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

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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 temperature the samples underwent an abrupt transformation as a result of which heat was released and larger x-ray detectable textured krypton crystallites were formed. In the as-grown Ar–H2 samples only the (111) reflections from the argon-rich phase were recorded. Warmup led to the same consequences, i.e. hydrogen effusion followed by 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 strong suppression of the quantum nature of hydrogen in the argon lattice environment. The experimental findings viewed as a whole can be construed as evidence of the fact that quench-condensed hydrogen-containing alloys morphologically resemble helium-impurity solids (gels) whose structure and morphology are currently being studied at Cornell University.