X-ray diffraction analysis of solid hydrogen

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The properties of compressed hydrogen are of fundamental interest in condensed matter and planetary physics. Recent optical studies up to 230 GPa have demonstrated that hydrogen has three solid phases, phase I, phase II and phase III.Structural studies of solid hydrogen by x-ray diffraction technique are severely restiricted because of the low scattering efficiency of hydrogen and the small size of sample at high pressure, and thus the structure of only phase I is crystallographically determined to be the hcp one by the single-crystal technique. Powder diffraction techniques, however, are still important because the fragmentation of single crystals is frequently caused at the structural transition accompanying a discontinuous change in volume. In this research, x-ray powder diffraction experiments at high pressures and low temperatures were carried out on the beam line BL10XU station at SPring-8. The wavelength was tuned with a Si (111) double-crystal monochromator to 0.6196 Å. An x-ray refractive lens made of molded PMMA was inserted in the x-ray path to enhance the density of the incident beam. The sample was exposed to an x-ray beam through a pinhole collimator of 26 μm diameter. Powder patterns were obtained by an angle-dispersive method with an image plate detector. Diamond anvil cell (DAC) were used for high pressure generation. X-ray powder diffraction studies on solid deuterium were carried out at 83 K and 94 GPa, at which conditions solid deuterium has been reported to transform into phase II.Three diffraction arcs from the sample were observed, which are assigned to the 100, 002 and 101 diffraction lines of the hcp lattice. Thus the center of each molecule is still on the hcp lattice in phase II.