  Pressure significantly modifies the microscopic interactions in condense
phase, leading to new patterns of bonding and unconventional chemistry. Both
argon and xenon possess closed-shell electronic structures, which renders them
chemically unreactive. Using unbiased structure searching techniques combined
with first-principles calculations, we demonstrate the reaction of argon with
xenon at a pressure as low as 1.1 GPa, producing a novel van der Waals compound
XeAr2, which crystallizes in the MgCu2-type Laves phase structure. Due to the
pressure-induced delocalization of the electrons in outermost shells, the
covalent Xe-Xe and Xe-Ar bonds have been detected which lead XeAr2 to be
unexpectedly stable without any phase transition or decomposition at least up
to 500 GPa.
