  The complex structures and electronic properties of alkali metals and their
alloys provide a natural laboratory for studying the interelectronic
interactions of metals under compression. A recent theoretical study (J. Phys.
Chem. Lett. 2019, 10, 3006) predicted an interesting pressure-induced
decomposition-recombination behavior of the Na2K compound over a pressure range
of 10 - 500 GPa. However, a subsequent experiment (Phys. Rev. B 2020, 101,
224108) reported the formation of NaK rather than Na2K at pressures above 5.9
GPa. To address this discordance, we study the chemical stability of different
stoichiometries of NaxK (x = 1/4, 1/3, 1/2, 2/3, 3/4, 4/3, 3/2 and 1 - 4) by
effective structure searching method combined with first-principles
calculations. Na2K is calculated to be unstable at 5 - 35 GPa due to the
decomposition reaction Na2K-> NaK + Na, coinciding well with the experiment.
NaK undergoes a combination-decomposition-recombination process accompanied by
an opposite charge-transfer behavior between Na and K with pressure. Besides
NaK, two hitherto unknown compounds NaK3 and Na3K2 are uncovered. NaK3 is a
typical metallic alloy, while Na3K2 is an electride with strong interstitial
electron localization.
