Novel polymerization of nitrogen in zinc nitrides at high pressures

Nitrogen-rich compounds containing polynitrogen are attractive candidates for high-energy-density materials. In this work, using first-principles calculations and a particle swarm optimization structural search method, four novel nitrogen-rich structures are predicted at high pressures, i.e., two ZnN₃phases with the same space groupP1 (low-pressure phase LP-ZnN₃and high-pressure phase HP-ZnN₃),Cmm2-ZnN₅andPcc2-ZnN₆, the energy density are estimated to be 1.41 kJ g^-1, 1.88 kJ g^-1, 4.07 kJ g^-1, and 2.60 kJ g^-1, respectively. LP-ZnN₃(54-72 GPa) and HP-ZnN₃(above 72 GPa) have the lowest enthalpies in all known ZnN₃phases, and the N₆chains in LP-ZnN₃polymerize into infinite nitrogen chains in HP-ZnN₃at 72 GPa, showing a narrow-band-gap-semiconductor to metallic phase transition. Interestingly,P1-ZnN₃has a superconducting transition temperature of 6.2 K at 50 GPa and 16.3 K at 100 GPa. InCmm2-ZnN₅andPcc2-ZnN₆, nitrogen atoms polymerize into three-dimensional network structures and network layers under high pressures. Those predicted structures may enrich the phase diagram of high-pressure zinc nitrides, and provide clues for synthesis and exploration of novel stable polymeric nitrogen.