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

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 N20.5- 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 (Tc) 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. The design of compounds with interesting coordination geometries, exotic oxidation states, and intriguing properties is of fundamental interest in physics, chemistry, and materials science.