Unconventional stoichiometric two-dimensional potassium nitrides with anion-driven metallicity and superconductivity

Abstract

The design of compounds with interesting coordination geometries, exotic oxidation states, and intriguing properties is of fundamental interest in physics, chemistry, and materials science. Herein, first-principle swarm-intelligence structure search calculations identify two unconventional stoichiometric KN₂ and KN₄ monolayers, in which the coordination number of K atoms increases from 6 to 12, becoming two-dimensional (2D) metal nitrides with the highest coordination number. Nitrogen motifs in stable K–N monolayers depend on the nitrogen content (e.g., N₂⁻ dimer in KN₂ and N₂^0.5− dimer in KN₄), and are accompanied by an electronic transition from superconducting to metallic. Intriguingly, the KN₂ monolayer shows a unique superconducting energy gap with a calculated critical temperature (T_c) of 4.3 K under ambient conditions. The superconductivity is mainly derived from strong electron–phonon coupling (EPC) of the N 2p electrons and low- and mid-frequency K and N atomic vibrational modes. Our work provides key insight into 2D metal-bearing nitrides and the correlation between non-stoichiometry and conductivity mechanisms.