  The application of high pressure can fundamentally modify the crystalline and
electronic structures of elements as well as their chemical reactivity, which
could lead to the formation of novel materials. Here, we explore the reactivity
of lithium with sodium under high pressure, using a swarm structure searching
techniques combined with first-principles calculations, which identify a
thermodynamically stable LiNa compound adopting an orthorhombic oP8 phase at
pressure above 355 GPa. The formation of LiNa may be a consequence of strong
concentration of electrons transfer from the lithium and the sodium atoms into
the interstitial sites, which also leads to opening a relatively wide band gap
for LiNa-op8. This is substantially different from the picture that share or
exchange electrons in common compounds and alloys. In addition, lattice-dynamic
calculations indicate that LiNa-op8 remains dynamically stable when pressure
decompresses down to 70 GPa.
