(color online) (a) Pressure dependent formation enthalpy of Li–Na calculated by PBE functional. Inset: the crystal structure of LiNa-oP8. (b) Enthalpies of formation of Li_mNa_n (m = 1, n = 2–5 and n = 1, m = 2–5) with respect to decomposition into elemental Li and Na at 400 GPa. The abscissa x is the fraction of Na in the structures. For elemental Li, the cmca-24 (80 GPa–185 GPa), cmca-56 (185 GPa–269 GPa) and p4₂mbc (> 269 GPa) structures^[15] are considered, and for Na, the fcc (65 GPa–103 GPa), oP8 (103 GPa–260 GPa) and hP4 (> 260 GPa) phases^[16] are used.