Prediction of Stable Ground-State Binary Sodium-Potassium Interalkalis under High Pressures

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 NaK (x = 1/4, 1/3, 1/2, 2/3, 3/4, 4/3, 3/2, and 1-4) by an 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.