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

Abstract

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.

EPR spectra of deuterated methyl radicals trapped in low temperature matrices

“…The quench-condensed films of Ar and Kr are known to contain an appreciable number of structural defects. For pure Kr quench-condensed onto a 5 K substrate, the average number of planes between stacking faults is estimated as equal to 11±3, which is a very high density [13]. The matrices studied in the present work have considerably lower annealing temperatures then Kr and Ar, and would contain a much lower density of structural defects.…”

“…The matrices studied in the present work have considerably lower annealing temperatures then Kr and Ar, and would contain a much lower density of structural defects. As for impurities, it is not a direct effect on the rotation of the molecule that is interesting, because it is hard to observe (see above), but an indirect effect through the influence on the matrix morphology [13]. In turn, this influence is more prominent for mixtures of two substances differing strongly in molecular parameters [13], like Kr:D 2 and Ar:D 2 , but not Ne:D 2 .…”

Infrared-active vibron bands associated with rare gas atom dopants isolated in solid parahydrogen

“…X-ray diffraction studies of solid p-H 2 doped with Ne, Ar, or Kr indicate that the molar volume of the doped solid increases and the c/a ratio of the hexagonal close-packed (hcp) lattice decreases for Ne and Ar and increases for Kr [6,[9][10][11]. The increase in molar volume for the Ne and Ar doped solids is in violation of Vegard's law which holds that a linear relation exists between the crystal lattice constant of an alloy and the concentration of the constituent elements [12].…”

Properties of solid hydrogen doped by heavy atomic and molecular impurities

“…The fact that the excess volume and its temperature dependence are the same for the nominal gas fractions of 1% and 10% implies that the true Kr concentration in solid H 2 must not extend 1%. Our previous evaluation [5] yields an upper limit value of 4%, which does not contradict the above reasoning. The very fact of the positive effect of Kr impurities seems to be quite natural because of the larger impurity size.…”

“…At low pressures, hydrogen-containing alloys with smaller molecular (atomic) species can be expected to form random binary systems that would in many aspects resemble helium-impurity gels [3,4]. Quench condensed Ar-H 2 mixtures at sufficiently high H 2 contents in the source gas show many properties that could be treated as pertaining to gels of that kind [5]. In strongly diluted H 2 -based mixtures one can expect the formation of van der Waals (VdW) complexes, loosely bound to the crystal environment due to quantum-crystal effects.…”