The unconventionally stoichiometric compounds in the Na–K system at high pressures

Recent discoveries of alkali metal structures with novel properties at high pressures have stimulated considerable interest in binary mixtures of these metals under compression. This work reports a broad investigation of the physical and chemical behaviors of different Na–K stoichiometries by first-principles calculations and swarm-intelligence structure searching methodology. In addition to five structures previously proposed, seven new structures with stoichiometries of NaK, NaK2, and NaK3 were uncovered. New NaK2 compounds with Pm and Pnma structures are potential superconductors with Tc of 12.17 K at 250 GPa and 6.26 K at 400 GPa, respectively. The NaK and NaK3 compounds show intriguing decomposition–recombination behavior. The newly found Cmcm structure of NaK, Pm structure of NaK2, and Pmmm and Cmcm structures of NaK3 are electrides with strong interstitial electron localization. What's more, interesting reversible charge transfer between Na and K occurred as the pressure increased. Our discoveries prove that pressure is a powerful tool for creating new exotic materials in the binary alkali compounds.