Search

The binary Xe–Ar system has been studied in a series of high pressure diamond anvil cell experiments up to 60 GPa at 300 K. In-situ x-ray powder diffraction and Raman spectroscopy indicate the formation of a van der Waals compound, XeAr2, at above 3.5 GPa. Powder x-ray diffraction analysis demonstrates that XeAr2 adopts a Laves MgZn2-type structure with space group P63/mmc and cell parameters a = 6.595 Å and c = 10.716 Å at 4 GPa. Density functional theory calculations support the structure determination, with agreement between experimental and calculated Raman spectra. Our DFT calculations suggest that XeAr2 would remain stable without a structural transformation or decomposition into elemental Xe and Ar up to at least 80 GPa. [ABSTRACT FROM AUTHOR]

The observation of high-temperature superconductivity in hydride sulfide (H2S) at high pressures has generated considerable interest in compressed hydrogen-rich compounds. High-pressure hydrogen selenide (H2Se) has also been predicted to be superconducting at high temperatures; however, its behaviour and stability upon compression remains unknown. In this study, we synthesize H2Se in situ from elemental Se and molecular H2 at pressures of 0.4 GPa and temperatures of 473 K. On compression at 300 K, we observe the high-pressure solid phase sequence (I-I0-IV) of H2Se through Raman spectroscopy and x-ray diffraction measurements, before dissociation into its constituent elements. Through the compression of H2Se in H2 media, we also observe the formation of a host-guest structure, (H2Se)2H2, which is stable at the same conditions as H2Se, with respect to decomposition. These measurements showthat the behaviour ofH2Se is remarkably similar to that ofH2S and provides further understanding of the hydrogen chalcogenides under pressure. [ABSTRACT FROM AUTHOR]

Molecular hydrogen forms the archetypical quantum solid. Its quantum nature is revealed by behavior which is classically impossible and by very strong isotope effects. Isotope effects between H2, D2, and HD molecules come from mass difference and the different quantum exchange effects: fermionic H2 molecules have antisymmetric wavefunctions, while bosonic D2 molecules have symmetric wavefunctions, and HD molecules have no exchange symmetry. To investigate how the phase diagram depends on quantum-nuclear effects, we use high-pressure and low-temperature in situ Raman spectroscopy to map out the phase diagrams of H2-HD-D2 with various isotope concentrations over a wide pressure-temperature (P-T) range. We find that mixtures of H2, HD, and D2 behave as an isotopic molecular alloy (ideal solution) and exhibit symmetry-breaking phase transitions between phases I and II and phase III. Surprisingly, all transitions occur at higher pressures for the alloys than either pure H2 or D2. This runs counter to any quantum effects based on isotope mass but can be explained by quantum trapping of high-kinetic energy states by the exchange interaction. [ABSTRACT FROM AUTHOR]