HIGH-PRESSURE MEASUREMENTS OF THE HE-NE BINARY PHASE-DIAGRAM AT 296-K - EVIDENCE FOR THE STABILITY OF A STOICHIOMETRIC NE(HE)2 SOLID

The binary phase diagram of He-Ne mixtures has been measured at 296 K in a diamond anvil cell. It is of the eutectic type with no fluid-fluid separation of phases. A homogeneous solid mixture is shown to be stable for a mole fraction of He equal to 2/3. Single-crystal synchrotron x-ray measurements indicate that this solid is ordered with 12 atoms in the unit cell. Gibbs free energy calculations support the attribution to the MgZn2 type structure. It is the first Laves phase observed in a van der Waals molecular compound.