A new high-pressure polymeric nitrogen phase in potassium azide

Abstract

To explore new stable polymeric nitrogen phases in alkali metal azides, the crystalline structures of potassium azide KN₃, are systematically investigated up to 400 GPa by using unbiased structure searching methods combined with first principles density functional calculations. Two high-pressure phases of KN₃, insulator C2/m phase with N³⁻ anions and metallic P6/mmm phase with “N₆” rings were uncovered above 20 GPa and 41 GPa, respectively, which are consistent with recent theoretical works. Above 274 GPa, a stable C2/m-N phase featuring polymerized N is identified for the first time and it is energetically much superior to the previously proposed C2/m-II structure. This C2/m-N structure consists of zig-zag N polymer nets which can be naturally viewed as the polymerization of “N₆” molecules rings in the low-pressure P6/mmm phase under increasing pressure. Furthermore, the structure evolutions and accompanied chemical bonding behavior of KN₃ under pressure are also discussed.