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 ${\mathrm{MgZn}}_2$ type structure. It is the first Laves phase observed in a van der Waals molecular compound,

• Received 2 November 1992

DOI:https://doi.org/10.1103/PhysRevLett.70.178