahedra, fcc and hcp were probed for different sizes up to approximately 3·10 4 atoms. The analysis was based on the comparison of precise experimental and calculated diffracted intensities with the help of the R (reliability) -factor minimization procedure. Highly reproducible discrete distribution functions of sizes and structures were found. The time-dependent evolution of diffractograms at earlier stages of growth was revealed. Initially distinct diffraction peaks gradually «disappeared» although the total electron beam absorption evidenced that deposited neon was preserved in the porous substrate. We ascribed this effect to diffusion-like gas penetration from larger to smaller pores which resulted in a highly dispersed or even disordered substance. Evidently, clusters initially grown during deposition were later soaked by a sponge-like substrate due to capillary forces.

Section: Introductionmentioning

confidence: 89%

Neon in carbon nanopores: wetting, growth mechanisms, and cluster structures

Krainyukova

2007

“…6). Later studies at Cornell [7][8][9] led to the conclusion that these materials are highly porous amorphous media made up of particles or small clusters surrounded by layers of helium, which could be considered solidified due to the external forces of the impurity-helium interactions. Since hydrogen cannot guarantee a heat removal like in superfluid 4 He, in order to get a hydrogen-impurity solid it seems worthwhile to try quench condensation of the corresponding mixture at high deposition rates.…”

Section: Introductionmentioning

confidence: 99%

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

Strzhemechny

Galtsov

Prokhvatilov

2003

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-H 2 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 ³ 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-H 2 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.

“…1 was placed into the VTI. The details of the experimental procedure were described in our previous work [17,19]. For sample preparation, a gas mixture of H 2 , D 2 , and He was transported from a room temperature gas handling system to the cryogenic region.…”

Section: Experimental Methodsmentioning

confidence: 99%

“…Our preliminary x-ray investigations of D 2 -He samples showed the presence of nanoclusters of D 2 in these solids [15]. Therefore it is reasonable to assume that the structure of Im-He solids formed from light impurities is similar to that of Im-He solids formed from heavy impurities (Ne, N 2 , Kr) [16,17]. In this work we have studied impurity-helium solids formed by light impurities, namely hydrogen and deuterium atoms and molecules.…”

Section: Introductionmentioning

confidence: 99%

ESR investigation of hydrogen and deuterium atoms in impurity-helium solids

Kiselev

Khmelenko

Bernard

et al. 2003

Self Cite

pendence of H and D atom concentrations has been investigated for Im-He samples with different initial ratios of hydrogen and deuterium ranging from 1:20 to 1:1.The satellite ESR lines associated with the dipolar coupling of electron spins of H and D atoms to the nuclear moments of the hydrogen nuclei found in neighboring molecules have been observed in Im-He solids. The forbidden hyperfine transition of atomic hydrogen involving the mutual spin flips of electrons and protons has also been observed.