Oxygen/noble gas binary phase diagrams at 296 K and high pressures

The binary phase diagrams of O₂/rare gas (He, Ne, Ar, and Xe) mixtures have been measured at 296 K up to 20 GPa in a diamond-anvil cell. The boundary lines were determined by visual observation and by the Raman frequency of the O₂ vibron mode. The O₂-He phase diagram is of eutectic type with a liquid-liquid miscibility gap and complete immiscibility in the solid phase. The O₂-Ne phase diagram is of eutectic type with partial immiscibility in the solid phase. The O₂-Ar phase diagram is of azeotrope type and three different structures of Ar/O₂ solid solutions have been identified. The O₂-Xe phase diagram is of peritectic with eutectic type. The stoichiometric compound Xe(O₂)₂ is observed and characterized as a laves phase with a cubic structure isomorphous to MgCu₂ . A qualitative understanding of these various diagrams is reached by the comparison of the effective hard-sphere diameters of the various species in the 10 GPa range. Some of the usefulness of the knowledge of these binary phase diagrams is highlighted: as a reference data set to test theoretical calculations on mixtures; to grow single crystal of O₂ in a rare-gas pressure medium; and to synthesize rare-gas oxides at high pressure.