As part of a systematic study of binary mixtures at high pressures, the system nitrogen-helium has been studied with a diamond anvil cell, using visual observation and quasi-isochoric pressure-temperature scanning. We observed 12 different two-phase equilibria and 8 different three-phase equilibria. These are presented in isobaric cross sections of the phase diagram. It is shown that the solid-fluid equilibria grow at the expense of the fluid-fluid equilibria. The liquidi of the solid-fluid equilibria at the nitrogen-rich side are described by simple Simon-Glatzel (SG) equations with composition dependent parameters. The fluid-fluid lines of constant composition are also fitted to SG equations. In view of the properties of the SG equation, this should permit reasonable extrapolations. The fluid-fluid separation curve shows a clear asymmetry with composition, which can qualitatively be understood from the behavior of a simple model system. The system shows the interesting phenomenon of solubility of helium in the nitrogen-rich solid. A thermodynamic argument is given to explain why helium dissolves preferentially in the orientationally ordered phase instead of in the, less dense, orientationally disordered phase. Finally, no conclusion can be drawn about the solubility of nitrogen in solid helium.
|Number of pages||23|
|Journal||Physica A: Statistical Mechanics and its Applications|
|Publication status||Published - 15 Mar 1992|